Aarhus Universitets segl

Articles

Solaro, C., Meyer, S., Fisher, K., Berengut, J. C., Fuchs, E. & Drewsen, M., 2020.
"Improved isotope-shift-based bounds on bosons beyond the Standard Model through measurements of the 2D3/22D5/2 interval in Ca+"
arXiv:2005.00529.
Abstract: We perform high-resolution spectroscopy of the 3d 2D3/2 − 3d 2D5/2 interval in all stable even isotopes of ACa+ (A = 40, 42, 44, 46 and 48) with an accuracy of ≈ 20 Hz using direct frequency-comb Raman spectroscopy. Combining these data with isotope shift measurements of the 4s 2S1/2 − 3d 2D5/2 transition, we carry out a King plot analysis with unprecedented sensitivity to coupling between electrons and neutrons by bosons beyond the Standard Model. Furthermore, we estimate the sensitivity to such bosons from equivalent spectroscopy in Ba+ and Yb+. Finally, the data yield isotope shifts of the 4s 2S1/2 − 3d 2D3/2 transition at 10 part-per-billion through combination with recent data of Knollmann et al..
BibTeX:
@Article{Solaro2020,
  author        = {Solaro, Cyrille and Meyer, Steffen and Fisher, Karin and Berengut, Julian C and Fuchs, Elina and Drewsen, Michael},
  title         = {Improved isotope-shift-based bounds on bosons beyond the Standard Model through measurements of the $\^{} 2$ D $ \_ $\{$3/2$\}$-\^{} 2$ D $ \_ $\{$5/2$\}$ $ interval in Ca $\^{}+ $},
  journal       = {arXiv preprint arXiv:2005.00529},
  year          = {2020},
}
Clausen, E. H., Jarlaud, V., Fisher, K., Meyer, S., Solaro, C. & Drewsen, M., 2020.
"Unresolved sideband photon recoil spectroscopy of molecular ions"
arXiv:2004.02959.
Abstract: We reflect on the prospect of exploiting the recoil associated with absorption and emission of photons to perform spectroscopy of a single molecular ion. For this recoil to be detectable, the molecular ion is sympathetically cooled by a laser-cooled atomic ion to near the quantum mechanical ground state of a common trapping potential. More specifically, we present a general framework for simulating the expected photon recoil spectra in regimes where either the natural transition linewidth Γt of the molecular ion or the spectral width ΓL of the exciting light source exceeds the motional angular frequencies (typically ~ 2π × 0.1 - 1 MHz) of the two-ion system. To exemplify the framework, we present two complementary cases: spectroscopy of the broad 3s 2S1/2 - 3p 2P3/2 electronic transition (Γt/2π = 41.8 MHz) of a single Mg+ ion at λ=279.6 nm by a narrow laser source (ΓL/2π < 1 MHz) and mid-infrared vibrational spectroscopy of the very narrow |v=0,J=1> - |v'=1,J'=0> transition (Γt/2π = 2.50 Hz) at λ = 6.17 μm in the Σ+ electronic ground state of MgH+ by a broadband laser source (ΓL/2π ≥ 50 MHz). The atomic ion Mg+ has been picked to introduce a simple system to make comparisons with experimental results while still capturing most of the physics involved in electronic excitations of molecular ions.
BibTeX:
@Article{Clausen2020,
  author        = {Emilie H. Clausen and Vincent Jarlaud and Karin Fisher and Steffen Meyer and Cyrille Solaro and Michael Drewsen},
  title         = {Unresolved sideband photon recoil spectroscopy of molecular ions},
  journal       = {arXiv preprint arXiv:2004.02959},
  year          = {2020},
  archiveprefix = {arXiv},
  eprint        = {2004.02959},
  primaryclass  = {quant-ph},
}
Solaro, C., Meyer, S., Fisher, K., DePalatis, M. & Drewsen, M., 2018.
"Direct Frequency-Comb-Driven Raman Transitions in the Terahertz Range"
Physical Review Letters. Vol. 120, pp. 253601.
Abstract: We demonstrate the use of a femtosecond frequency comb to coherently drive stimulated Raman transitions between terahertz-spaced atomic energy levels. More specifically, we address the 3d 2D3/2 and 3d 2D5/2 fine structure levels of a single trapped 40Ca+ ion and spectroscopically resolve the transition frequency to be νD=1,819,599,021,534±8  Hz. The achieved accuracy is nearly a factor of five better than the previous best Raman spectroscopy, and is currently limited by the stability of our atomic clock reference. Furthermore, the population dynamics of frequency-comb-driven Raman transitions can be fully predicted from the spectral properties of the frequency comb, and Rabi oscillations with a contrast of 99.3(6)% and millisecond coherence time have been achieved. Importantly, the technique can be easily generalized to transitions in the sub-kHz to tens of THz range and should be applicable for driving, e.g., spin-resolved rovibrational transitions in molecules and hyperfine transitions in highly charged ions..
BibTeX:
@article{Solaro2018,
  title = {Direct Frequency-Comb-Driven Raman Transitions in the Terahertz Range},
  author = {Solaro, C. and Meyer, S. and Fisher, K. and DePalatis, M. V. and Drewsen, M.},
  journal = {Phys. Rev. Lett.},
  volume = {120},
  issue = {25},
  pages = {253601},
  numpages = {6},
  year = {2018},
  month = {Jun},
  publisher = {American Physical Society},
  doi = {10.1103/PhysRevLett.120.253601},
  url = {https://link.aps.org/doi/10.1103/PhysRevLett.120.253601}
}
Albert, M., Dantan, A. & Drewsen, M., 2018.
"Transient dynamics in cavity electromagnetically induced transparency with ion Coulomb crystals"
J. Mod. Opt. Vol. 65, pp. 602.
Abstract: We experimentally investigate the transient dynamics of an optical cavity field interacting with large ion Coulomb crystals in a situation of electromagnetically induced transparency (EIT). EIT is achieved by injecting a probe field at the single photon level and a more intense control field with opposite circular polarization into the same mode of an optical cavity to couple Zeeman substates of a metastable level in 40Ca+ ions. The EIT interaction dynamics are investigated both in the frequency-domain – by measuring the probe field steady state reflectivity spectrum – and in the time-domain – by measuring the progressive buildup of transparency. The experimental results are observed to be in excellent agreement with theoretical predictions taking into account the inhomogeneity of the control field in the interaction volume, and confirm the high degree of control on light-matter interaction that can be achieved with ion Coulomb crystals in optical cavities.
BibTeX:
@article{doi:10.1080/09500340.2017.1401135,
author = {Magnus Albert and Aurélien Dantan and Michael Drewsen},
title = {Transient dynamics in cavity electromagnetically induced transparency with ion Coulomb crystals},
journal = {Journal of Modern Optics},
volume = {65},
number = {5-6},
pages = {602-612},
year  = {2018},
publisher = {Taylor & Francis},
doi = {10.1080/09500340.2017.1401135},
URL = {https://doi.org/10.1080/09500340.2017.1401135},
eprint = {https://doi.org/10.1080/09500340.2017.1401135}
}
Aharon, N., Drewsen, M., & Retzker, A., 2017.
"Enhanced quantum sensing with multi level structures of trapped ions"
Quantum Sci. Technol. Vol. 2, pp. 034006.
Abstract: We present a method of enhanced sensing of AC magnetic fields. The method is based on the construction of a robust qubit by the application of continuous driving fields. Specifically, magnetic noise and power fluctuations of the driving fields do not operate within the robust qubit subspace, hence robustness to both external and controller noise is achieved. The scheme is applicable to either a single ion or an ensemble of ions. We consider trapped-ion based implementation via the dipole transitions, which is relevant for several types of ions, such as the 40Ca+, 88Sr+ and the 138Ba+ ions. Taking experimental errors into account, we conclude that the coherence time of the robust qubit can be improved by up to ~4 orders of magnitude compared to the coherence time of the bare states. We show how the robust qubit can be utilised for the task of sensing AC magnetic fields in the range ~ 0.1-100 MHz with an improvement of ~2 orders of magnitude of the sensitivity. In addition, we present a microwave-based sensing scheme that is suitable for ions with a hyperfine structure, such as the 9Be+,25Mg+,43Ca+,87Sr+,137Ba+,111Cd+,171Yb+ and the 199Hg+ ions. This scheme enables the enhanced sensing of high-frequency fields at the GHz level.
BibTeX:
@article{2058-9565-2-3-034006,
  author={N Aharon and M Drewsen and A Retzker},
  title={Enhanced quantum sensing with multi-level structures of trapped ions},
  journal={Quantum Science and Technology},
  volume={2},
  number={3},
  pages={034006},
  url={http://stacks.iop.org/2058-9565/2/i=3/a=034006},
  year={2017},
  abstract={We present a method of enhanced sensing of AC magnetic fields. The method is based on the construction of a robust qubit by the application of continuous driving fields. Specifically, magnetic noise and power fluctuations of the driving fields do not operate within the robust qubit subspace, hence robustness to both external and controller noise is achieved. The scheme is applicable to either a single ion or an ensemble of ions. We consider trapped-ion based implementation via the dipole transitions, which is relevant for several types of ions, such as the ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn1.gif] {${}^{40}{\mathrm{Ca}}^{+}$} , ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn2.gif] {${}^{88}{\mathrm{Sr}}^{+}$} and the ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn3.gif] {${}^{138}{\mathrm{Ba}}^{+}$} ions. Taking experimental errors into account, we conclude that the coherence time of the robust qubit can be improved by up to ∼4 orders of magnitude compared to the coherence time of the bare states. We show how the robust qubit can be utilised for the task of sensing AC magnetic fields in the range ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn4.gif] {$\sim 0.1\,-\,100\,\mathrm{MHz}$} with an improvement of ∼2 orders of magnitude of the sensitivity. In addition, we present a microwave-based sensing scheme that is suitable for ions with a hyperfine structure, such as the ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn5.gif] {${}^{9}{\mathrm{Be}}^{+}$} , ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn6.gif] {${}^{25}{\mathrm{Mg}}^{+}$} , ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn7.gif] {${}^{43}{\mathrm{Ca}}^{+}$} , ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn8.gif] {${}^{87}{\mathrm{Sr}}^{+}$} , ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn9.gif] {${}^{137}{\mathrm{Ba}}^{+}$} , ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn10.gif] {${}^{111}{\mathrm{Cd}}^{+}$} , ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn11.gif] {${}^{171}{\mathrm{Yb}}^{+}$} and the ##IMG## [http://ej.iop.org/images/2058-9565/2/3/034006/qstaa771aieqn12.gif] {${}^{199}{\mathrm{Hg}}^{+}$} ions. This scheme enables the enhanced sensing of high-frequency fields at the GHz level.}
}
Talvard, T., Westergaard, P. G., DePalatis, M. V., Mortensen, N. F., Drewsen, M., Goth, B. & Hald, J., 2017.
"Enhancement of the performance of a fiber-based frequency comb by referencing to an acetylene-stabilized fiber laser"
Optics Express. Vol. 25, pp. 2259.
Abstract: We demonstrate a significant improvement in the performance of a fiber-based frequency comb when a GPS-disciplined Rb clock is replaced with an acetylene-stabilized laser as the frequency reference. We have developed a compact, maintenance-free acetylene-stabilized fiber laser with a sub-kHz short-term linewidth and an Allan deviation below 3 x 10^-13 for integration times above 1 s. Switching the comb reference from the Rb clock to the acetylene-stabilized laser improves both comb tooth linewidth and Allan deviation by about two orders of magnitude. Furthermore, long-term measurements of the acetylene-stabilized laser frequency with reference to the GPS-disciplined clock indicate a potential relative frequency uncertainty of 2 x 10^-12.
BibTeX:
@article{Talvard2017,
author = {Thomas Talvard and Philip G. Westergaard and Michael V. DePalatis and Nicolai F. Mortensen and Michael Drewsen and Bjarke G{\o}th and Jan Hald},
journal = {Opt. Express},
keywords = {Metrology; Lasers and laser optics; Mode-locked lasers; Laser stabilization ; Erbium doped fiber amplifiers; Fiber lasers; Frequency combs; Laser sources; Optical components; Saturation spectroscopy},
number = {3},
pages = {2259--2269},
publisher = {OSA},
title = {Enhancement of the performance of a fiber-based frequency comb by referencing to an acetylene-stabilized fiber laser},
volume = {25},
month = {Feb},
year = {2017},
url = {http://www.opticsexpress.org/abstract.cfm?URI=oe-25-3-2259},
doi = {10.1364/OE.25.002259},
abstract = {We demonstrate a significant improvement in the performance of a fiber-based frequency comb when a GPS-disciplined Rb clock is replaced with an acetylene-stabilized laser as the frequency reference. We have developed a compact, maintenance-free acetylene-stabilized fiber laser with a sub-kHz short-term linewidth and an Allan deviation below 3\&\#x000D7;10\&\#x02212;13 for integration times above 1 s. Switching the comb reference from the Rb clock to the acetylene-stabilized laser improves both comb tooth linewidth and Allan deviation by about two orders of magnitude. Furthermore, long-term measurements of the acetylene-stabilized laser frequency with reference to the GPS-disciplined clock indicate a potential relative frequency uncertainty of 2 \&\#x000D7; 10\&\#x02212;12.},
}
Berglund, J.M., Drewsen, M. & Koch, C.P., 2015.
"Femtosecond wavepacket interferometry using the rotational dynamics of a trapped cold molecular ion"
New Journal of Physics. Vol. 17(2), pp. 025007.
Abstract: A Ramsey-type interferometer is suggested, employing a cold trapped ion and two time-delayed off-resonant femtosecond laser pulses. The laser light couples to the molecular polarization anisotropy, inducing rotational wavepacket dynamics. An interferogram is obtained from the delay dependent populations of the final field-free rotational states. Current experimental capabilities for cooling and preparation of the initial state are found to yield an interferogram visibility of more than 80%. The interferograms can be used to determine the polarizability anisotropy with an accuracy of about ±2%, respectively ±5%, provided the uncertainty in the initial populations and measurement errors are confined to within the same limits.
BibTeX:
@article{Berglund2015,
  author = {J Martin Berglund and Michael Drewsen and Christiane P Koch},
  title = {Femtosecond wavepacket interferometry using the rotational dynamics of a trapped cold molecular ion},
  year = {2015},
  volume = {17},
  number = {2},
  pages = {025007},
  url = {http://dx.doi.org/10.1088/1367-2630/17/2/025007}
}
Drewsen, M., 2015.
"Ion Coulomb crystals "
Physica B: Condensed Matter . Vol. 460(0), pp. 105 - 113.
