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Events

2019

April 26 10:15-11:00 in 1525-626

Nuclear spins for quantum contextuality tests and multiplexed quantum network protocols

Suzanne van Dam,  TU Delft

The nitrogen-vacancy (NV) centre in diamond is a promising node for a quantum network, as it combines a coherent spin state with an optical interface. Moreover, surrounding nuclear spins can be used in quantum computation protocols, for tests of quantum foundations, or as a quantum memory. In this seminar, we employ these spins in two ways. First, I describe the first implementation of repeated three-qubit parity measurements and how we use these to perform foundational quantum contextuality tests [1]. Second, I discuss how the rate of entangling protocols can benefit from multiplexing using nuclear spins in a multi-qubit quantum network node [2].

[1] S.B. van Dam, J. Cramer, T.H. Taminiau, and R. Hanson, arXiv:1902.08842

[2] S.B. van Dam, P.C. Humphreys, F. Rozpedek, S. Wehner, and R. Hanson, Quantum Science and Technology, 2, 034002 (2017)

Coffee/tea and bread rolls from 10:00

2018

Dec 4 15:15-16:00 in 1525-626

Vortex Momentum Distribution in Hydrogen

Guizhong Zhang, Tianjin University, China

This presentation will start with a brief introduction to Tianjin University of China. Following that, the talk will be concerned with attosceond and strong-field physics. As is well known for a few-cycle attosecond light pulse, the carrier envelope phase (CEP) has a direct and significant impact on its electric field and strong field processes induced by isolated attosecond pulse (IAP). By numerically investigating the angular photoelectron momentum spectra generated by overlapping IAP and infrared (IR) pulses, we show a scheme for retrieving the CEP value through the rotation angle of the momentum pattern. Another interesting observation for momentum distributions is their vortex shape created and experimentally corroborated by two time-delayed and oppositely circularly polarized attosecond pulses. We also report our numerical experiments on the formation of the vortex momentum distributions and their symmetry distortion when the dynamic Stark effect is taken into account. All the simulation has been performed by deploying the strong field approximation (SFA) theory and on hydrogen atom which has an analytical expression for the dipole transition matrix element.

Coffee/tea and cake from 15:00

Nov 15 10:15-11:00 in 1520-737

Are there non-trivial quantum effects in Biology? A discussion on light harvesting processes

Susana F. Huelga, Uni-Ulm, Germany

Quantum biology is an emerging field of research that concerns itself with the experimental and theoretical exploration of non-trivial quantum phenomena in biological systems (See references below for recent reviews on the subject). We will present an overview aimed to bring out fundamental assumptions and questions in the field, using light harvesting as a prototypical biological process. We will identify basic design principles and develop a key underlying theme -- the dynamics of quantum dynamical networks in the presence of an environment and the fruitful interplay that the two may enter.

A fundamental element in the discussion is the formulation of a microscopic model able to explain the observed persisting oscillatory features in the spectral response of different pigment-protein complexes at ambient temperatures. Along delocalized electronic excitations, we argue that quantum coherent interactions with near-resonant vibrations are instrumental for explaining long lived coherence and may contribute to light-harvesting performance.

Experimental results on both natural and artificial systems will be shown to be in agreement with this vibronic model which therefore provides an archetypical framework for the field.

S. F. Huelga and M.B. Plenio, Contemp. Phys. 54, 181 (2013) G. D. Scholes et al, Nature  543, 647 (2017)

E. Romero,    V.~I. Novoderezhkin and R. van Grondelle,  Nature 543, 355 (2017)

Coffee/tea and bread rolls from 10:05

June 18 15:15-16:00 in 1525-626

Quantum sensing with diamond spins

Ulrik Lund Andersen, Department of Physics, Technical University of Denmark

Due to its outstanding electronic properties at room temperature, the Nitrogen-Vacancy (NV) center hosted by a diamond crystal has generated increasing interest in quantum information science and in quantum sensing. The NV center has been deployed in numerous quantum information protocols and in measuring extremely small magnetic fields from e.g. neurons and single protons – all performed at ambient temperature and pressure. In this talk we report on an increased sensing efficiency using optimized measurement protocols and improved designs, and I will outline a path towards measuring neurons from human brains.

