Over the last decade, the investigation of cold atomic gases has been one of the most active areas in physics. From the production of Bose-Einstein Condensates in 1995, to the demonstration of superfluidity in a strongly interacting mixture of degenerate Fermi gases, the experimental control of these systems has progressed dramatically. This research is driven by the desire to understand strongly interacting and strongly correlated systems, with applications in solid-state physics.
One key tool in these investigation is the optical lattice. It is formed by interfering laser beams and gives rise to a potential that is strikingly similar to the crystalline structure of solid state systems. However, the lattice spacing and depth can be controlled and allows for the investigation of condensed matter phenomena in a very pure environment. Thus, atoms in an optical lattice can provide a "simulator" for solid state many-body systems and may allow us to answer unsolved questions of the field.
Published in PRL:
Quantum systems are typically characterized by the inherent fluctuation of their physical observables. However, fluctuations in interacting quantum systems are not well understood theoretically and have resisted experimental measurement efforts. In our paper we report the characterization of atom number fluctuations in weakly interacting Bose-Einstein condensates. In particular we observe fluctuations reduced by 27% below the canonical expectation, revealing the microcanonical nature of our system!
Read or paper at APS or on arXiv.
(04/2021)
Fluctuations are a key property of both classical and quantum systems. While the fluctuations are well understood for many quantum systems at zero temperature, the case of an interacting quantum system at finite temperature still poses numerous challenges. Despite intense theoretical investigations of atom number fluctuations in Bose-Einstein condensates (BECs), their amplitude in experimentally relevant interacting systems is still poorly understood. Moreover, technical limitations have prevented their experimental observation to date. Here we report the first observation of these fluctuations. Our experiments are based on a stabilization technique, which allows for the preparation of ultracold thermal clouds at the shot noise level, thereby eliminating numerous technical noise sources. Furthermore, we make use of the correlations established by the evaporative cooling process to precisely determine the fluctuations and the sample temperature. This allows us to observe a telltale signature: the sudden increase in fluctuations of the condensate atom number close to the critical temperature.
Read our paper on arXiv.
(12/2018)
We have developed an imaging technique which can measure the atom number below the atom shot noise level. This work is closely related to our recent work on feedback stabilization of atom numbers. We use Faraday imaging which allows multiple images of the same cloud to be acquired. To describe the expected noise, we have developed a model based on photon shot noise and single atom loss. For clouds containing N~5×106 atoms, a precision more than a factor of two below the atom shot noise level is achieved.
Our manuscript on this work has been published in Journal of Physics B as part of a special issue on addressing quantum many-body problems with cold atoms and molecules.
Read the manuscript in Jour. Phys. B or on arXiv!
(01/2017)
Experiments with ultracold atoms inherently suffer from shot-to-shot atom number fluctuations which limit the precision. The UQGG Lattice team have demonstrated a technique for preparing a large cloud of a specific number of atoms with unprecedented low uncertainty. The usual atom number fluctuation of about 10% are reduced to below 0.1%!
During the experimental procedure, a series of non-destructive Faraday images probe the number of atoms in the cloud. A field programmable gate array provides online data analysis and performs feedback by removing atoms from the cloud until the desired number of atoms are reached. Finally, a second series of Faraday images confirm the number of atoms remaining in the cloud.
By creating similar atom clouds reproducibly, this newly developed technique can potientially improve the performance of atomic clocks and other high-precision measurements or simply just reduce the number of hours the typical graduate student has to spend in the lab to obtain data of sufficiently high quality.
The results have been published in Physical Review Letters as an Editors' Suggestion. Additionally, the work is featured in Physics, where a Focus article was written. The article can be found on arXiv as well.
(08/2016)
The quantum speed limit sets the minimum time required to transfer a quantum system completely into a given target state. At shorter times the higher operation speed results in a loss of fidelity. Here we quantify the trade-off between the fidelity and the duration in a system driven by a time-varying control. The problem is addressed in the framework of Hilbert space geometry offering an intuitive interpretation of optimal control algorithms. This approach leads to a necessary criterion for control optimality applicable as a measure of algorithm convergence. The time fidelity trade-off expressed in terms of the direct Hilbert velocity provides a robust prediction of the quantum speed limit and allows one to adapt the control optimization such that it yields a predefined fidelity. The results are verified numerically in a multilevel system with a constrained Hamiltonian and a classification scheme for the control sequences is proposed based on their optimizability. (12/2015)
Published in Physical Review A, Rapid comm.!