Abstract: Abstract The following text will give a brief introduction to the physics of the spatially ordered structures, so-called Coulomb crystals, that appear when confined ions are cooled to sufficiently low temperatures. It will as well briefly comment on the very diverse scientific applications of such crystals, which have emerged the past two decades. While this document lacks figures, it includes a substantial number of references in which more detailed information can be found. It is the hope that the text will stimulate the reader to dig deeper into one or more of the discussed subjects and inspire her/him to think about new potential applications.
BibTeX:
@article{Drewsen2015105,
  author = {Michael Drewsen},
  title = {Ion Coulomb crystals },
  year = {2015},
  volume = {460},
  number = {0},
  pages = {105 - 113},
  note = {Special Issue on Electronic Crystals (ECRYS-2014) },
  url = {http://www.sciencedirect.com/science/article/pii/S0921452614008849},
  doi = {http://dx.doi.org/10.1016/j.physb.2014.11.050}
}
Hansen, A.K., Versolato, O.O., Klosowski, L., Kristensen, S.B., Gingell, A., Schwarz, M., Windberger, A., Ullrich, J., Lopez-Urrutia, J.R.C. & Drewsen, M., 2014.
"Efficient rotational cooling of Coulomb-crystallized molecular ions by a helium buffer gas"
Nature. Vol. advance online publication, pp. -.
Abstract: The preparation of cold molecules is of great importance in many contexts, such as fundamental physics investigations, high-resolution spectroscopy of complex molecules, cold chemistry and astrochemistry. One versatile and widely applied method to cool molecules is helium buffer-gas cooling in either a supersonic beam expansion or a cryogenic trap environment. Another more recent method applicable to trapped molecular ions relies on sympathetic translational cooling, through collisional interactions with co-trapped, laser-cooled atomic ions, into spatially ordered structures called Coulomb crystals, combined with laser-controlled internal-state preparation6. Here we present experimental results on helium buffer-gas cooling of the rotational degrees of freedom of MgH+ molecular ions, which have been trapped and sympathetically cooled in a cryogenic linear radio-frequency quadrupole trap. With helium collision rates of only about ten per second—that is, four to five orders of magnitude lower than in typical buffer-gas cooling settings—we have cooled a single molecular ion to a rotational temperature of 7.5+0.9/-0.7  kelvin, the lowest such temperature so far measured. In addition, by varying the shape of, or the number of atomic and molecular ions in, larger Coulomb crystals, or both, we have tuned the effective rotational temperature from about 7 kelvin to about 60 kelvin by changing the translational micromotion energy of the ions. The extremely low helium collision rate may allow for sympathetic sideband cooling of single molecular ions, and eventually make quantum-logic spectroscopy of buffer-gas-cooled molecular ions feasible. Furthermore, application of the present cooling scheme to complex molecular ions should enable single- or few-state manipulations of individual molecules of biological interest.
BibTeX:
@article{Hansen2014,
  author = {Hansen, A. K. and Versolato, O. O. and Klosowski, L. and Kristensen, S. B. and Gingell, A. and Schwarz, M. and Windberger, A. and Ullrich, J. and Lopez-Urrutia, J. R. Crespo and Drewsen, M.},
  title = {Efficient rotational cooling of Coulomb-crystallized molecular ions by a helium buffer gas},
  publisher = {Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.},
  year = {2014},
  volume = {advance online publication},
  pages = {--},
  url = {http://dx.doi.org/10.1038/nature12996}
}
Linnet, R., Leroux, I., Dantan, A. & Drewsen, M., 2014.
"Sub-micron positioning of trapped ions with respect to the absolute center of a standing-wave cavity field"
Applied Physics B. Vol. 114(1-2), pp. 295-301.
Abstract: We demonstrate that it is possible, with sub-micron precision, to locate the absolute center of a Fabry–Pérot resonator oriented along the radiofrequency-field-free axis of a linear Paul trap through the application of two simultaneously resonating optical fields. In particular, we apply a probe field, which is near-resonant with an electronic transition of trapped ions, simultaneously with an off-resonant strong field acting as a periodic AC Stark-shifting potential. Through the resulting spatially modulated fluorescence signal, we can find the cavity center of an 11.7-mm-long symmetric Fabry–Pérot cavity with a precision of ±135 nm, which is smaller than the periodicity of the individual standing-wave fields. This can, e.g., be used to position the minimum of the axial trap potential with respect to the center of the cavity at any location along the cavity mode.
BibTeX:
@article{raey,
  author = {Linnet, RasmusB. and Leroux, IanD. and Dantan, Aurélien and Drewsen, Michael},
  title = {Sub-micron positioning of trapped ions with respect to the absolute center of a standing-wave cavity field},
  publisher = {Springer Berlin Heidelberg},
  year = {2014},
  volume = {114},
  number = {1-2},
  pages = {295-301},
  url = {http://dx.doi.org/10.1007/s00340-013-5666-0},
  doi = {http://dx.doi.org/10.1007/s00340-013-5666-0}
}
Shi, M., Herskind, P.F., Drewsen, M. & Chuang, I.L., 2013.
"Microwave quantum logic spectroscopy and control of molecular ions"
New Journal of Physics. Vol. 15(11), pp. 113019.
Abstract: A general method for rotational microwave spectroscopy and control of polar molecular ions via direct microwave addressing is considered. Our method makes use of spatially varying ac Stark shifts, induced by far off-resonant, focused laser beams to achieve an effective coupling between the rotational state of a molecular ion and the electronic state of an atomic ion. In this setting, the atomic ion is used for read-out of the molecular ion state, in a manner analogous to quantum logic spectroscopy based on Raman transitions. In addition to high-precision spectroscopy, this setting allows for rotational ground state cooling, and can be considered as a candidate for the quantum information processing with polar molecular ions. All elements of our proposal can be realized with currently available technology.
BibTeX:
@article{1367-2630-15-11-113019,
  author = {M Shi and P F Herskind and M Drewsen and I L Chuang},
  title = {Microwave quantum logic spectroscopy and control of molecular ions},
  year = {2013},
  volume = {15},
  number = {11},
  pages = {113019},
  url = {http://iopscience.iop.org/1367-2630/15/11/113019/}
}
Clausen, C., Sangouard, N. & Drewsen, M., 2013.
"Analysis of a photon number resolving detector based on fluorescence readout of an ion Coulomb crystal quantum memory inside an optical cavity"
New Journal of Physics. Vol. 15(2), pp. 025021.
Abstract: The ability to detect single photons with a high efficiency is a crucial requirement for various quantum information applications. By combining the storage process of a quantum memory for photons with fluorescence-based quantum state measurement, it is, in principle, possible to achieve high-efficiency photon counting in large ensembles of atoms. The large number of atoms can, however, pose significant problems in terms of noise stemming from imperfect initial state preparation and off-resonant fluorescence. We identify and analyse a concrete implementation of a photon number resolving detector based on an ion Coulomb crystal inside a moderately high-finesse optical cavity. The cavity enhancement leads to an effective optical depth of 15 for a finesse of 3000 with only about 1500 ions interacting with the light field. We show that these values allow for essentially noiseless detection with an efficiency larger than 93%. Moderate experimental parameters allow for repetition rates of about 3Â kHz, limited by the time needed for fluorescence collection and re-cooling of the ions between trials. Our analysis may lead to the first implementation of a photon number resolving detector in atomic ensembles.
BibTeX:
@article{1367-2630-15-2-025021,
  author = {Christoph Clausen and Nicolas Sangouard and Michael Drewsen},
  title = {Analysis of a photon number resolving detector based on fluorescence readout of an ion Coulomb crystal quantum memory inside an optical cavity},
  year = {2013},
  volume = {15},
  number = {2},
  pages = {025021},
  url = {http://iopscience.iop.org/1367-2630/15/2/025021/}
}
Versolato, O.O., Schwarz, M., Hansen, A.K., Gingell, A.D., Windberger, A., Kłosowski, L., Ullrich, J., Jensen, F., Crespo López-Urrutia, J.R. & Drewsen, M., 2013.
"Decay Rate Measurement of the First Vibrationally Excited State of MgH+ in a Cryogenic Paul Trap"
Phys. Rev. Lett.. Vol. 111, pp. 053002.
Abstract: We present a method to measure the decay rate of the first excited vibrational state of polar molecular ions that are part of a Coulomb crystal in a cryogenic linear Paul trap. Specifically, we have monitored the decay of the |ν=1,J=1⟩X towards the |ν=0,J=0⟩X level in MgH+ by saturated laser excitation of the |ν=0,J=2⟩X-|ν=1,J=1⟩X transition followed by state selective resonance enhanced two-photon dissociation out of the |ν=0,J=2⟩X level. The experimentally observed rate of 6.32(0.69)  s-1 is in excellent agreement with the theory value of 6.13(0.03)  s-1 (this Letter). The technique enables the determination of decay rates, and thus absorption strengths, with an accuracy at the few percent level.
BibTeX:
@article{PhysRevLett.111.053002,
  author = {Versolato, O. O. and Schwarz, M. and Hansen, A. K. and Gingell, A. D. and Windberger, A. and Kłosowski, L. and Ullrich, J. and Jensen, F. and Crespo López-Urrutia, J. R. and Drewsen, M.},
  title = {Decay Rate Measurement of the First Vibrationally Excited State of MgH+ in a Cryogenic Paul Trap},
  publisher = {American Physical Society},
  year = {2013},
  volume = {111},
  pages = {053002},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.111.053002},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.111.053002}
}
Aharon, N., Drewsen, M. & Retzker, A., 2013.
"General Scheme for the Construction of a Protected Qubit Subspace"
Phys. Rev. Lett.. Vol. 111, pp. 230507.
Abstract: We present a new robust decoupling scheme suitable for levels with either half-integer or integer angular momentum states. Through continuous dynamical decoupling techniques, we create a protected qubit subspace, utilizing a multistate qubit construction. Remarkably, the multistate system can also be composed of multiple substates within a single level. Our scheme can be realized with state-of-the-art experimental setups and thus has immediate applications for quantum information science. While the scheme is general and relevant for a multitude of solid-state and atomic systems, we analyze its performance for the case composed of trapped ions. Explicitly, we show how single qubit gates and an ensemble coupling to a cavity mode can be implemented efficiently. The scheme predicts a coherence time of ∼1  s, as compared to typically a few milliseconds for the bare states.
BibTeX:
@article{PhysRevLett.111.230507,
  author = {Aharon, N. and Drewsen, M. and Retzker, A.},
  title = {General Scheme for the Construction of a Protected Qubit Subspace},
  publisher = {American Physical Society},
  year = {2013},
  volume = {111},
  pages = {230507},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.111.230507},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.111.230507}
}
Versolato, O., Schwarz, M., Windberger, A., Ullrich, J., Schmidt, P., Drewsen, M. & Crespo López-Urrutia, J., 2013.
"Cold highly charged ions in a cryogenic Paul trap"
Hyperfine Interactions. Vol. 214(1-3), pp. 189-194.
Abstract: Narrow optical transitions in highly charged ions (HCIs) are of particular interest for metrology and fundamental physics, exploiting the high sensitivity of HCIs to new physics. The highest sensitivity for a changing fine structure constant ever predicted for a stable atomic system is found in Ir17 + . However, laser spectroscopy of HCIs is hindered by the large (∼ 10^6 K) temperatures at which they are produced and trapped. An unprecedented improvement in such laser spectroscopy can be obtained when HCIs are cooled down to the mK range in a linear Paul trap. We have developed a cryogenic linear Paul trap in which HCIs will be sympathetically cooled by 9Be +  ions. Optimized optical access for laser light is provided while maintaining excellent UHV conditions. The Paul trap will be connected to an electron beam ion trap (EBIT) which is able to produce a wide range of HCIs. This EBIT will also provide the first experimental input needed for the determination of the transition energies in Ir17 + , enabling further laser-spectroscopic investigations of this promising HCI.
BibTeX:
@article{versolato2013,
  author = {Versolato, O.O. and Schwarz, M. and Windberger, A. and Ullrich, J. and Schmidt, P.O. and Drewsen, M. and Crespo López-Urrutia, J.R.},
  title = {Cold highly charged ions in a cryogenic Paul trap},
  publisher = {Springer Netherlands},
  year = {2013},
  volume = {214},
  number = {1-3},
  pages = {189-194},
  url = {http://dx.doi.org/10.1007/s10751-013-0806-9},
  doi = {http://dx.doi.org/10.1007/s10751-013-0806-9}
}
Dantan, A., 2012.
"Optical quantum swapping in a coherent atomic medium"
EPL (Europhysics Letters). Vol. 97(3), pp. 34010.
Abstract: We propose to realize a passive optical quantum swapping device which allows for the exchange of the quantum fluctuations of two bright optical fields interacting with a coherent atomic medium in an optical cavity. The device is based on a quantum interference process between the fields within the cavity bandwidth arising from coherent population trapping in the atomic medium.
BibTeX:
@article{0295-5075-97-3-34010,
  author = {A. Dantan},
  title = {Optical quantum swapping in a coherent atomic medium},
  year = {2012},
  volume = {97},
  number = {3},
  pages = {34010},
  url = {http://iopscience.iop.org/0295-5075/97/3/34010/}
}
Zangenberg, K.R., Dantan, A. & Drewsen, M., 2012.
"Spatial mode effects in a cavity EIT-based quantum memory with ion Coulomb crystals"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 45(12), pp. 124011.
Abstract: Quantum storage and retrieval of light in ion Coulomb crystals using cavity electromagnetically induced transparency are investigated theoretically. It is found that when both the control and the probe fields are coupled to the same spatial cavity mode, their transverse mode profile affects the quantum memory efficiency in a non-trivial way. Under such conditions, the control-field parameters and crystal dimensions that maximize the memory efficiency are calculated.
BibTeX:
@article{0953-4075-45-12-124011,
  author = {Kasper R Zangenberg and Aurelien Dantan and Michael Drewsen},
  title = {Spatial mode effects in a cavity EIT-based quantum memory with ion Coulomb crystals},
  year = {2012},
  volume = {45},
  number = {12},
  pages = {124011},
  url = {http://iopscience.iop.org/0953-4075/45/12/124011/}
}
Landa, H., Drewsen, M., Reznik, B. & Retzker, A., 2012.
"Modes of oscillation in radiofrequency Paul traps"
New Journal of Physics. Vol. 14(9), pp. 093023.