Coffee/Tea and cake will be served from 15:00

May 25 10:15-11:00 in 1525-626

Mean-field treatment of continuously probed and pumped atomic ensembles

Klemens Hammerer, University of Hannover, Germany

A number of experiments demonstrated the possibility of observing fascinating quantum effects with continuously probed and pumped atomic ensembles. These experiments deal with collective spin excitations on top of a spin polarized reference state serving as an effective ground state in the space of collective excitations. When collective excitations are generated or probed with CW light a certain degree of CW optical pumping has to be applied compensating probe induced depumping in order to maintain a mean spin polarization. The resulting interplay of continuous probe and pump gives rise to interesting collective dynamics of the atomic spin going well beyond single atom physics in optically dense atomic ensembles. Determining the spin polarized reference state in itself becomes a non-trivial problem of non-equilibrium open-system many-body quantum dynamics. Drawing an analogy to superradiant lasers I will introduce a systematic mean-field treatment taking into account any desired order of n-particle correlations for determining the atomic steady states as well as their two-time correlation functions. I will discuss results for ensembles of Caesium atoms.

Coffee/tea and bread rolls from 10:00

May 25 11:15-12:00 in 1525-626

Thermodynamics and Information at the nanoscale

Janet Anders, University of Exeter, UK  

Thermodynamic laws have been key for the design of useful everyday devices from car engines and fridges to power plants and solar cells. Technology’s continuing miniaturisation to the nanoscale is expected to soon enter regimes where standard thermodynamic laws do not apply. I will briefly introduce quantum thermodynamics, an emerging research field that aims to uncover the thermodynamic laws that govern small ensembles of systems that follow non-equilibrium dynamics and can host quantum properties [1]. I will then discuss a nanoscale thermodynamic experiment with heated optically trapped nanospheres in a dilute gas [2]. By developing a new theoretical model that captures the non-equilibrium situation of the particles, we were able to measure the surface temperature of the trapped spheres and observe temperature gradients on the nanoscale. In the second part of the talk I will discuss recent theoretical advances in defining thermodynamic work in the quantum regime. By introducing a process that removes quantum coherences we were able to show that work cannot only be extracted from classical non-equilibrium systems, additional work can be extracted from quantum coherences [3].

[1] Quantum thermodynamics, S. Vinjanampathy, J. Anders, Contemporary Physics 57, 545 (2016).
[2] Nanoscale temperature measurements using non-equilibrium Brownian dynamics of a levitated nanosphere, J. Millen, T. Deesuwan, P. Barker, J. Anders, Nature Nanotechnology 9, 425 (2014). 
[3] Coherence and measurement in quantum thermodynamics, P. Kammerlander, J. Anders, Scientific Reports 6, 22174 (2016).

May 2 11:15-12:00 in 1520-732

‘New’ tools in open quantum systems: Collision Models

Steve Campbell, INFN Sezione di Milano & Università degli Studi di Milano

In this talk we will examine the details of collision models, which are versatile tools for modelling open quantum systems. First, we use them to identify the relevant system-environment correlations that lead to a non-Markovian evolution. Using this information equivalent expressions of the dynamics can be formulated with regards to these "relevant" correlations and we will introduce the notion of memory depth where these correlations are established between the system and a suitably sized memory rendering the overall system+memory evolution Markovian. Furthermore, we will extend the framework to assess the thermodynamic aspects of non-Markovian open quantum systems and show some other exciting applications of these tools.