We have investigated spin dynamics in a 2D quantum gas. Through spin-changing collisions, two clouds with opposite spin orientations are spontaneously created in a Bose-Einstein condensate. After ballistic expansion, both clouds acquire ring-shaped density distributions with superimposed angular density modulations. The density distributions depend on the applied magnetic field and are well explained by a simple Bogoliubov model. We show that the two clouds are anti-correlated in momentum space. The observed momentum correlations pave the way towards the creation of an atom source with non-local Einstein-Podolsky-Rosen entanglement. (05/2014)
Accepted for publication in PRA!
Within the combined potential of an optical lattice and a harmonic magnetic trap, it is possible to form matter wave packets by intensity modulation of the lattice. The modulation technique also allows for a controllable transfer (de-excitation) of atoms from such wave packets to a state bound by the lattice. Thus, it acts as a beam splitter for matter waves that can selectively address different bands, enabling the preparation of atoms in selected localized states. Here, we use the de-excitation for precision spectroscopy of the anharmonicity of the magnetic trap. Finally, we demonstrate that lattice modulation can be used to excite matter wave packets to even higher momenta. http://arxiv.org/abs/1306.1082 (08/13)
Accepted for publication in Review of Scientific Instruments!
We introduce an easily implementable method for non-destructive measurements of ultracold atomic clouds based on dark field imaging of spatially resolved Faraday rotation. The dependence on laser detuning, atomic density and temperature is characterized in a detailed comparison with theory. Due to low destructiveness, the same cloud can be imaged up to 2000 times. The technique is applied to avoid the effect of shot-to-shot fluctuations in atom number calibration, to demonstrate single-run vector magnetic field imaging and singlerun spatial imaging of the system’s dynamic behavior. This paves the way towards quantum state engineering using feedback control of ultracold atoms. http://arxiv.org/abs/1301.3018 (07/2013)
We demonstrate the experimental realization of quasi-free wave packets of ultra-cold atoms bound by an external harmonic trap. The wave packets are produced by modulating the intensity of an optical lattice containing a Bose–Einstein condensate. The evolution of these wave packets is monitored in situ and their six-photon reflection at a band gap is observed. In direct analogy with pump–probe spectroscopy, a probe pulse allows for the resonant de-excitation of the wave packet into states localized around selected lattice sites at a long distance from the main component. New J. Phys. 14 083013 (2012) (08/2012)
We report on measurements of splitting Bose-Einstein condensates by using a time-dependent optical lattice potential. In this work we demonstrate the division of a BEC into a set of equally populated components and we apply time-dependent optical Bragg mirrors to a BEC oscillating in a harmonic trap. In addition a combination of multiple Bragg reflections and Landau-Zener tunneling allows for the generation of macroscopic arrays of condensates. arXiv:1203.6683 and Phys. Rev. A 85, 033626 (2012) (03/2012)
Our first paper based entirely on data taken at Århus is available online.
In this paper we demonstrate the experimental realization of quasi-free wave packets of ultra-cold atoms bound by an external harmonic trap. The wave packets are produced by modulating the intensity of an optical lattice containing a Bose-Einstein condensate. We monitor the evolution of these wave packets in-situ and observe a reflection on a band gap. In direct analogy with pump-probe spectroscopy, a probe pulse allows for the resonant de-excitation of the wave packet into localized lattice states at a long, controllable distance of more than 100 lattice sites from the main component. arXiv:1107.1643 (07/2011)
The first BEC was realized in our new laboratory on the 3.5.2011 roughly 5 months after making the last BEC in our old lab and moving the experiment! The number of atoms and their temeperature is looking good and we can now fine tune it! (05/2011)
The first MOT was realized in our new laboratory on the 16.2.2011 only 9 working weeks after moving the experiment! The atom number has grwn quickly since and currently we start our experiments with 5*109 Rb atoms (02/2011)
The lattice experiment has successfully moved to its new laboratory underneath the physics building! The experiment was moved under vacuum and with most optical elements installed. This should allow us to resume our experiments with moderate rebuilding. We hope to be back up and running in the late spring! Have a look at all the images from the move! (12/2010)
Bloch oscillations can provide a precise probe of disordered media. We have therefore investigated disorder induced damping of Bloch oscillationsin optical lattices. The spatially inhomogeneous force responsible for the damping is due to a combination of a disordered optical and a magnetic gradient potential. We show that the inhomogeneity of this force results in a broadening of the quasimomentum spectrum, which in turn causes damping of the centre-of-mass oscillation. New J. Phys. 10 045027 (2008)