Abstract: We examine the time-dependent dynamics of ion crystals in radiofrequency traps. The problem of stable trapping of general three-dimensional crystals is considered and the validity of the pseudopotential approximation is discussed. We analytically derive the micromotion amplitude of the ions, rigorously proving well-known experimental observations. We use a recently proposed method to find the modes that diagonalize the linearized time-dependent dynamical problem. This allows one to obtain explicitly the (‘Floquet–Lyapunov’) transformation to coordinates of decoupled linear oscillators. We demonstrate the utility of the method by analyzing the modes of a small ‘peculiar’ crystal in a linear Paul trap. The calculations can be readily generalized to multispecies ion crystals in general multipole traps, and time-dependent quantum wavefunctions of ion oscillations in such traps can be obtained.
BibTeX:
@article{1367-2630-14-9-093023,
  author = {H Landa and M Drewsen and B Reznik and A Retzker},
  title = {Modes of oscillation in radiofrequency Paul traps},
  year = {2012},
  volume = {14},
  number = {9},
  pages = {093023},
  url = {http://iopscience.iop.org/1367-2630/14/9/093023/}
}
Landa, H., Drewsen, M., Reznik, B. & Retzker, A., 2012.
"Classical and quantum modes of coupled Mathieu equations"
Journal of Physics A: Mathematical and Theoretical. Vol. 45(45), pp. 455305.
Abstract: We expand the solutions of linearly coupled Mathieu equations in terms of infinite-continued matrix inversions, and use it to find the modes which diagonalize the dynamical problem. This allows obtaining explicitly the (Floquet-Lyapunov) transformation to coordinates in which the motion is that of decoupled linear oscillators. We use this transformation to solve the Heisenberg equations of the corresponding quantum-mechanical problem, and find the quantum wavefunctions for stable oscillations, expressed in configuration space. The obtained transformation and quantum solutions can be applied to more general linear systems with periodic coefficients (coupled Hill equations, periodically driven parametric oscillators), and to nonlinear systems as a starting point for convenient perturbative treatment of the nonlinearity.
BibTeX:
@article{1751-8121-45-45-455305,
  author = {H Landa and M Drewsen and B Reznik and A Retzker},
  title = {Classical and quantum modes of coupled Mathieu equations},
  year = {2012},
  volume = {45},
  number = {45},
  pages = {455305},
  url = {http://iopscience.iop.org/1751-8121/45/45/455305/}
}
Drewsen, M., Matthey, T., Mortensen, A. & Petter Hansen, J., 2012.
"Direct imaging of thermally excited metastable structures of ion Coulomb clusters"
ArXiv e-prints.
Abstract: Coulomb crystallisation of large ensembles of ions has in the past years been intensively studied experimentally with many spectacular results of relevance to infinite systems in one-, two-, and three-dimensions.While strings of a few ions have proven to be very attractive objects in quantum information processing, larger Coulomb crystals have very recently found applications within other aspects the dynamics of quantum systems. Smaller finite ensembles of cold identical charged particles confined by a harmonic potential furthermore constitute very special types of clusters due to the pure repulsive long-range inter-particle forces. Here, we report on the direct imaging of metastable structures of Coulomb clusters consisting of a few thousands confined and laser-cooled 40Ca+ ions. The observations are attributed to structural excitations due to finite temperatures, a feature likely to appear in clusters of short-range interacting particles, but yet not observed directly.
BibTeX:
@article{2012arXiv1202.2544D,
  author = {Drewsen, M. and Matthey, T. and Mortensen, A. and Petter Hansen, J. },
  title = {Direct imaging of thermally excited metastable structures of ion Coulomb clusters},
  year = {2012},
  url = {http://arxiv.org/abs/1202.2544}
}
Hansen, A.K., Sørensen, M.A., Staanum, P.F. & Drewsen, M., 2012.
"Single-Ion Recycling Reactions"
Angewandte Chemie International Edition. Vol. 51(32), pp. 7960-7962.
Abstract: A single ion is enough: Ion reaction rates and reaction product branching ratios could be determined through repeated regeneration of the original target ion by photodissociation after each reaction. The product molecule was identified through nondestructive mass spectrometry. Finally, the target ion was regenerated through photodissociation of the molecular ion (see picture).
BibTeX:
@article{ANIE:ANIE201203550,
  author = {Hansen, Anders K. and Sørensen, Magnus A. and Staanum, Peter F. and Drewsen, Michael},
  title = {Single-Ion Recycling Reactions},
  publisher = {WILEY-VCH Verlag},
  year = {2012},
  volume = {51},
  number = {32},
  pages = {7960--7962},
  url = {http://dx.doi.org/10.1002/anie.201203550},
  doi = {http://dx.doi.org/10.1002/anie.201203550}
}
Miroshnychenko, Y., Nielsen, O., Thorsen, A. & Drewsen, M., 2012.
"Profiling of micrometer-sized laser beams in restricted volumes"
Appl. Opt.. Vol. 51(13), pp. 2341-2345.
Abstract: We present a method for determining the three-dimensional intensity distribution of directed laser radiation with micrometer resolution in restricted volumes. Our method is based on the incoupling and guiding properties of optical fibers, with the current version requiring only a few hundred micrometers across the measuring volume. We characterize the performance of the method and experimentally demonstrate profiling of micrometer-sized laser beams. We discuss the limiting factors and routes toward a further increase of the resolution and beam profiling in even more restricted volumes. Finally, as an application example, we present profiling of laser beams inside a micro ion trap with integrated optical fibers.
BibTeX:
@article{Miroshnychenko:12,
  author = {Yevhen Miroshnychenko and Otto Nielsen and Aske Thorsen and Michael Drewsen},
  title = {Profiling of micrometer-sized laser beams in restricted volumes},
  publisher = {OSA},
  year = {2012},
  volume = {51},
  number = {13},
  pages = {2341--2345},
  url = {https://www.osapublishing.org/ao/abstract.cfm?URI=ao-51-13-2341},
  doi = {http://dx.doi.org/10.1364/AO.51.002341}
}
Dantan, A., Albert, M. & Drewsen, M., 2012.
"All-cavity electromagnetically induced transparency and optical switching: Semiclassical theory"
Phys. Rev. A. Vol. 85, pp. 013840.
Abstract: The transmission of a probe field experiencing electromagnetically induced transparency and optical switching in an atomic medium enclosed in an optical cavity is investigated. Using a semiclassical input-output theory for the interaction between an ensemble of four-level atoms and three optical cavity fields coupled to the same spatial cavity mode, we derive the steady-state transmission spectra of the probe field and discuss the dynamics of the intracavity field buildup. The analytical and numerical results are in good agreement with recent experiments with ion Coulomb crystals [ M. Albert et al. Nature Photon. 5 633 (2011)].
BibTeX:
@article{PhysRevA.85.013840,
  author = {Dantan, Aurélien and Albert, Magnus and Drewsen, Michael},
  title = {All-cavity electromagnetically induced transparency and optical switching: Semiclassical theory},
  publisher = {American Physical Society},
  year = {2012},
  volume = {85},
  pages = {013840},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.85.013840},
  doi = {http://dx.doi.org/10.1103/PhysRevA.85.013840}
}
Albert, M., Marler, J.P., Herskind, P.F., Dantan, A. & Drewsen, M., 2012.
"Collective strong coupling between ion Coulomb crystals and an optical cavity field: Theory and experiment"
Phys. Rev. A. Vol. 85, pp. 023818.
Abstract: A detailed description and theoretical analysis of experiments achieving coherent coupling between an ion Coulomb crystal and an optical cavity field are presented. The various methods used to measure the coherent coupling rate between large ion Coulomb crystals in a linear quadrupole radiofrequency ion trap and a single-field mode of a moderately high-finesse cavity are described in detail. Theoretical models based on a semiclassical approach are applied in assessment of the experimental results of P. F. Herskind et al.[ Nature Phys. 5 494 (2009)] and of complementary new measurements. Generally, a very good agreement between theory and experiments is obtained.
BibTeX:
@article{PhysRevA.85.023818,
  author = {Albert, M. and Marler, J. P. and Herskind, P. F. and Dantan, A. and Drewsen, M.},
  title = {Collective strong coupling between ion Coulomb crystals and an optical cavity field: Theory and experiment},
  publisher = {American Physical Society},
  year = {2012},
  volume = {85},
  pages = {023818},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.85.023818},
  doi = {http://dx.doi.org/10.1103/PhysRevA.85.023818}
}
Horak, P., Dantan, A. & Drewsen, M., 2012.
"Optically induced structural phase transitions in ion Coulomb crystals"
Phys. Rev. A. Vol. 86, pp. 043435.
Abstract: We investigate numerically the structural dynamics of ion Coulomb crystals confined in a three-dimensional harmonic trap when influenced by an additional one-dimensional optically induced periodical potential. We demonstrate that transitions between thermally excited crystal structures, such as body-centered cubic and face-centered cubic, can be suppressed by a proper choice of the potential depth and periodicity. Furthermore, by varying the harmonic trap parameters and/or the optical potential in time, controlled transitions between crystal structures can be obtained with close to unit efficiency.
BibTeX:
@article{PhysRevA.86.043435,
  author = {Horak, Peter and Dantan, Aurélien and Drewsen, Michael},
  title = {Optically induced structural phase transitions in ion Coulomb crystals},
  publisher = {American Physical Society},
  year = {2012},
  volume = {86},
  pages = {043435},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.86.043435},
  doi = {http://dx.doi.org/10.1103/PhysRevA.86.043435}
}
Poulsen, G., Miroshnychenko, Y. & Drewsen, M., 2012.
"Efficient ground-state cooling of an ion in a large room-temperature linear Paul trap with a sub-Hertz heating rate"
Phys. Rev. A. Vol. 86, pp. 051402.
Abstract: We demonstrate efficient resolved sideband laser cooling (99±1% ground-state population) of a single 40Ca+ ion in a large linear Paul trap (electrode spacing of 7 mm) operated at an rf drive frequency of just 3.7 MHz. For ion oscillation frequencies in the range 280–585 kHz, heating rates below or about one motional quantum per second have been measured at room temperature. The results, obtained under these unconventional sideband cooling conditions, pave the way for a range of new types of cold ion experiments, including spectroscopy of molecular ions as well as ultracold chemistry.
BibTeX:
@article{PhysRevA.86.051402,
  author = {Poulsen, G. and Miroshnychenko, Y. and Drewsen, M.},
  title = {Efficient ground-state cooling of an ion in a large room-temperature linear Paul trap with a sub-Hertz heating rate},
  publisher = {American Physical Society},
  year = {2012},
  volume = {86},
  pages = {051402},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.86.051402},
  doi = {http://dx.doi.org/10.1103/PhysRevA.86.051402}
}
Xuereb, A., Genes, C. & Dantan, A., 2012.
"Strong Coupling and Long-Range Collective Interactions in Optomechanical Arrays"
Phys. Rev. Lett.. Vol. 109, pp. 223601.
Abstract: We investigate the collective optomechanics of an ensemble of scatterers inside a Fabry-Pérot resonator and identify an optimized configuration where the ensemble is transmissive, in contrast to the usual reflective optomechanics approach. In this configuration, the optomechanical coupling of a specific collective mechanical mode can be several orders of magnitude larger than the single-element case, and long-range interactions can be generated between the different elements since light permeates throughout the array. This new regime should realistically allow for achieving strong single-photon optomechanical coupling with massive resonators, realizing hybrid quantum interfaces, and exploiting collective long-range interactions in arrays of atoms or mechanical oscillators.
BibTeX:
@article{PhysRevLett.109.223601,
  author = {Xuereb, André and Genes, Claudiu and Dantan, Aurélien},
  title = {Strong Coupling and Long-Range Collective Interactions in Optomechanical Arrays},
  publisher = {American Physical Society},
  year = {2012},
  volume = {109},
  pages = {223601},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.109.223601},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.109.223601}
}
Linnet, R.B., Leroux, I.D., Marciante, M., Dantan, A. & Drewsen, M., 2012.
"Pinning an Ion with an Intracavity Optical Lattice"
Phys. Rev. Lett.. Vol. 109, pp. 233005.
Abstract: We report one-dimensional pinning of a single ion by an optical lattice. A standing-wave cavity produces the lattice potential along the rf-field-free axis of a linear Paul trap. The ion’s localization is detected by measuring its fluorescence when excited by standing-wave fields with the same period, but different spatial phases. The experiments agree with an analytical model of the localization process, which we test against numerical simulations. For the best localization achieved, the ion’s average coupling to the cavity field is enhanced from 50% to 81(3)% of its maximum possible value, and we infer that the ion is bound in a lattice well with over 97% probability.
BibTeX:
@article{PhysRevLett.109.233005,
  author = {Linnet, Rasmus B. and Leroux, Ian D. and Marciante, Mathieu and Dantan, Aurélien and Drewsen, Michael},
  title = {Pinning an Ion with an Intracavity Optical Lattice},
  publisher = {American Physical Society},
  year = {2012},
  volume = {109},
  pages = {233005},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.109.233005},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.109.233005}
}
Schwarz, M., Versolato, O.O., Windberger, A., Brunner, F.R., Ballance, T., Eberle, S.N., Ullrich, J., Schmidt, P.O., Hansen, A.K., Gingell, A.D., Drewsen, M. & Lopez-Urrutia, J.R.C., 2012.
"Cryogenic linear Paul trap for cold highly charged ion experiments"
Review of Scientific Instruments. Vol. 83(8), pp. 083115.
Abstract: Storage and cooling of highly charged ions require ultra-high vacuum levels obtainable by means of cryogenic methods. We have developed a linear Paul trap operating at 4 K capable of very long ion storage times of about 30 h. A conservative upper bound of the H2 partial pressure of about 10−15 mbar (at 4 K) is obtained from this. External ion injection is possible and optimized optical access for lasers is provided, while exposure to black body radiation is minimized. First results of its operation with atomic and molecular ions are presented. An all-solid state laser system at 313 nm has been set up to provide cold Be+ ions for sympathetic cooling of highly charged ions.
BibTeX:
@article{schwarz:083115,
  author = {M. Schwarz and O. O. Versolato and A. Windberger and F. R. Brunner and T. Ballance and S. N. Eberle and J. Ullrich and P. O. Schmidt and A. K. Hansen and A. D. Gingell and M. Drewsen and J. R. Crespo Lopez-Urrutia},
  title = {Cryogenic linear Paul trap for cold highly charged ion experiments},
  publisher = {AIP},
  year = {2012},
  volume = {83},
  number = {8},
  pages = {083115},
  url = {http://scitation.aip.org/content/aip/journal/rsi/83/8/10.1063/1.4742770},
  doi = {http://dx.doi.org/10.1063/1.4742770}
}
Albert, M., Dantan, A. & Drewsen, M., 2011.
"Cavity electromagnetically induced transparency and all-optical switching using ion Coulomb crystals"
Nature Photonics. Vol. 5, pp. 633.