January 26 10:15-11:00 in 1525-626

Large deviations and non-equilibrium in open quantum systems

Juan Garrahan, University of Nottingham, UK

There will be coffee, tea and bread rolls from 10:00

January 26 11:10-11:55 in 1525-626

Compressed quantum computation and multipartite entanglement

Barbara Kraus, University of Innsbruck, Austria

2017

December 14 13:15-16:45 in 1520-737

QUSCOPE Seminar

Programme

November 30 14:15-16:00 in 1520-737

Coherent Population Oscillation-based Light Storage

Etienne Brion, Laboratoire Aimé Cotton, France

Coffee/tea and cake at 14:00
November 7 13:15-16:00 in 1525-626

Mini-symposium with VKR Centre QMATH

See programme

Coffee/tea and cake at 14:30
May 16 10:15-11:00 in 1520-732

Refocusing schemes of two qubit gates for trapped ions

Itsik Cohen,  Hebrew University of Jerusalem, Israel

Entanglement between two trapped ions is carried via a vibrational phonon mediator. This relatively slow, second order interaction is usually most fragile to noise effects, e.g., single body noise terms and two body noise terms. To compensate for single body noise effects during the two-qubit interaction, dynamical decoupling pulses are often applied at times when the qubits are disentangled from the phonon bus. Therefore, applying dynamical decoupling pulses with  Ωr repetition rate, to refocus a lower frequency noise, would prolong the gate duration linearly with  Ωr. In the first part of my talk I will show how to utilize dynamical decoupling pulses, such that the phonon-mediated entangling gate is performed in the strong coupling regime; namely, the gate duration is increased by a reduced factor of less than π/2.

On the other hand, compensating for the amplitude noise, which inflicts a random two qubit term, has proven more challenging. The main obstacle is that the two body noise time scale is shorter than the two qubit gate itself which prevents the use of standard refocusing methods. In the second part of my talk I will present two approaches to tackle this problem. The first one makes the use of composite pulses, applied as dynamical decoupling on the building blocks of ultrafast entangling gates; whereas the second approach uses a measurement and feedback based method to refocus two qubit gates.

Coffee/tea and bread rolls from 10:00
April 28 14:15-15:00 in 1593-012

Specialized iNANO Lecture: Intramolecular dynamics of single molecules in free diffusion

Hideo Mabuchi, Stanford University

Biomolecular systems such as multiprotein complexes or biopolymers can span several tens to several hundreds of nanometers, but the dynamics of such “mesocale” molecules remain challenging to probe. We have developed a single-molecule technique that uses Tracking Fluorescence Correlation Spectroscopy (tFCS) to measure the conformation and dynamics of molecular assemblies specifically at the mesoscale level (∼100-1000 nm). tFCS is non-perturbative, as molecules, which are tracked in real-time, are untethered and freely diffusing. To achieve sub-diffraction spatial resolution, we use a feedback scheme which allows us to maintain the molecule at an optimal position within the laser intensity gradient. We find that tFCS is sufficiently sensitive to measure the distance fluctuations between two sites within a DNA molecule separated by distances as short as 1000 bp. We demonstrate that tFCS detects changes in the compaction of reconstituted chromatin, and can assay transient protein mediated interactions between distant sites in an individual DNA molecule. Our measurements highlight the impact that tFCS can have in the study of a wide variety of biochemical processes involving mesoscale conformational dynamics.

See: http://biorxiv.org/content/early/2017/03/24/120311

Mabuchi Lab: https://minty2.stanford.edu/wp/

April 25 10:15-11:00 in 1520-616

Ceramics

Hideo Mabuchi, Stanford

  Macro- and micro-crystalline glazes are well known in the ceramic arts.  We are much less aware of the key role that *nanoscale* crystals, which are too small to see individually with the unaided eye, play in the development of surface color in atmospheric firing – wood, salt and soda – of unglazed clay.  In this talk I will show and discuss microscope images that illustrate our contemporary understanding of “flashing” and related phenomena based on nano-crystal growth during kiln cooling.  The story will be told in simple non-technical terms, but at the end of the talk I will indicate some intriguing potential connections (via zeolites and nanoparticle chains) to high-tech materials that engineers are investigating for power storage and for removal of chemical contaminants from gas emissions and water.  Some loose analogies will likewise emerge between atmospheric firing and experimental petrology.