Abstract: The control of one light field by another, ultimately at the single photon level, is a challenging task which has numerous interesting applications within nonlinear optics and quantum information science. Due to the extremely weak direct interactions between optical photons in vacuum, this type of control can in practice only be achieved through highly nonlinear interactions within a medium. Electromagnetic induced transparency (EIT) constitutes one such means to obtain the extremely strong nonlinear coupling needed to facilitate interactions between two faint light fields. Here, we demonstrate for the first time EIT as well as all-optical EIT-based light switching using ion Coulomb crystals situated in an optical cavity. Unprecedented narrow cavity EIT feature widths down to a few kHz and a change from essentially full transmission to full absorption of the probe field within a window of only 100 kHz are achieved. By applying a weak switching field, we furthermore demonstrate nearly perfect switching of the transmission of the probe field. These results represent important milestones for future realizations of quantum information processing devices, such as high-efficiency quantum memories, single-photon transistors and single-photon gates.
BibTeX:
@article{Albert2011,
  author = {Albert, Magnus and Dantan, Aurelien and Drewsen, Michael},
  title = {Cavity electromagnetically induced transparency and all-optical switching using ion Coulomb crystals},
  year = {2011},
  volume = {5},
  pages = {633},
  url = {http://www.nature.com/nphoton/journal/v5/n10/abs/nphoton.2011.214.html},
  doi = {http://dx.doi.org/10.1038/nphoton.2011.214}
}
Genes, C., Ritsch, H., Drewsen, M. & Dantan, A., 2011.
"Atom-membrane cooling and entanglement using cavity electromagnetically induced transparency"
Physical Review A. Vol. 84, pp. 051801.
Abstract: We investigate a hybrid optomechanical system composed of a micromechanical oscillator as a movable membrane and an atomic three-level ensemble within an optical cavity. We show that a suitably tailored cavity field response via electromagnetically induced transparency (EIT) in the atomic medium allows for strong coupling of the membrane's mechanical oscillations to the collective atomic ground-state spin. This facilitates ground-state cooling of the membrane motion, quantum state mapping, and robust atom-membrane entanglement even for cavity widths larger than the mechanical resonance frequency.
BibTeX:
@article{Genes2011,
  author = {Genes, Claudio and Ritsch, Helmut and Drewsen, Michael and Dantan, Aurelien},
  title = {Atom-membrane cooling and entanglement using cavity electromagnetically induced transparency},
  year = {2011},
  volume = {84},
  pages = {051801},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.84.051801},
  doi = {http://dx.doi.org/10.1103/PhysRevA.84.051801}
}
Dantan, A., Marler, J.P., Albert, M., Guénot, D. & Drewsen, M., 2010.
"Noninvasive Vibrational Mode Spectroscopy of Ion Coulomb Crystals through Resonant Collective Coupling to an Optical Cavity Field"
Physical Review Letters. Vol. 105(10), pp. 103001+.
Abstract: We report on a novel noninvasive method to determine the normal mode frequencies of ion Coulomb crystals in traps based on the resonance enhanced collective coupling between the electronic states of the ions and an optical cavity field at the single photon level. Excitations of the normal modes are observed through a Doppler broadening of the resonance. An excellent agreement with the predictions of a zero-temperature uniformly charged liquid plasma model is found. The technique opens up for investigations of the heating and damping of cold plasma modes, as well as the coupling between them.
BibTeX:
@article{Dantan2010Noninvasive,
  author = {Dantan, A. and Marler, J. P. and Albert, M. and Guénot, D. and Drewsen, M.},
  title = {Noninvasive Vibrational Mode Spectroscopy of Ion Coulomb Crystals through Resonant Collective Coupling to an Optical Cavity Field},
  publisher = {American Physical Society},
  year = {2010},
  volume = {105},
  number = {10},
  pages = {103001+},
  url = {http://dx.doi.org/10.1103/PhysRevLett.105.103001},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.105.103001}
}
Staanum, P.F., Hojbjerre, K., Skyt, P.S., Hansen, A.K. & Drewsen, M., 2010.
"Rotational laser cooling of vibrationally and translationally cold molecular ions"
Nature Physics. Vol. 6(4), pp. 271-274.
Abstract: Stationary molecules in well-defined internal states are of broad interest for physics and chemistry. In physics, this includes metrology, quantum computing and many-body quantum mechanics, whereas in chemistry, state-prepared molecular targets are of interest for uni-molecular reactions with coherent light fields, for quantum-state-selected bi-molecular reactions and for astrochemistry. Here, we demonstrate rotational ground-state cooling of vibrationally and translationally cold MgH+ ions, using a laser-cooling scheme based on excitation of a single rovibrational transition. A nearly 15-fold increase in the rotational ground-state population of the X  1Σ+ electronic ground-state potential has been obtained. The resulting ground-state population of 36.7±1.2% is equivalent to that of a thermal distribution at about 20 K. The obtained cooling results imply that cold molecular-ion experiments can now be carried out at cryogenic temperatures in room-temperature set-ups.
BibTeX:
@article{Staanum2010Rotational,
  author = {Staanum, Peter F. and Hojbjerre, Klaus and Skyt, Peter S. and Hansen, Anders K. and Drewsen, Michael},
  title = {Rotational laser cooling of vibrationally and translationally cold molecular ions},
  publisher = {Nature Publishing Group},
  year = {2010},
  volume = {6},
  number = {4},
  pages = {271--274},
  url = {http://dx.doi.org/10.1038/nphys1604},
  doi = {http://dx.doi.org/10.1038/nphys1604}
}
Dantan, A., Albert, M., Marler, J.P., Herskind, P.F. & Drewsen, M., 2009.
"Large ion Coulomb crystals: A near-ideal medium for coupling optical cavity modes to matter"
Physical Review A (Atomic, Molecular, and Optical Physics). Vol. 80(4), pp. 041802+.
Abstract: We present an investigation of the coherent coupling of various transverse field modes of an optical cavity to ion Coulomb crystals. The obtained experimental results, which include the demonstration of identical collective coupling rates for different transverse modes of a cavity field to ions in the same large Coulomb crystal, are in excellent agreement with theoretical predictions. The results furthermore suggest that Coulomb crystals in the future may serve as near-ideal media for high-fidelity multimode quantum information processing and communication purposes, including the generation and storage of single-photon qubits encoded in different transverse modes.
BibTeX:
@article{Dantan2009Large,
  author = {Dantan, A. and Albert, M. and Marler, J. P. and Herskind, P. F. and Drewsen, M.},
  title = {Large ion Coulomb crystals: A near-ideal medium for coupling optical cavity modes to matter},
  publisher = {APS},
  year = {2009},
  volume = {80},
  number = {4},
  pages = {041802+},
  url = {http://dx.doi.org/10.1103/PhysRevA.80.041802},
  doi = {http://dx.doi.org/10.1103/PhysRevA.80.041802}
}
Herskind, P.F., Dantan, A., Albert, M., Marler, J.P. & Drewsen, M., 2009.
"Positioning of the rf potential minimum line of a linear Paul trap with micrometer precision"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 42(15), pp. 154008+.
Abstract: We demonstrate a general technique to achieve a precise radial displacement of the nodal line of the radiofrequency (rf) field in a linear Paul trap. The technique relies on the selective adjustment of the load capacitance of the trap electrodes, achieved through the addition of capacitors to the basic resonant rf circuit used to drive the trap. Displacements of up to 100 um with micrometer precision are measured using a combination of fluorescence images of ion Coulomb crystals and coherent coupling of such crystals to a mode of an optical cavity. The displacements are made without measurable distortion of the shape or structure of the Coulomb crystals, as well as without introducing excess heating commonly associated with the radial displacement of crystals by adjustment through static potentials. We expect this technique to be of importance for future developments of microtrap architectures and ion-based cavity QED.
BibTeX:
@article{Herskind2009Positioning,
  author = {Herskind, P. F. and Dantan, A. and Albert, M. and Marler, J. P. and Drewsen, M.},
  title = {Positioning of the rf potential minimum line of a linear Paul trap with micrometer precision},
  year = {2009},
  volume = {42},
  number = {15},
  pages = {154008+},
  url = {http://dx.doi.org/10.1088/0953-4075/42/15/154008},
  doi = {http://dx.doi.org/10.1088/0953-4075/42/15/154008}
}
Herskind, P.F., Dantan, A., Marler, J.P., Albert, M. & Drewsen, M., 2009.
"Realization of collective strong coupling with ion Coulomb crystals in an optical cavity"
Nat Phys. Vol. 5(7), pp. 494-498.
Abstract: Cavity quantum electrodynamics (CQED) focuses on understanding the interactions between matter and the electromagnetic field in cavities at the quantum level. In the past years, CQED has attracted attention especially owing to its importance for the field of quantum information. At present, photons are the best carriers of quantum information between physically separated sites and quantum-information processing using stationary qubits is most promising, with the furthest advances having been made with trapped ions. The implementation of complex quantum-information-processing networks hence requires devices to efficiently couple photons and stationary qubits. Here, we present the first CQED experiments demonstrating that the collective strong-coupling regime can be reached in the interaction between a solid in the form of an ion Coulomb crystal and an optical field. The obtained coherence times are in the millisecond range and indicate that Coulomb crystals positioned inside optical cavities are promising for realizing a variety of quantum-information devices, including quantum repeaters and quantum memories for light. Moreover, cavity optomechanics19 using Coulomb crystals might enable the exploration of similar phenomena investigated using more traditional solids, such as micro-mechanical oscillators.
BibTeX:
@article{Herskind2009Realization,
  author = {Herskind, Peter F. and Dantan, Aurelien and Marler, Joan P. and Albert, Magnus and Drewsen, Michael},
  title = {Realization of collective strong coupling with ion Coulomb crystals in an optical cavity},
  publisher = {Nature Publishing Group},
  year = {2009},
  volume = {5},
  number = {7},
  pages = {494--498},
  url = {http://dx.doi.org/10.1038/nphys1302},
  doi = {http://dx.doi.org/10.1038/nphys1302}
}
Højbjerre, K., Hansen, A.K., Skyt, P.S., Staanum, P.F. & Drewsen, M., 2009.
"Rotational state resolved photodissociation spectroscopy of translationally and vibrationally cold MgH+ ions: toward rotational cooling of molecular ions"
New Journal of Physics. Vol. 11(5), pp. 055026+.
Abstract: The first steps toward the implementation of a simple scheme for rotational cooling of MgH+ ions based on rotational state optical pumping is considered. The various aspects of such an experiment are described in detail, and the rotational state-selective dissociation spectra of translationally and vibrationally cold MgH+ ions are presented, with and without the optical pumping laser being present. While rotational cooling is as yet not evident, first results showed evidence of a change in the rotational distribution in the presence of the optical pumping laser.
BibTeX:
@article{Hojbjerre2009Rotational,
  author = {Højbjerre, K. and Hansen, A. K. and Skyt, P. S. and Staanum, P. F. and Drewsen, M.},
  title = {Rotational state resolved photodissociation spectroscopy of translationally and vibrationally cold MgH^+ ions: toward rotational cooling of molecular ions},
  publisher = {IOP Publishing},
  year = {2009},
  volume = {11},
  number = {5},
  pages = {055026+},
  url = {http://dx.doi.org/10.1088/1367-2630/11/5/055026},
  doi = {http://dx.doi.org/10.1088/1367-2630/11/5/055026}
}
Poschinger, U.G., Huber, G., Ziesel, F., Deiss, M., Hettrich, M., Schulz, S.A., Singer, K., Poulsen, G., Drewsen, M., Hendricks, R.J. & Schmidt-Kaler, F., 2009.
"Coherent manipulation of a 40Ca+ spin qubit in a micro ion trap"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 42(15), pp. 154013+.
Abstract: We demonstrate the implementation of a spin qubit with a single 40Ca+ ion in a micro ion trap. The qubit is encoded in the Zeeman ground state levels mJ = +1/2 and mJ = [?]1/2 of the S1/2 state of the ion. We show sideband cooling close to the vibrational ground state and demonstrate the initialization and readout of the qubit levels with 99.5% efficiency. We employ a Raman transition close to the S1/2-P1/2 resonance for coherent manipulation of the qubit. We observe single qubit rotations with 96% fidelity and gate times below 5 us. Rabi oscillations on the blue motional sideband are used to extract the phonon number distribution. The dynamics of this distribution is analysed to deduce the trap-induced heating rate of 0.3(1) phonons ms[?]1.
BibTeX:
@article{Poschinger2009Coherent,
  author = {Poschinger, U. G. and Huber, G. and Ziesel, F. and Deiss, M. and Hettrich, M. and Schulz, S. A. and Singer, K. and Poulsen, G. and Drewsen, M. and Hendricks, R. J. and Schmidt-Kaler, F.},
  title = {Coherent manipulation of a ^40Ca^+ spin qubit in a micro ion trap},
  year = {2009},
  volume = {42},
  number = {15},
  pages = {154013+},
  url = {http://dx.doi.org/10.1088/0953-4075/42/15/154013},
  doi = {http://dx.doi.org/10.1088/0953-4075/42/15/154013}
}
Champenois, C., Hagel, G., Knoop, M., Houssin, M., Zumsteg, C., Vedel, F. & Drewsen, M., 2008.
"Two-step Doppler cooling of a three-level ladder system with an intermediate metastable level"
Physical Review A (Atomic, Molecular, and Optical Physics). Vol. 77(3), pp. 033411+.
Abstract: Doppler laser cooling of a three-level ladder system using two near-resonant laser fields is analyzed in the case of the intermediate level being metastable while the upper level is short lived. Analytical as well as numerical results, e.g., obtainable scattering rates and achievable temperatures are presented. When appropriate, comparisons with two-level single-photon Doppler laser cooling is made. These results are relevant to recent experimental Doppler laser cooling investigations addressing intercombination lines in alkaline-earth-metal atoms and quadrupole transitions in alkaline-earth-metal ions.
BibTeX:
@article{Champenois2008Twostep,
  author = {Champenois, C. and Hagel, G. and Knoop, M. and Houssin, M. and Zumsteg, C. and Vedel, F. and Drewsen, M.},
  title = {Two-step Doppler cooling of a three-level ladder system with an intermediate metastable level},
  publisher = {APS},
  year = {2008},
  volume = {77},
  number = {3},
  pages = {033411+},
  url = {http://dx.doi.org/10.1103/PhysRevA.77.033411},
  doi = {http://dx.doi.org/10.1103/PhysRevA.77.033411}
}
Hendricks, R.J., Sørensen, J.L., Champenois, C., Knoop, M. & Drewsen, M., 2008.
"Doppler cooling of calcium ions using a dipole-forbidden transition"
Physical Review A (Atomic, Molecular, and Optical Physics). Vol. 77(2), pp. 021401+.
Abstract: Doppler cooling of calcium ions has been experimentally demonstrated using the S1/2D5/2 dipole-forbidden transition. Scattering forces and fluorescence levels a factor of 5 smaller than for the usual Doppler cooling on the dipole-allowed S1/2P1/2 transition have been achieved. Since the light scattered from the ions can be monitored at (violet) wavelengths that are very different from the excitation wavelengths, single ions can be detected with an essentially zero background level. This, as well as other features of the cooling scheme, can be extremely valuable for ion-trap-based quantum-information processing.
BibTeX:
@article{Hendricks2008Doppler,
  author = {Hendricks, Richard J. and Sørensen, Jens L. and Champenois, Caroline and Knoop, Martina and Drewsen, Michael},
  title = {Doppler cooling of calcium ions using a dipole-forbidden transition},
  publisher = {APS},
  year = {2008},
  volume = {77},
  number = {2},
  pages = {021401+},
  url = {http://dx.doi.org/10.1103/PhysRevA.77.021401},
  doi = {http://dx.doi.org/10.1103/PhysRevA.77.021401}
}
Herskind, P., Dantan, A., Langkilde-Lauesen, M., Mortensen, A., Sørensen, J. & Drewsen, M., 2008.
"Loading of large ion Coulomb crystals into a linear Paul trap incorporating an optical cavity"
Applied Physics B: Lasers and Optics. Vol. 93, pp. 373-379.
Abstract: Abstract  We report on the loading of large ion Coulomb crystals into a linear Paul trap incorporating a high-finesse optical cavity (ℱ∼3000). We show that, even though the 3-mm diameter dielectric cavity mirrors are placed between the trap electrodes and separated by only 12 mm, it is possible to produce in situ ion Coulomb crystals containing more than 105 calcium ions of various isotopes and with lengths of up to several millimeters along the cavity axis. We show that the number of ions inside the cavity mode is, in principle, high enough to achieve strong collective coupling between the ion Coulomb crystal and the cavity field. The results thus represent an important step towards ion trap based Cavity Quantum ElectroDynamics (CQED) experiments using cold ion Coulomb crystals.
BibTeX:
@article{Herskind2008,
  author = {Herskind, P. and Dantan, A. and Langkilde-Lauesen, M. and Mortensen, A. and Sørensen, J. and Drewsen, M.},
  title = {Loading of large ion Coulomb crystals into a linear Paul trap incorporating an optical cavity},
  year = {2008},
  volume = {93},
  pages = {373--379},
  url = {http://dx.doi.org/10.1007/s00340-008-3199-8},
  doi = {http://dx.doi.org/10.1007/s00340-008-3199-8}
}
Højbjerre, K., Offenberg, D., Bisgaard, C.Z., Stapelfeldt, H., Staanum, P.F., Mortensen, A. & Drewsen, M., 2008.
"Consecutive photodissociation of a single complex molecular ion"
Physical Review A (Atomic, Molecular, and Optical Physics). Vol. 77(3), pp. 030702+.
Abstract: Using the positively charged aniline ion (C6H5NH2+" align="middle"/>) as a test molecule, we demonstrate that it is possible to study consecutive photodissociation of complex molecular ions at the single molecule level in an ion trap. When a single C6H5NH2+" align="middle"/> ion is exposed to laser light at 397  nm and 294  nm, direct or consecutive photodissociation leads to the production of a range of smaller polyatomic molecular ions such as C5H6+" align="middle"/> and C3H3+" align="middle"/>. The applied method is very versatile and can, e.g., be used in combination with free electron lasers or synchrotron radiation sources.
BibTeX:
@article{Hojbjerre2008Consecutive,
  author = {Højbjerre, K. and Offenberg, D. and Bisgaard, C. Z. and Stapelfeldt, H. and Staanum, P. F. and Mortensen, A. and Drewsen, M.},
  title = {Consecutive photodissociation of a single complex molecular ion},
  publisher = {APS},
  year = {2008},
  volume = {77},
  number = {3},
  pages = {030702+},
  url = {http://dx.doi.org/10.1103/PhysRevA.77.030702},
  doi = {http://dx.doi.org/10.1103/PhysRevA.77.030702}
}
Staanum, P.F., Højbjerre, K., Wester, R. & Drewsen, M., 2008.
"Probing Isotope Effects in Chemical Reactions Using Single Ions"
Physical Review Letters. Vol. 100(24), pp. 243003+.
Abstract: Isotope effects in reactions between Mg+ in the 3p 2P3/2 excited state and molecular hydrogen at thermal energies are studied through single reaction events. From only 250 reactions with HD, the branching ratio between formation of MgD+ and MgH+ is found to be larger than 5. From an additional 65 reactions with H2 and D2 we find that the overall fragmentation probability of the intermediate MgH2+" align="middle"/>, MgHD+, or MgD2+" align="middle"/> complexes is the same. Our study shows that few single ion reactions can provide quantitative information on ion-neutral reactions. Hence, the method is well suited for reaction studies involving rare species, e.g., rare isotopes or short-lived unstable elements.
BibTeX:
@article{Staanum2008Probing,
  author = {Staanum, Peter F. and Højbjerre, Klaus and Wester, Roland and Drewsen, Michael},
  title = {Probing Isotope Effects in Chemical Reactions Using Single Ions},
  publisher = {APS},
  year = {2008},
  volume = {100},
  number = {24},
  pages = {243003+},
  url = {http://dx.doi.org/10.1103/PhysRevLett.100.243003},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.100.243003}
}
Drewsen, M., 2007.
"Cooling, identification and spectroscopy of super-heavy element ions"
European Physics Journal D. Vol. 45(1), pp. 125-127.
Abstract: We briefly discuss some possibilities for cooling, identification and spectroscopy of super-heavy element (SHE) ions based on recent results obtained from studies of atomic and molecular ions in linear rf traps. Since these investigations only relied on the charge and the mass of the ion of interest, we believe it should be straight forward to adopt most of the techniques for SHE ion research.
BibTeX:
@article{Drewsen2007Cooling,
  author = {Drewsen, Michael},
  title = {Cooling, identification and spectroscopy of super-heavy element ions},
  year = {2007},
  volume = {45},
  number = {1},
  pages = {125--127},
  url = {http://dx.doi.org/10.1140/epjd/e2007-00162-1},
  doi = {http://dx.doi.org/10.1140/epjd/e2007-00162-1}
}
Hendricks, R.J., Grant, D.M., Herskind, P.F., Dantan, A. & Drewsen, M., 2007.
"An all-optical ion-loading technique for scalable microtrap architectures"
Applied Physics B: Lasers and Optics. Vol. 88(4), pp. 507-513.
Abstract: An experimental demonstration of a novel all-optical technique for loading ion traps, which has particular application to microtrap architectures, is presented. The technique is based on photoionisation of an atomic beam created by pulsed laser ablation of a calcium target, and provides improved temporal control compared to traditional trap loading methods. Ion loading rates as high as 125 ions per second have so far been observed. Also described are observations of trap loading where Rydberg state atoms are photoionised by the ion Doppler cooling laser.
BibTeX:
@article{Hendricks2007Alloptical,
  author = {Hendricks, R. J. and Grant, D. M. and Herskind, P. F. and Dantan, A. and Drewsen, M.},
  title = {An all-optical ion-loading technique for scalable microtrap architectures},
  year = {2007},
  volume = {88},
  number = {4},
  pages = {507--513},
  url = {http://dx.doi.org/10.1007/s00340-007-2698-3},
  doi = {http://dx.doi.org/10.1007/s00340-007-2698-3}
}
Herskind, P., Lindballe, J., Clausen, C., Sørensen, J.L. & Drewsen, M., 2007.
"Second-harmonic generation of light at 544 and 272 nm from an ytterbium-doped distributed-feedback fiber laser"
OPTICS LETTERS. Vol. 32(3), pp. 268-270.
Abstract: We report external cavity second-harmonic generation of light at 544 and 272 nm based on an ytterbium-doped distributed-feedback fiber laser. The nonlinear crystal used to generate light at 544 nm is LiNbO3, and the maximum output of the cavity is 845 mW, corresponding to a conversion efficiency of 55 In a second frequency-doubling step, using a beta-BaBa2O4 crystal, we generate up to 115 mW of light at 272 nm with a conversion efficiency of 14 (c) 2007 Optical Society of America.
BibTeX:
@article{Herskind2007,
  author = {Herskind, Peter and Lindballe, Jens and Clausen, Christoph and Sørensen, Jens L. and Drewsen, Michael},
  title = {Second-harmonic generation of light at 544 and 272 nm from an ytterbium-doped distributed-feedback fiber laser},
  year = {2007},
  volume = {32},
  number = {3},
  pages = {268--270},
  url = {http://dx.doi.org/10.1364/OL.32.000268},
  doi = {http://dx.doi.org/10.1364/OL.32.000268}
}
Møller, D., Sørensen, J.L., Thomsen, J.B. & Drewsen, M., 2007.
"Efficient qubit detection using alkaline-earth-metal ions and a double stimulated Raman adiabatic process"
Physical Review A (Atomic, Molecular, and Optical Physics). Vol. 76(6), pp. 062321+.
Abstract: We present a scheme for robust and efficient projection measurement of a qubit consisting of the two magnetic sublevels in the electronic ground state of alkaline-earth-metal ions. The scheme is based on two stimulated Raman adiabatic passages involving four partially coherent laser fields. We show how the efficiency depends on experimentally relevant parameters: Rabi frequencies, pulse widths, laser linewidths, one- and two-photon detunings, residual laser power, laser polarization, and ion motion.
BibTeX:
@article{Moller2007Efficient,
  author = {Møller, Ditte and Sørensen, Jens L. and Thomsen, Jakob B. and Drewsen, Michael},
  title = {Efficient qubit detection using alkaline-earth-metal ions and a double stimulated Raman adiabatic process},
  publisher = {APS},
  year = {2007},
  volume = {76},
  number = {6},
  pages = {062321+},
  url = {http://dx.doi.org/10.1103/PhysRevA.76.062321},
  doi = {http://dx.doi.org/10.1103/PhysRevA.76.062321}
}
Mortensen, A., Nielsen, E., Matthey, T. & Drewsen, M., 2007.
"Radio frequency field-induced persistent long-range ordered structures in two-species ion Coulomb crystals"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 40(15), pp. F223-F229.
Abstract: We report on the observations of strikingly persistent (lifetimes of ∼10 s) three-dimensional long-range ordered structures in the central 40 Ca+ ion component of 40 Ca+ –44 Ca+ two-species ion Coulomb crystals in a linear Paul trap. Molecular dynamics simulations strongly indicate that the observed structures are a hitherto unpredicted consequence of an effective anisotropy in the inter-particle interaction induced by the radio frequency quadrupole- trapping field. The results have implications for such diverse research fields as cold molecular ion studies and quantum information processing.
BibTeX:
@article{Mortensen2007,
  author = {Mortensen, A. and Nielsen, E. and Matthey, T. and Drewsen, M.},
  title = {Radio frequency field-induced persistent long-range ordered structures in two-species ion Coulomb crystals},
  publisher = {Institute of Physics Publishing},
  year = {2007},
  volume = {40},
  number = {15},
  pages = {F223--F229},
  url = {http://dx.doi.org/10.1088/0953-4075/40/15/F01},
  doi = {http://dx.doi.org/10.1088/0953-4075/40/15/F01}
}
Bertelsen, A., Jørgensen, S. & Drewsen, M., 2006.
"The rotational temperature of polar molecular ions in Coulomb crystals"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 39(5), pp. L83-L89.
Abstract: With MgH+ ions as a test case, we investigate to what extent the rotational motion of smaller polar molecular ions sympathetically cooled into Coulomb crystals in linear Paul traps couples to the translational motions of the ion ensemble. By comparing the results obtained from rotational resonance-enhanced multiphoton photo-dissociation experiments with data from theoretical simulations, we conclude that the effective rotational temperature exceeds the translational temperature (<100 mK) by more than two orders of magnitude, indicating a very weak coupling.
BibTeX:
@article{Bertelsen2006Rotational,
  author = {Bertelsen, Anders and Jørgensen, Solvejg and Drewsen, Michael},
  title = {The rotational temperature of polar molecular ions in Coulomb crystals},
  year = {2006},
  volume = {39},
  number = {5},
  pages = {L83--L89},
  url = {http://dx.doi.org/10.1088/0953-4075/39/5/L02},
  doi = {http://dx.doi.org/10.1088/0953-4075/39/5/L02}
}
Mortensen, A., Nielsen, E., Matthey, T. & Drewsen, M., 2006.
"Observation of Three-Dimensional Long-Range Order in Small Ion Coulomb Crystals in an rf Trap"
Physical Review Letters. Vol. 96(10), pp. 103001+.
Abstract: Three-dimensional long-range ordered structures in smaller and near-spherically symmetric Coulomb crystals of 40Ca+ ions confined in a linear rf Paul trap have been observed when the number of ions exceeds 1000 ions. This result is unexpected from ground state molecular dynamics (MD) simulations, but found to be in agreement with MD simulations of metastable ion configurations. Previously, three-dimensional long-range ordered structures have only been reported in Penning traps in systems of 50 000 ions or more.
BibTeX:
@article{Mortensen2006Observation,
  author = {Mortensen, A. and Nielsen, E. and Matthey, T. and Drewsen, M.},
  title = {Observation of Three-Dimensional Long-Range Order in Small Ion Coulomb Crystals in an rf Trap},
  publisher = {APS},
  year = {2006},
  volume = {96},
  number = {10},
  pages = {103001+},
  url = {http://dx.doi.org/10.1103/PhysRevLett.96.103001},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.96.103001}
}
Sørensen, J.L., Møller, D., Iversen, T., Thomsen, J.B., Jensen, F., Staanum, P., Voigt, D. & Drewsen, M., 2006.
"Efficient coherent internal state transfer in trapped ions using stimulated Raman adiabatic passage"
New Journal of Physics. Vol. 8(11), pp. 261.
Abstract: We demonstrate experimentally how the process of stimulated Raman adiabatic passage (STIRAP) can be utilized for efficient coherent internal state transfer in single trapped and laser-cooled 40Ca+ ions. The transfer from the D3/2 to the D5/2 state, is detected by a fluorescence measurement revealing the population not transferred to the D5/2 state. A coherent population transfer efficiency at the level of 95% in a set-up allowing for the internal state detection of individual ions in a string has been obtained.