Coffe and tea will be available from 10:00
February 10 10:15-11:00 in 1525-626

Strong coupling of an electron ensemble on the surface of liquid helium to a microwave cavity

Denis Konstaninov, Okinawa Institute of Science and Technology, Okinawa, Japan

Recently there has been a significant interest in the strong coupling of an ensemble of quantum particles to the electro-magnetic modes of a resonator. Besides traditional systems used in cavity QED experiments such as Rydberg atoms, strong coupling regime has been recently studied in various electron and nuclear spin ensembles, as well as two-dimensional electron systems (2DES) in semiconductors [1,2]. The hallmark of the strong coupling regime is the splitting in the resonator spectrum revealed in the signal reflected from or transmitted through the resonator. In case of a collection of N quantum particles this splitting scales as the square root of N. Besides general interest in the fundamental problem of light-matter interaction, the particular interest in the strong coupling regime comes from the quantum information processing as strong coupling to a high-Q resonator enables coherent information transfer between, for example, a qubit and quantum system excitations. Therefore, most of the recent observations of strong coupling have been interpreted as pure quantum phenomena. However, it is rarely mentioned that the strong coupling between large N-particle ensemble and the coherent state of electromagnetic mode in a resonator can be described completely classically in many cases. We present experimental observation of the strong coupling between cyclotron mode of 2DES on the surface of liquid helium and a microwave cavity resonator. The splitting in the eigenspectrum of coupled motion is observed in the cavity reflection signal, as well as in the ac current of electrons detected by measuring their bolometric photoresponse [3]. A simple model based on classical mechanics and electrodynamics accounts for all experimental features including the observed splitting. The square root scaling of the splitting follows naturally from our model.  Thus, our work reproduces all main features of the strong coupling regime for a large N-particle 2DES, including those reported in Refs. [1,2], but puts them on a completely classical ground.

[1] G. Scalari, Science 335, 1323 (2012).

[2] Q. Zhang et al., Nature Physics 12, 1005 (2016).

[3] L. V. Abdurakhimov et al., Phys. Rev. Lett. 117, 056803 (2016).
Coffe and tea will be available from 10:00
January 13 11:15-12:00 in 1525-626

Optimization of real-world qubit measurements

Benjamin D'Anjou, Department of Physics, McGill University, Montreal QC, Canada

The last decade has witnessed the rapid development of a variety of quantum bit (qubit) implementations for use in emerging quantum information technologies ranging from fault-tolerant quantum computation to quantum metrology. In these applications, it is often necessary or desirable to read out the state of the qubits with the highest accuracy and in the shortest amount of time. Achieving these goals generally requires 1) an understanding of the physics of the measurement noise and 2) an optimal inference procedure tailored to that noise. In this talk, I will discuss various aspects of the optimization of qubit readout, including single-shot readout [1], adaptive decisions [2], and soft-decision decoding [3]. I will illustrate these aspects with the help of several experimentally relevant examples.

[1] B. D'Anjou & W.A. Coish, "Optimal post-processing for a generic single-shot qubit readout", Phys. Rev. A 89 012313 (2014)
[2] B. D'Anjou, L. Kuret, L. Childress & W.A. Coish, "Maximal adaptive-decision speedups in quantum-state readout", Phys. Rev. X 6 011017 (2016)
[3] B. D'Anjou & W.A. Coish, "Soft decoding of a qubit readout apparatus", Phys. Rev. Lett 113 230402 (2014).

Coffe and tea will be available from 11:00

2016

December 9 13:15-22 in 1520-732

QUSCOPE Seminar

Program

October 14 11:15-12:00 in 1520-733

Encryption and quantum computing with continuous variables

Ulrik Lund Andersen, Technical University of Denmark

October 4 10:15-11:00 in 1520-733

The role of quantum measurement in quantum thermodynamics

Alexia Auffèves, Institut Néel - CNRS, Grenoble, France

In this talk I will propose a new formalism to investigate stochastic thermodynamics in the quantum regime, where stochasticity and irreversibility primarily come from quantum measurement. In the absence of any bath, a purely quantum component to heat exchange is defined, that corresponds to energy fluctuations caused by measurement back-action. Energetic and entropic signatures of measurement induced irreversibility are then investigated for canonical experiments of quantum optics, and the energetic cost of counter-acting decoherence is characterized on a simple state-stabilizing protocol. By placing quantum measurement in a central position, our formalism contributes to bridge a gap between experimental quantum optics and quantum thermodynamics.

September 14-15  in 1520-626

Thesis Defense and Workshop on Strong-field and Ultrafast Physics

Final programme

June 22 15:15-16:00 in 1525-323

Clean quantum and classical communication protocols

Matthias Christandl, Department of Mathematical Sciences, University of Copenhagen

To compute a two-party function cleanly , the players must not only correctly calculate the function's value but also return all registers to their initial states at the end of the communication protocol. Such protocols provide methods for implementing distributed computations, can be safely run in coherent superposition and provide lower bounds on the communication complexity of their non-clean counterparts.

Here we present clean protocols for calculating the Inner Product of two n-bit strings, showing that (in the absence of pre-shared entanglement) at most n+O(√n) bits or n + 4 qubits of communication are required. These provide new methods for implementing distribute CNOT or controlled-Z gates in parallel whilst minimizing the amount of communication. While evaluating a function cleanly is harder to perform than just computing the function, it can always be done using at most 2 n bits of communication for functions of n-bit strings. In contrast to the Inner Product, we show that nearly all Boolean functions require close to the maximal 2 n bits of classical communication to compute cleanly.

There will be coffee, tea and cake from 15:00.

April 5 15:15-16:00 in 1525-323

Transfer of spectral purity from the optics to the microwave, THz and Optics domains using an optical frequency comb

Yann Le Coq,  LNE-SYRTE, Observatoire de Paris, France

Optical frequency combs are used routinely in national metrology institutes and elsewhere for measuring and comparing optical frequency standards (optical clocks). Beyond this "traditional" use, they can also be utilized to transfer the spectral purity of a state-of-the-art optical source to other frequency domains, thereby generating an ultra-low phase noise source in this other frequency domain. I will describe our experiments that do so, targeting the microwave (~10 GHz), teraHertz (~30 THz) and optics (~200 THz) domains. I will describe several techniques that are used to ensure the comb adds a negligible extra noise in the frequency conversion and show how these techniques allow to create, both in the microwave and Teraherz domains, spectrally ultra-pure sources with record low phase noise levels, several orders of magnitude below more traditional technologies.


March 10 9:30-18:30 in 1525-626

QUSCOPE WORKSHOP ON ”ULTRAFAST AND ULTRASTRONG”

Final programme

January 8 10:15 in 1520-732

One-dimensional Bose gases in momentum space : equilibrium and out-of-equilibrium behavior

Isabelle Bouchoule, Institut d’Optique, France

On our atom-chip experiment, using the focussing technique, we are able to observe single one-dimensional (1D) Bose gases in momentum-space. We investigated the momentum distribution at equilibrium across the quasi-condensation crossover. Moreover, analyzing statistical noise on the pictures, we measured the two-body correlations in momentum-space. While the momentum distribution does not qualitatively change across the quasi-condensate transition, due to the lack of long-range order in finite temperature 1D Bose gas, the two-body correlations in momentum-space present a clear signature of the quasi-condensation phenomena. Finally, measurements in momentum space are very valuable to study out-of-equilibrium dynamics. In our experiment, we revisited the breathing mode of a harmonically confined 1D Bose gas and show for the first time the self-reflection mechanism that occurs in the quasi-condensate regime.