BibTeX:
@article{Sorensen2006Efficient,
  author = {Sørensen, Jens L. and Møller, Ditte and Iversen, Theis and Thomsen, Jakob B. and Jensen, Frank and Staanum, Peter and Voigt, Dirk and Drewsen, Michael},
  title = {Efficient coherent internal state transfer in trapped ions using stimulated Raman adiabatic passage},
  publisher = {Institute of Physics Publishing},
  year = {2006},
  volume = {8},
  number = {11},
  pages = {261},
  url = {http://dx.doi.org/10.1088/1367-2630/8/11/261},
  doi = {http://dx.doi.org/10.1088/1367-2630/8/11/261}
}
Vogelius, I.S., Madsen, L.B. & Drewsen, M., 2006.
"Probabilistic state preparation of a single molecular ion by projection measurement"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 39(19), pp. S1259-S1265.
Abstract: We propose how to prepare a single molecular ion in a specific internal quantum state in a situation where the molecule is trapped and sympathetically cooled by an atomic ion and where its internal degrees of freedom are initially in thermal equilibrium with the surroundings. The scheme is based on the conditional creation of correlation between the internal state of the molecule and the translational state of the collective motion of the two ions, followed by a projection measurement of this collective mode by atomic ion shelving techniques. We estimate that state preparation in a large number of internal states is possible.
BibTeX:
@article{Vogelius2006Probabilistic,
  author = {Vogelius, I. S. and Madsen, L. B. and Drewsen, M.},
  title = {Probabilistic state preparation of a single molecular ion by projection measurement},
  year = {2006},
  volume = {39},
  number = {19},
  pages = {S1259--S1265},
  url = {http://dx.doi.org/10.1088/0953-4075/39/19/S31},
  doi = {http://dx.doi.org/10.1088/0953-4075/39/19/S31}
}
Vogelius, I.S., Madsen, L.B. & Drewsen, M., 2006.
"Rotational cooling of molecular ions through laser-induced coupling to the collective modes of a two-ion Coulomb crystal"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 39(19), pp. S1267-S1280.
Abstract: We show that the rotational degree of freedom of a polar heteronuclear molecular ion can be cooled through an optical coupling to the collective motional modes of the molecular ion and a simultaneously trapped and laser cooled atomic ion. Since the dissipative part of the rotational cooling is realized through laser cooling of the two-ion systems motional modes, the scheme should be applicable to a large range of molecules. As a test case for our cooling scheme we consider rotational cooling of a MgH+ ion trapped with a laser cooled 40Ca+ ion.
BibTeX:
@article{Vogelius2006Rotational,
  author = {Vogelius, I. S. and Madsen, L. B. and Drewsen, M.},
  title = {Rotational cooling of molecular ions through laser-induced coupling to the collective modes of a two-ion Coulomb crystal},
  year = {2006},
  volume = {39},
  number = {19},
  pages = {S1267--S1280},
  url = {http://dx.doi.org/10.1088/0953-4075/39/19/S32},
  doi = {http://dx.doi.org/10.1088/0953-4075/39/19/S32}
}
Jørgensen, S., Drewsen, M. & Kosloff, R., 2005.
"Intensity and wavelength control of a single molecule reaction: Simulation of photodissociation of cold-trapped MgH+"
The Journal of Chemical Physics. Vol. 123(9), pp. 094302+.
Abstract: Photodissociation of cold magnesium hydride ions MgH^+ leading to either Mg^++H or Mg^+H^+ is simulated from first principles. The purpose is to study the possibility of single molecule control of the products in the presence of two laser fields. The system evolves on four electronic potential-energy curves, X^1 Sigma , A^1 Sigma, B^1 Pi  , and C^1 Sigma  . These potential-energy curves are calculated from first principles using multireference self-consistent field theory. The accuracy of the electronic potential curves has been checked by calculating the energies of the rovibrational eigenstates and comparing them to experimental findings. The photodissociation dynamics has furthermore been simulated by solving the time-dependent Schrödinger equation. It is shown that the branching ratio of the two dissociation channels, Mg++H or Mg+H+, can be controlled by changing the intensity and wavelength of the two driving laser fields. pyright2005 American Institute of Physics
BibTeX:
@article{Jorgensen2005Intensity,
  author = {Jørgensen, Solvejg and Drewsen, Michael and Kosloff, Ronnie},
  title = {Intensity and wavelength control of a single molecule reaction: Simulation of photodissociation of cold-trapped MgH^+},
  publisher = {AIP},
  year = {2005},
  volume = {123},
  number = {9},
  pages = {094302+},
  url = {http://dx.doi.org/10.1063/1.2011398},
  doi = {http://dx.doi.org/10.1063/1.2011398}
}
Kjærgaard, N., Mølhave, K. & Drewsen, M., 2005.
"Design and construction of a linear Paul trap for the study of crystalline beams"
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. Vol. 540(1), pp. 209-214.
Abstract: We comment on the paper ” Design and fabrication of a linear Paul trap for the study space-charge-dominated beams” by Takai et al. (Nucl. Instr. and Meth. 532 (2004) 508) and describe a preceding experimental setup which shows many similarities.
BibTeX:
@article{Kjaergaard2005Design,
  author = {Kjærgaard, N. and Mølhave, K. and Drewsen, M.},
  title = {Design and construction of a linear Paul trap for the study of crystalline beams},
  year = {2005},
  volume = {540},
  number = {1},
  pages = {209--214},
  url = {http://dx.doi.org/10.1016/j.nima.2004.12.026},
  doi = {http://dx.doi.org/10.1016/j.nima.2004.12.026}
}
Bertelsen, A., Vogelius, I.S., Jørgensen, S., Kosloff, R. & Drewsen, M., 2004.
"Photo-dissociation of Cold MgH ionsTowards rotational temperature measurements and controlled dissociation"
The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics. Vol. 31(2), pp. 403-408.
Abstract: Molecular ions sympathetically cooled into a Coulomb crystal by laser cooled atomic ions represent in many ways an ideal target for molecular physics studies due to achievable low translational temperatures (∼10 mK) and strong spatial localization (∼1 μm). In particular, in experiments with focused laser beams, both these features can be extremely useful. Here, we present results from experiments on photo-dissociation of MgH + ions in Coulomb crystals, and discuss possible avenues for using such processes for rotational temperature measurements as well as for manipulating the branching ratio of the dissociation channels Mg + H + and Mg + + H through intensity and frequency control of the laser beams involved.
BibTeX:
@article{Bertelsen2004Photodissociation,
  author = {Bertelsen, A. and Vogelius, I. S. and Jørgensen, S. and Kosloff, R. and Drewsen, M.},
  title = {Photo-dissociation of Cold MgH ionsTowards rotational temperature measurements and controlled dissociation},
  year = {2004},
  volume = {31},
  number = {2},
  pages = {403--408},
  url = {http://dx.doi.org/10.1140/epjd/e2004-00152-9},
  doi = {http://dx.doi.org/10.1140/epjd/e2004-00152-9}
}
Drewsen, M., Mortensen, A., Lindballe, J., Mølhave, K. & Kjærgaard, N., 2004.
"Dynamically excited single-component ion Coulomb crystals in linear Paul traps"
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. Vol. 532(1-2), pp. 237-240.
Abstract: This paper briefly discusses a few experiments regarding dynamically excited crystalline structures in a linear Paul trap. The dynamically excited structures are created either by applying a torque to an initially stationary crystal by utilizing the radiation pressure force from a near-resonant laser beam or by operating the trap at parameters where parametric resonances melt normal stationary crystals.
BibTeX:
@article{Drewsen2004Dynamically,
  author = {Drewsen, M. and Mortensen, A. and Lindballe, J. and Mølhave, K. and Kjærgaard, N.},
  title = {Dynamically excited single-component ion Coulomb crystals in linear Paul traps},
  year = {2004},
  volume = {532},
  number = {1-2},
  pages = {237--240},
  url = {http://dx.doi.org/10.1016/j.nima.2004.06.079},
  doi = {http://dx.doi.org/10.1016/j.nima.2004.06.079}
}
Drewsen, M., Mortensen, A., Martinussen, R., Staanum, P. & Sørensen, J.L., 2004.
"Nondestructive Identification of Cold and Extremely Localized Single Molecular Ions"
Physical Review Letters. Vol. 93(24), pp. 243201+.
Abstract: We demonstrate a simple and nondestructive method for identification of a single molecular ion sympathetically cooled by a single laser cooled atomic ion in a linear Paul trap. The technique is based on a precise nondestructive determination of the molecular ion mass through a measurement of the eigenfrequency of a common motional mode of the two ions. The demonstrated mass resolution is sufficiently high that molecular ion mass doublets can potentially be distinguished from each other. The obtained results represent an important step towards single molecule gas phase chemical physics.
BibTeX:
@article{Drewsen2004Nondestructive,
  author = {Drewsen, M. and Mortensen, A. and Martinussen, R. and Staanum, P. and Sørensen, J. L.},
  title = {Nondestructive Identification of Cold and Extremely Localized Single Molecular Ions},
  publisher = {American Physical Society},
  year = {2004},
  volume = {93},
  number = {24},
  pages = {243201+},
  url = {http://dx.doi.org/10.1103/PhysRevLett.93.243201},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.93.243201}
}
Mortensen, A., Lindballe, J.J.T., Jensen, I.S., Staanum, P., Voigt, D. & Drewsen, M., 2004.
"Isotope shifts of the 4s2 1S0 → 4s5p 1P1 transition and hyperfine splitting of the 4s5p 1P1 state in calcium"
Physical Review A. Vol. 69(4), pp. 042502+.
Abstract: Using a technique based on production of ion Coulomb crystals; the isotope shifts of the 4 s 2 1 S 0 →4 s 5 p 1 P 1 transition for all naturally occurring isotopes of calcium as well as the hyperfine splitting of the 4 s 5 p 1 P 1 state in 43 Ca have been measured. The field shift and specific mass shift coefficients as well as the hyperfine structure constants for 43 Ca have been derived from the data.
BibTeX:
@article{Mortensen2004Isotope,
  author = {Mortensen, A. and Lindballe, J. J. T. and Jensen, I. S. and Staanum, P. and Voigt, D. and Drewsen, M.},
  title = {Isotope shifts of the 4s^2 ^1S_0  4s5p ^1P_1 transition and hyperfine splitting of the 4s5p ^1P_1 state in calcium},
  publisher = {American Physical Society},
  year = {2004},
  volume = {69},
  number = {4},
  pages = {042502+},
  url = {http://dx.doi.org/10.1103/PhysRevA.69.042502},
  doi = {http://dx.doi.org/10.1103/PhysRevA.69.042502}
}
Staanum, P., Drewsen, M. & Mølmer, K., 2004.
"Geometric quantum gate for trapped ions based on optical dipole forces induced by Gaussian laser beams"
Physical Review A. Vol. 70(5), pp. 052327+.
Abstract: We propose an implementation of quantum logic gates via internal state dependent displacements of ions in a linear Paul trap caused by optical dipole forces. Based on a general quantum analysis of the system dynamics we consider specific implementations with alkaline-earth-metal ions. For experimentally realistic parameters gate infidelities as low as 10 −4 can be obtained.
BibTeX:
@article{Staanum2004Geometric,
  author = {Staanum, Peter and Drewsen, Michael and Mølmer, Klaus},
  title = {Geometric quantum gate for trapped ions based on optical dipole forces induced by Gaussian laser beams},
  publisher = {American Physical Society},
  year = {2004},
  volume = {70},
  number = {5},
  pages = {052327+},
  url = {http://dx.doi.org/10.1103/PhysRevA.70.052327},
  doi = {http://dx.doi.org/10.1103/PhysRevA.70.052327}
}
Staanum, P., Jensen, I.S., Martinussen, R.G., Voigt, D. & Drewsen, M., 2004.
"Lifetime measurement of the metastable 3d 2D5/2 state in the 40Ca+ ion using the shelving technique on a few-ion string"
Physical Review A. Vol. 69(3), pp. 032503+.
Abstract: We present a measurement of the lifetime of the metastable 3 d  2 D 5∕2 state in the 40 Ca + ion; using the so-called shelving technique on a string of five Doppler laser-cooled ions in a linear Paul trap. A detailed account of the data analysis is given; and systematic effects due to unwanted excitation processes and collisions with background gas atoms are discussed and estimated. From a total of 6805 shelving events; we obtain a lifetime τ=1149±14(stat.)±4(sys.)ms ; a result which is in agreement with the most recent measurements.
BibTeX:
@article{Staanum2004Lifetime,
  author = {Staanum, Peter and Jensen, Inger S. and Martinussen, Randi G. and Voigt, Dirk and Drewsen, Michael},
  title = {Lifetime measurement of the metastable 3d ^2D_5/2 state in the ^40Ca^+ ion using the shelving technique on a few-ion string},
  publisher = {American Physical Society},
  year = {2004},
  volume = {69},
  number = {3},
  pages = {032503+},
  url = {http://dx.doi.org/10.1103/PhysRevA.69.032503},
  doi = {http://dx.doi.org/10.1103/PhysRevA.69.032503}
}
Vogelius, I.S., Madsen, L.B. & Drewsen, M., 2004.
"Rotational cooling of molecules using lamps"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 37(22), pp. 4571-4574.
Abstract: We investigate theoretically the application of tailored incoherent far-infrared fields in combination with laser excitation of a single rovibrational transition for rotational cooling of translationally cold polar diatomic molecules. The cooling schemes are effective on a timescale shorter than typical unperturbed trapping times in ion traps and comparable to obtainable confinement times of neutral molecules.
BibTeX:
@article{Vogelius2004Rotational,
  author = {Vogelius, I. S. and Madsen, L. B. and Drewsen, M.},
  title = {Rotational cooling of molecules using lamps},
  year = {2004},
  volume = {37},
  number = {22},
  pages = {4571--4574},
  url = {http://dx.doi.org/10.1088/0953-4075/37/22/015},
  doi = {http://dx.doi.org/10.1088/0953-4075/37/22/015}
}
Vogelius, I.S., Madsen, L.B. & Drewsen, M., 2004.
"Rotational cooling of heteronuclear molecular ions with Σ1, Σ2, Σ3, and Π2 electronic ground states"
Physical Review A. Vol. 70(5), pp. 053412+.