January 7 11:15 in 1520-732

Dipolar QED (dQED): an alternative paradigm for quantum optics

Charles Adams, Durham University, UK

In the strong coupling regime of cavity QED, a single photon modifies the optical response of the atom-cavity system to subsequent photons. An analogous effect can occur in atomic ensembles without the need of a cavity, if the atoms interact strongly. For example a single photon stored as a Rydberg excitation can switch the optical response of the ensemble to subsequent photons. In this talk we review some applications of this dipolar QED effect in both the quantum and classical regimes.

2015

December 17 9:15-22:00 in 1525-323

QUSCOPE meeting

Program


Material from QUSCOPE meeting December 2015

Time-prop-presentation

Split operator

December 16 14:15

Fractional quantum Hall physics in lattice systems

Anne E.B. Nielsen

The fractional quantum Hall effect can be displayed by electrons moving in two dimensions in a strong magnetic field and at very low temperatures. The effect is intriguing for several reasons. First, the systems can be used as a resistance standard. Second, it is necessary to develop new tools to investigate the systems. It has, e.g., been necessary to rethink the way phase transitions are treated. Third, the systems can host quasi-particles that are anyons. Anyons are neither bosons nor fermions, and their properties can, e.g., be exploited in quantum computers. Fundamental interest and the hope to find easier and more robust ways to realize the fractional quantum Hall effect experimentally have triggered a lot of effort towards clarifying under which conditions the effect can occur. In particular lattice systems have received much attention. In this talk, I will first give an introduction to topology, anyons, and the fractional quantum Hall effect. I will then discuss strategies to obtain fractional quantum Hall physics in lattice systems.

December 14 15:15

Thermalization in a 1D Rydberg gas: validity of the microcanonical ensemble hypothesis

Ruben Cohen, Laboratoire Aimé Cotton, Orsay, France

We question the microcanonical hypothesis, often made to account for the thermalization of complex closed quantum systems, on the specific example of a chain of two-level atoms optically driven by a resonant laser beam and strongly interacting via Rydberg-Rydberg dipole-dipole interactions. Along its (necessarily unitary) evolution, this system is indeed expected to thermalize, i.e. observables, such as the number of excitations, stop oscillating and reach equilibrium-like expectation values. The latter are often calculated through assuming the system can be effectively described by a thermal-like microcanonical state. Here, we compare the distribution of excitations in the chain calculated either according to the microcanonical assumption or through direct exact numerical simulation. This allows us to show the limitations of the thermal equilibrium hypothesis and precise its applicability conditions.

November 16 15:15

Optimized remote entanglement and stabilization with circuit QED

Felix Motzoi, University of California, Berkeley and Saarland University, Germany

I discuss different resources for generating long-distance entanglement between superconducting qubits using circuit-QED. We start with non-local quantum trajectories of a common dispersive parity measurement between remote transmons for which entanglement can be built up stochastically. Then, by including feedback control conditioned on undesired measurements outcomes, the objective state can be reached deterministically. To reach the desired state and remain stabilized in that state in the presence of decoherence, all other states must be rotated non-trivially. We show how to do this by making the target state be the only fixed point of both coherent and dissipative dynamics. Finally, it is shown that conditioned feedback can be replaced by "spooky" environmental backaction when the joint measurement is destructive.

August 31 15:15

Strongly interacting bosons on optical lattice ladders with flux

Ulrich Schollwöck, University of Munich

In this talk I will study the rich phase diagrams emerging for two-leg and three-leg ladders in optical lattices with interacting bosons exposed to strong magnetic flux (as is hard to achieve in real solids): Meissner phases, vortex phases, vortex lattices, ... Among other things, one observes the reversal of the direction of the chiral (Meissner) currents compared to the naively expected direction. The mechanisms behind this observation are all due to interaction effects.