Abstract: The translational motion of molecular ions can be effectively cooled sympathetically to translational temperatures below 100  mK in ion traps through Coulomb interactions with laser-cooled atomic ions. The rovibrational degrees of freedom; however; are expected to be largely unaffected during translational cooling. We have previously proposed schemes for cooling of the internal degrees of freedom of such translationally cold but internally hot heteronuclear diatomic ions in the simplest case of 1 Σ electronic ground-state molecules. Here we present a significant simplification of these schemes and make a generalization to the most frequently encountered electronic ground states of heteronuclear molecular ions: 1 Σ ; 2 Σ ; 3 Σ ; and 2 Π. The schemes are relying on one or two laser-driven transitions with the possible inclusion of a tailored incoherent far-infrared radiation field.
BibTeX:
@article{Vogelius2006Rotational_b,
  author = {Vogelius, I. S. and Madsen, L. B. and Drewsen, M.},
  title = {Rotational cooling of heteronuclear molecular ions with 1, 2, 3, and 2 electronic ground states},
  publisher = {American Physical Society},
  year = {2004},
  volume = {70},
  number = {5},
  pages = {053412+},
  url = {http://dx.doi.org/10.1103/PhysRevA.70.053412},
  doi = {http://dx.doi.org/10.1103/PhysRevA.70.053412}
}
Drewsen, M., Jensen, I., Lindballe, J., Nissen, N., Martinussen, R., Mortensen, A., Staanum, P. & Voigt, D., 2003.
"Ion Coulomb crystals: a tool for studying ion processes"
International Journal of Mass Spectrometry. Vol. 229(1-2), pp. 83-91.
Abstract: Ion processes can be studied by a wide variety of experimental setups ranging from small ion trap devises to large storage rings. In Penning as well as Paul (rf) traps, the effective oscillation frequencies of trapped particles have, e.g., extensively been used to gain information of the charge to mass ratio of produced atomic and molecular ions. For such investigations, space charges due to the simultaneous presence of multiple ions, is a limiting factor, and hence low density or single particle experiments are preferred. The purpose of the present article is to discuss the prospects of using traps in the opposite regime where space charges dominate and so-called ion Coulomb crystals form (at temperatures about 10 mK). The experimental results presented below show that in this regime, differences in single particle oscillation frequencies of simultaneously trapped ions lead to a spatial segregation of the ion species which can be used as means to study a large variety of ion processes. Since the detection relies generally on observations of changes in the spatial structure of the Coulomb crystals formed by the cold, trapped ions, it can often be non-destructive, i.e., the product ions of a certain process will stay trapped and be available for further experimentations. It is the intention of the present article to discuss, via some examples, the usefulness of ion Coulomb crystals as a tool in investigations of ion processes more than it is the aim to provide new data.
BibTeX:
@article{Drewsen2003Ion,
  author = {Drewsen, M. and Jensen, I. and Lindballe, J. and Nissen, N. and Martinussen, R. and Mortensen, A. and Staanum, P. and Voigt, D.},
  title = {Ion Coulomb crystals: a tool for studying ion processes},
  year = {2003},
  volume = {229},
  number = {1-2},
  pages = {83--91},
  url = {http://dx.doi.org/10.1016/S1387-3806(03)00259-8},
  doi = {http://dx.doi.org/10.1016/S1387-3806(03)00259-8}
}
Drewsen, M., Jensen, I.S., Kjærgaard, N., Lindballe, J., Mortensen, A., Mølhave, K. & Voigt, D., 2003.
"Non-stationary Coulomb crystals in linear Paul traps"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 36(3), pp. 525-532.
Abstract: Ion Coulomb crystals form when trapped ions are cooled below a certain critical temperature. In radiofrequency (RF) traps, such crystals are usually stationary (i.e. non-rotating) with shapes similar to those of cold, homogeneously charged liquids when the number of ions is sufficient for three-dimensional structures to appear. 'Dynamically stable' structures can, however, also be created either by directly applying a torque to an initially stationary crystal or by choosing trap parameters just within the stability region where stationary crystals will melt due to parametric resonances between the RF trap field and ion plasma modes.
BibTeX:
@article{Drewsen2003Nonstationary,
  author = {Drewsen, Michael and Jensen, Inger S. and Kjærgaard, Niels and Lindballe, Jens and Mortensen, Anders and Mølhave, Kristian and Voigt, Dirk},
  title = {Non-stationary Coulomb crystals in linear Paul traps},
  year = {2003},
  volume = {36},
  number = {3},
  pages = {525--532},
  url = {http://dx.doi.org/10.1088/0953-4075/36/3/310},
  doi = {http://dx.doi.org/10.1088/0953-4075/36/3/310}
}
Kjærgaard, N. & Drewsen, M., 2003.
"Observation of a Structural Transition for Coulomb Crystals in a Linear Paul Trap"
Physical Review Letters. Vol. 91(9), pp. 095002+.
Abstract: A structural transition for laser cooled ion Coulomb crystals in a linear Paul trap just above the stability limit of parametrically resonant excitation of bulk plasma modes has been observed. In contrast to the usual spheroidal shell structures present below the stability limit; the ions arrange in a ” string-of-disks” configuration. The spheroidal envelopes of the string-of-disks structures are in agreement with results from cold fluid theory usually valid for ion Coulomb crystals if the ion systems are assumed to be rotating collectively.
BibTeX:
@article{Kjaergaard2003Observation,
  author = {Kjærgaard, Niels and Drewsen, Michael},
  title = {Observation of a Structural Transition for Coulomb Crystals in a Linear Paul Trap},
  publisher = {American Physical Society},
  year = {2003},
  volume = {91},
  number = {9},
  pages = {095002+},
  url = {http://dx.doi.org/10.1103/PhysRevLett.91.095002},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.91.095002}
}
Matthey, T., Hansen, J.P. & Drewsen, M., 2003.
"Coulomb Bicrystals of Species with Identical Charge-to-Mass Ratios"
Physical Review Letters. Vol. 91(16), pp. 165001+.
Abstract: Structures of cold bicomponent Coulomb systems of particles with identical charge-to-mass ratios and common oscillation frequency in a spherical harmonic potential are studied by molecular dynamics simulations with up to 10 6 particles. For most initial conditions and cooling rates; the final state becomes a completely mixed core surrounded by a set of nearly degenerated double shells of separate species. For an equal amount of the two species; it is found that the ground state for larger systems consists of a simple cubic structured core surrounded by outer double-shell structures.
BibTeX:
@article{Matthey2003Coulomb,
  author = {Matthey, T. and Hansen, J. P. and Drewsen, M.},
  title = {Coulomb Bicrystals of Species with Identical Charge-to-Mass Ratios},
  publisher = {American Physical Society},
  year = {2003},
  volume = {91},
  number = {16},
  pages = {165001+},
  url = {http://dx.doi.org/10.1103/PhysRevLett.91.165001},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.91.165001}
}
Hornekær, L. & Drewsen, M., 2002.
"Formation process of large ion Coulomb crystals in linear Paul traps"
Physical Review A. Vol. 66(1), pp. 013412+.
Abstract: We report on the results on various aspects of the formation of ion Coulomb crystals in linear Paul traps; for plasmas containing from a few hundred to several thousand laser-cooled 24 Mg + ions. The presented studies include observations of changes in emitted fluorescence; changes in plasma aspect ratio; and the formation of shell structure; during the transition from the cloudlike to the crystalline state. The competition between rf heating and laser cooling is observed to result in a very dramatic transition from the cloudlike disordered state to a liquidlike state. After the liquid state is reached ordering into shell structure takes place as a continuous process at even higher plasma coupling parameters. This is in contrast to earlier observations of few-ion plasmas in hyperbolic rf traps.
BibTeX:
@article{Hornekaer2002Formation,
  author = {Hornekær, L. and Drewsen, M.},
  title = {Formation process of large ion Coulomb crystals in linear Paul traps},
  publisher = {American Physical Society},
  year = {2002},
  volume = {66},
  number = {1},
  pages = {013412+},
  url = {http://dx.doi.org/10.1103/PhysRevA.66.013412},
  doi = {http://dx.doi.org/10.1103/PhysRevA.66.013412}
}
Kjærgaard, N., Mølhave, K. & Drewsen, M., 2002.
"Stability of Coulomb crystals in a linear Paul trap with storage-ring-like confinement"
Physical Review E. Vol. 66(1), pp. 015401+.
Abstract: We report experiments on the stability of ion Coulomb crystals in a linear Paul trap with storage-ring-like confinement. The transverse dynamics of charged particles in a trap of this type is analogous to that of a fast beam traveling through a channel with periodic; magnetic alternating gradient confinement. The experimentally observed stability conditions for stationary crystals comply remarkably well with current theory of crystalline plasmas and beams.
BibTeX:
@article{Kjaergaard2002Stability,
  author = {Kjærgaard, Niels and Mølhave, Kristian and Drewsen, Michael},
  title = {Stability of Coulomb crystals in a linear Paul trap with storage-ring-like confinement},
  publisher = {American Physical Society},
  year = {2002},
  volume = {66},
  number = {1},
  pages = {015401+},
  url = {http://dx.doi.org/10.1103/PhysRevE.66.015401},
  doi = {http://dx.doi.org/10.1103/PhysRevE.66.015401}
}
Staanum, P. & Drewsen, M., 2002.
"Trapped-ion quantum logic utilizing position-dependent ac Stark shifts"
Physical Review A. Vol. 66(4), pp. 040302+.
Abstract: We present a scheme utilizing position-dependent ac Stark shifts for doing quantum logic with trapped ions. By a proper choice of direction; position; and size; as well as power and frequency of a far-off-resonant Gaussian laser beam; specific ac Stark shifts can be assigned to the individual ions; making them distinguishable in frequency space. In contrast to previous all-optical based quantum gates with trapped ions; the present scheme enables individual addressing of single ions and selective addressing of any pair of ions for two-ion quantum gates; without using tightly focused laser beams. Furthermore; the decoherence rate due to off-resonant excitations can be made negligible as compared with other sources of decoherence.
BibTeX:
@article{Staanum2002Trappedion,
  author = {Staanum, Peter and Drewsen, Michael},
  title = {Trapped-ion quantum logic utilizing position-dependent ac Stark shifts},
  publisher = {American Physical Society},
  year = {2002},
  volume = {66},
  number = {4},
  pages = {040302+},
  url = {http://dx.doi.org/10.1103/PhysRevA.66.040302},
  doi = {http://dx.doi.org/10.1103/PhysRevA.66.040302}
}
Vogelius, I.S., Madsen, L.B. & Drewsen, M., 2002.
"Blackbody-Radiation–Assisted Laser Cooling of Molecular Ions"
Physical Review Letters. Vol. 89(17), pp. 173003+.
Abstract: The translational motion of molecular ions can be effectively cooled sympathetically to temperatures below 100 mK in ion traps through Coulomb interactions with laser-cooled atomic ions. The distribution of internal rovibrational states; however; gets in thermal equilibrium with the typically much higher temperature of the environment within tens of seconds. We consider a concept for rotational cooling of such internally hot; but translationally cold; heteronuclear diatomic molecular ions. The scheme relies on a combination of optical pumping from a few specific rotational levels into a ” dark state” with redistribution of rotational populations mediated by blackbody radiation.
BibTeX:
@article{Vogelius2002BlackbodyRadiationchar21Assisted,
  author = {Vogelius, I. S. and Madsen, L. B. and Drewsen, M.},
  title = {Blackbody-Radiationar21Assisted Laser Cooling of Molecular Ions},
  publisher = {American Physical Society},
  year = {2002},
  volume = {89},
  number = {17},
  pages = {173003+},
  url = {http://dx.doi.org/10.1103/PhysRevLett.89.173003},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.89.173003}
}
Hornekær, L., Kjærgaard, N., Thommesen, A.M. & Drewsen, M., 2001.
"Structural Properties of Two-Component Coulomb Crystals in Linear Paul Traps"
Physical Review Letters. Vol. 86(10), pp. 1994-1997.
Abstract: We report on structural properties of two-component Coulomb crystals in a linear Paul trap. The crystals consist of two laser cooled ion species; 24 Mg + and 40 Ca + . The lighter 24 Mg + ions form an inner cylindrical crystal structure surprisingly similar to that of an infinitely long single component crystal; while the outermost shell of the surrounding 40 Ca + ions have a spheroidal shape; which is highly insensitive to the presence of the 24 Mg + ions. Observed changes in the radial separation of the two ion species with the radius of the inner cylindrical crystal is explained by a simple model.
BibTeX:
@article{Hornekaer2001Structural,
  author = {Hornekær, L. and Kjærgaard, N. and Thommesen, A. M. and Drewsen, M.},
  title = {Structural Properties of Two-Component Coulomb Crystals in Linear Paul Traps},
  publisher = {American Physical Society},
  year = {2001},
  volume = {86},
  number = {10},
  pages = {1994--1997},
  url = {http://dx.doi.org/10.1103/PhysRevLett.86.1994},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.86.1994}
}
Kjærgaard, N. & Drewsen, M., 2001.
"Crystalline beam emulations in a pulse-excited linear Paul trap"
Physics of Plasmas. Vol. 8(4), pp. 1371-1375.
BibTeX:
@article{Kjaergaard2001Crystalline,
  author = {Kjærgaard, Niels and Drewsen, Michael},
  title = {Crystalline beam emulations in a pulse-excited linear Paul trap},
  publisher = {AIP},
  year = {2001},
  volume = {8},
  number = {4},
  pages = {1371--1375},
  url = {https://aip.scitation.org/action/doSearch?SeriesKey=php&AllField=+Crystalline+beam+emulations+in+a+pulse-excited+linear+Paul+trap+&ConceptID=}
}
Drewsen, M. & Brøner, A., 2000.
"Harmonic linear Paul trap: Stability diagram and effective potentials"
Physical Review A. Vol. 62(4), pp. 045401+.
Abstract: We present the single-particle stability diagram for the radial motion in a linear Paul trap in a situation where the applied axial dc potential gives rise to a harmonic defocusing radial potential. Although most linear Paul trap experiments have been conducted in a regime where this approximation is reasonably valid; the effect of the axial confinement on the stability of the radial motion has not previously been analyzed. The defocusing effect in both radial directions leads to a stability diagram different from that of the two-dimensional quadrupole mass filter; and hence points toward new studies of few ion dynamics. Expressions for the effective or pseudopotentials for one and two charged particles are presented and discussed.
BibTeX:
@article{Drewsen2000Harmonic,
  author = {Drewsen, M. and Brøner, A.},
  title = {Harmonic linear Paul trap: Stability diagram and effective potentials},
  publisher = {American Physical Society},
  year = {2000},
  volume = {62},
  number = {4},
  pages = {045401+},
  url = {http://dx.doi.org/10.1103/PhysRevA.62.045401},
  doi = {http://dx.doi.org/10.1103/PhysRevA.62.045401}
}
Kjærgaard, N., Hornekær, L., Thommesen, A.M., Videsen, Z. & Drewsen, M., 2000.
"Isotope selective loading of an ion trap using resonance-enhanced two-photon ionization"
Applied Physics B: Lasers and Optics. Vol. 71(2), pp. 207-210.
Abstract: We have demonstrated that resonance-enhanced two-photon ionization of atomic beams provides an effective tool for isotope selective loading of ions into a linear Paul trap. Using a tunable, narrow-bandwidth, continuous wave (cw) laser system for the ionization process, we have succeeded in producing Mg+ and Ca+ ions at rates controlled by the atomic beam flux, the laser intensity, and the laser frequency detuning from resonance. We have observed that with a proper choice of control parameters, it is rather easy to load a specific number of ions into a string. This observation has direct applications in quantum optics and quantum computation experiments. Furthermore, resonant photo-ionization loading facilitates the formation of large isotope-pure Coulomb crystals.
BibTeX:
@article{Kjaergaard2000Isotope,
  author = {Kjærgaard, N. and Hornekær, L. and Thommesen, A. M. and Videsen, Z. and Drewsen, M.},
  title = {Isotope selective loading of an ion trap using resonance-enhanced two-photon ionization},
  year = {2000},
  volume = {71},
  number = {2},
  pages = {207--210},
  url = {http://dx.doi.org/10.1007/s003400000296},
  doi = {http://dx.doi.org/10.1007/s003400000296}
}
Madsen, N., Bowe, P., Drewsen, M., Hornekær, L., Kjægaard, N., Labrador, A., Nielsen, J., Schiffer, J., Shi, P. & Hangst, J., 2000.
"Density distribution in laser-cooled bunched beams"
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. Vol. 441(1-2), pp. 203-208.
Abstract: Knowledge of the density distribution of a bunched beam is important for controlling and damping instabilities which often depend on the distribution function (A.W. Chao, in Physics of Collective Beam Instabilities in High Energy Accelerators, Wiley, New York, 1993). In this paper, experimental studies of the longitudinal bunch shapes and temperatures of laser-cooled bunched beams are presented. These studies are combined with measurements of the transverse density distribution of the beam using a novel technique (Madsen et al., Proceedings of the Sixth European Particle Accelerator Conference, EPAC'98, Stockholm, IOP Publishing, Bristol 1998, p. 1046). The bunch shapes for various temperatures comply with a model where the transverse temperature is assumed to be high. However, in the longitudinally space-charge limited case, it is observed that the maximum attainable density is much lower than it would be in a completely space-charge limited beam. Furthermore, it is observed that this maximum density decreases with increasing current. These observations might be important for the beam quality obtainable with laser-cooling.
BibTeX:
@article{Madsen2000Density,
  author = {Madsen, N. and Bowe, P. and Drewsen, M. and Hornekær, L. and Kjægaard, N. and Labrador, A. and Nielsen, J. and Schiffer, J. and Shi, P. and Hangst, J.},
  title = {Density distribution in laser-cooled bunched beams},
  year = {2000},
  volume = {441},
  number = {1-2},
  pages = {203--208},
  url = {http://dx.doi.org/10.1016/S0168-9002(99)01134-1},
  doi = {http://dx.doi.org/10.1016/S0168-9002(99)01134-1}
}
Madsen, D.N., Balslev, S., Drewsen, M., Kjærgaard, N., Videsen, Z. & Thomsen, J.W., 2000.
"Measurements on photo-ionization of 3s3p 1P1 magnesium atoms"
Journal of Physics B: Atomic, Molecular and Optical Physics. Vol. 33(22), pp. 4981-4988.
Abstract: This paper presents experimental studies of resonant two-photon ionization of Mg via the 3s2 1S0-3s3p 1P1 transition using 285.2 nm CW light. Dimensionless ratios of the cross section for ionization into a continuum state of S character relative to ionization into a state of D character are extracted from data obtained in two independent experiments. We obtain the values 0.102+-0.003 and 0.096+-0.012, respectively, and compare these to current theoretical predictions.
BibTeX:
@article{Madsen2000Measurements,
  author = {Madsen, D. N. and Balslev, S. and Drewsen, M. and Kjærgaard, N. and Videsen, Z. and Thomsen, J. W.},
  title = {Measurements on photo-ionization of 3s3p ^1P_1 magnesium atoms},
  year = {2000},
  volume = {33},
  number = {22},
  pages = {4981--4988},
  url = {http://dx.doi.org/10.1088/0953-4075/33/22/302},
  doi = {http://dx.doi.org/10.1088/0953-4075/33/22/302}
}
Mølhave, K. & Drewsen, M., 2000.
"Demonstration of the continuous quantum Zeno effect in optical pumping"
Physics Letters A. Vol. 268(1-2), pp. 45-49.
Abstract: A continuous quantum Zeno effect is observed in optical pumping on the 3s 2 S 1/2 –3p 2 P 1/2 transition of the 24 Mg + ion. Independently of the strength of a magnetic field perpendicular to a circularly polarized light beam, optical pumping into the 3s 2 S 1/2 , M J =+1/2 state can in principle be obtained without absorption.
BibTeX:
@article{Molhave2000Demonstration,
  author = {Mølhave, K. and Drewsen, M.},
  title = {Demonstration of the continuous quantum Zeno effect in optical pumping},
  year = {2000},
  volume = {268},
  number = {1-2},
  pages = {45--49},
  url = {http://dx.doi.org/10.1016/S0375-9601(00)00166-3},
  doi = {http://dx.doi.org/10.1016/S0375-9601(00)00166-3}
}
Mølhave, K. & Drewsen, M., 2000.
"Formation of translationally cold MgH+ and MgD+ molecules in an ion trap"
Physical Review A. Vol. 62(1), pp. 011401+.
Abstract: We have produced and cooled the molecular ions MgH + and MgD + in a linear Paul trap. These ions were generated by the photochemical reactions Mg + (3 p 2 P 3/2 )+H 2  (D 2 )→MgH +  (MgD + )+H (D); and identified by the radial separation in the trap of ions with different charge-to-mass ratios. The molecular translational motion was cooled sympathetically by Coulomb interaction with laser-cooled Mg + ions to a temperature estimated to be below 100 mK. Ordered structures (ion crystals) containing more than 1000 ions; with more than 95% being molecular ions; were obtained. Such translationally cold and well-localized samples of molecular ions could become very useful for molecular physics and chemistry.
BibTeX:
@article{Molhave2000Formation,
  author = {Mølhave, K. and Drewsen, M.},
  title = {Formation of translationally cold MgH^+ and MgD^+ molecules in an ion trap},
  publisher = {American Physical Society},
  year = {2000},
  volume = {62},
  number = {1},
  pages = {011401+},
  url = {http://dx.doi.org/10.1103/PhysRevA.62.011401},
  doi = {http://dx.doi.org/10.1103/PhysRevA.62.011401}
}
Schiffer, J.P., Drewsen, M., Hangst, J.S. & Hornekær, L., 2000.
"Temperature, ordering, and equilibrium with time-dependent confining forces"
Proceedings of the National Academy of Sciences of the United States of America. Vol. 97(20), pp. 10697-10700.
Abstract: The concepts of temperature and equilibrium are not well defined in systems of particles with time-varying external forces. An example is a radio frequency ion trap, with the ions laser cooled into an ordered solid, characteristic of sub-mK temperatures, whereas the kinetic energies associated with the fast coherent motion in the trap are up to 7 orders of magnitude higher. Simulations with 1,000 ions reach equilibrium between the degrees of freedom when only aperiodic displacements (secular motion) are considered. The coupling of the periodic driven motion associated with the confinement to the nonperiodic random motion of the ions is very small at low temperatures and increases quadratically with temperature.
BibTeX:
@article{Schiffer2000Temperature,
  author = {Schiffer, J. P. and Drewsen, M. and Hangst, J. S. and Hornekær, L.},
  title = {Temperature, ordering, and equilibrium with time-dependent confining forces},
  year = {2000},
  volume = {97},
  number = {20},
  pages = {10697--10700},
  url = {http://www.pnas.org/content/97/20/10697.abstract}
}
Bowe, P., Hornekær, L., Brodersen, C., Drewsen, M., Hangst, J.S. & Schiffer, J.P., 1999.
"Sympathetic Crystallization of Trapped Ions"
Physical Review Letters. Vol. 82(10), pp. 2071-2074.
Abstract: We have created multispecies Coulomb crystals in a linear Paul trap containing up to a few hundred ions of which more than 50% were cooled only sympathetically through the Coulomb interaction with laser-cooled Mg + ions. In an extreme case; one laser-cooled ion maintained order in a 15 ion string. Ion species segregation was obtained by radiation pressure. Previous experiments and molecular dynamics simulations suggest the temperature is 10 mK or lower. These results indicate that a wide range of atomic and molecular ions can be cooled and localized in linear Paul traps which is important for improvements in spectroscopic studies of such ions.
BibTeX:
@article{Bowe1999,
  author = {Bowe, P. and Hornekær, L. and Brodersen, C. and Drewsen, M. and Hangst, J. S. and Schiffer, J. P.},
  title = {Sympathetic Crystallization of Trapped Ions},
  publisher = {American Physical Society},
  year = {1999},
  volume = {82},
  number = {10},
  pages = {2071--2074},
  url = {http://dx.doi.org/10.1103/PhysRevLett.82.2071},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.82.2071}
}
Kjærgaard, N. & Drewsen, M., 1999.
"Tapered laser cooling of stored coasting ion beams"
Physics Letters A. Vol. 260(6), pp. 507-511.
Abstract: We propose a scheme for tapered laser cooling of coasting ion beams in storage rings. Tapered cooling has recently been shown to be crucial for attaining crystalline ion beams. The scheme proposed here, based on a relative displacement of a co- and a counterpropagating Gaussian laser beam, gives a radial variation in the equilibrium velocities to which particles are cooled. The variation is approximately linear in a relatively large range transverse to the laser beams. Expressions for the spatially dependent equilibrium velocities and the range of the tapered cooling forces are derived. We discuss the dependence on laser beam parameters as well as the limitations of this cooling scheme.
BibTeX:
@article{Kjaergaard1999Tapered,
  author = {Kjærgaard, N. and Drewsen, M.},
  title = {Tapered laser cooling of stored coasting ion beams},
  year = {1999},
  volume = {260},
  number = {6},
  pages = {507--511},
  url = {http://dx.doi.org/10.1016/S0375-9601(99)00581-2},
  doi = {http://dx.doi.org/10.1016/S0375-9601(99)00581-2}
}
Madsen, N., Bowe, P., Drewsen, M., Hornekær, L.H., Kjærgaard, N., Labrador, A., Nielsen, J.S., Schiffer, J.P., Shi, P. & Hangst, J.S., 1999.
"Density Limitations in a Stored Laser-Cooled Ion Beam"
Physical Review Letters. Vol. 83(21), pp. 4301-4304.
Abstract: We present a new technique for transverse beam profile diagnostics of a stored ion beam; imaging the fluorescence light from a laser-excited ion beam onto a high resolution charge-coupled device detector. This technique has much higher sensitivity and spatial resolution than conventional techniques. Using this method we have obtained evidence for space-charge-dominated behavior of a stored; laser-cooled beam of 24 Mg + ions. However; the transverse size of the longitudinally cooled beam is larger than that expected for a space-charge limited beam. This seems to confirm expectations from molecular dynamics simulations; showing that to reach a crystalline beam; other techniques have to be applied.
BibTeX:
@article{Madsen1999Density,
  author = {Madsen, N. and Bowe, P. and Drewsen, M. and Hornekær, L. H. and Kjærgaard, N. and Labrador, A. and Nielsen, J. S. and Schiffer, J. P. and Shi, P. and Hangst, J. S.},
  title = {Density Limitations in a Stored Laser-Cooled Ion Beam},
  publisher = {American Physical Society},
  year = {1999},
  volume = {83},
  number = {21},
  pages = {4301--4304},
  url = {http://dx.doi.org/10.1103/PhysRevLett.83.4301},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.83.4301}
}
Drewsen, M., Brodersen, C., Hornekær, L., Hangst, J.S. & Schiffer, J.P., 1998.
"Large Ion Crystals in a Linear Paul Trap"
Physical Review Letters. Vol. 81(14), pp. 2878-2881.
Abstract: Plasmas of Mg + ions; containing more than 10 5 ions; have been observed to reach well-ordered (crystalline) states by applying laser cooling. The crystals are highly elongated with up to ten concentric cylindrical shells surrounding a central string. Such large structures have not previously been observed in a Paul trap. The amplitude of the micromotion of the ions can be larger than the shell spacings. As the diameter changes along the crystals; sharp transitions are observed when new shells form; in good agreement with molecular dynamics simulations. The predictions from simulations of how ordering develops with decreasing temperature are also confirmed.
BibTeX:
@article{Drewsen1998Large,
  author = {Drewsen, M. and Brodersen, C. and Hornekær, L. and Hangst, J. S. and Schiffer, J. P.},
  title = {Large Ion Crystals in a Linear Paul Trap},
  publisher = {American Physical Society},
  year = {1998},
  volume = {81},
  number = {14},
  pages = {2878--2881},
  url = {http://dx.doi.org/10.1103/PhysRevLett.81.2878},
  doi = {http://dx.doi.org/10.1103/PhysRevLett.81.2878}
}
Hornekær, L., Drewsen, M., Brodersen, C., Schiffer, J.P. & Hangst, J.S., 1998.
"Ion crystals in a linear Paul trap"
Hyperfine Interactions. Vol. 115(1), pp. 37-40.
Abstract: Abstract  Large ion crystals containing as many as 105 laser-cooled 24Mg+ ions arranged in 10 cylindrical shells around a central string have been observed in a linear Paul trap. For smaller crystals consisting of approximately 3500 ions, the varying linear charge densities along the trap axis give rise to well-defined transition regions between the different shell-structures. These structures and the transition regions as a function of linear density are in good agreement with molecular dynamics simulations. The micromotion of the ions in such smaller crystals is also investigated through comparison with MD-simulations. Finally, the degree and progression of ordering in the plasma have been observed for various temperatures and compared with MD-simulations. Good qualitative agreement is again obtained.
BibTeX:
@article{Hornekaer1998Ion,
  author = {Hornekær, L. and Drewsen, M. and Brodersen, C. and Schiffer, J. P. and Hangst, J. S.},
  title = {Ion crystals in a linear Paul trap},
  year = {1998},
  volume = {115},
  number = {1},
  pages = {37--40},
  url = {http://dx.doi.org/10.1023/A:1012676017179},
  doi = {http://dx.doi.org/10.1023/A:1012676017179}
}