August 31 and
September 1
Study group and mini-course on Matrix Product States in Aarhus

By the initiative of PhD students Alexander Holm Kiilerich and Jens Jakob Sørensen, a study group (5 ECTS for students) will be open for all students, post docs and staff. Professor Ulrich Schollwöck from Munich will present a mini-course and a research talk on August 31 and September 1 in Aarhus. This event can be attended separately or as part of the Study Group, and we will organize some social activity around the lectures.

August 11
9:00
QUSCOPE Center Meeting in Aalborg

The whole QUSCOPE center will meet in Aalborg at 9:00 (we arrange joint transport from Aarhus), and we will end the day with a good dinner before the Aarhus gang will return in the evening. If you have already made other arrangements for that date, please tell Lars, Thomas or Klaus. More details will follow, and we may ask for input to the scientific program from some of you.

June 4
14:15 in 1525-323

Hybrid quantum systems: NV-centers, quantum dots and recent fantasies about rare-earth doped crystals
Signe Seidelin, Institut Néel, CNRS, Grenoble, France

An exciting challenge of modern physics is to investigate the behavior of a material object - for instance a mechanical oscillator - placed in a non-classical state. One approach consists in exploiting a hybrid quantum system based on a mechanical oscillator coupled to an atom-like object. Diverse coupling mechanisms between these two radically different degrees of freedom have been demonstrated by the community in recent years [1], such as magnetic, capacitive, opto-mechanical, or via surface potentials, etc. As a starting point, I will briefly present our first hybrid system, which consisted in in a single Nitrogen Vacancy (NV) defect hosted in a diamond nanocrystal positioned at the extremity of a vibrating nanowire [2]. The coupling was achieved by an external magnetic gradient placed near the nanowire. However, a more stable, and potentially stronger coupling mechanism is based on material strain. Here, the oscillator is a bulk object containing an embedded artificial atom (dopant, quantum dot, ...) which is sensitive to the mechanical strain of the surrounding material. Vibrations of the oscillator result in a time-varying strain field that modulates the energy levels of the embedded structure. Using a quantum dot embedded in a photonic nanowire, we did a proof-of-principle experiment in which we demonstrated a coupling based on material strain [3]. However, due to the relatively large spectral linewidth of quantum dots in general, other systems might prove more suitable for reaching the so-called resolved-sideband regime. This regime - a pre-requisite for some active cooling schemes for mechanical oscillators - requires a linewidth of the emitter well below the mechanical oscillation frequency. I will discuss some ideas (fantasies?) on using rare-earth doped crystals as mechanical oscillators, which might hold promise to reach even deeper into this regime. More precisely, we are currently studying Eu3+  (in an Y2SiO5 matrix) which has an optical transition with the narrowest linewidth known for a solid-state emitter [4], and a transition which is directly sensitive to mechanical strain [5].

[1] M. Aspelmeyer, P. Meystre, and K. Schwab, Quantum optomechanics, Physics Today 65, 29 (2012)
[2] O. Arcizet, V. Jacques, A. Siria, P. Poncharal, P. Vincent, and S. Seidelin, Nature Physics 7, 879 (2011)
[3] I. Yeo et al., Strain-mediated coupling in a quantum dot–mechanical oscillator hybrid system, Nature Nanotechnology 9, 106 (2014)
[4] R. Yano, M. Mitsunaga, and N. Uesugi, Ultralong optical dephasing time in Eu3+:Y2SiO5, Optics   Letters, 16, 1884 (1991)
[5] M. J. Thorpe et al., Frequency stabilization to 6 x10-16 via spectral-hole burning, Nature Photonics, 5, 688 (2011)

May 5
13:15 in 1520-732

Dynamical phase transitions as a resource for quantum enhanced metrology
Katarzyna Macieszczak , University of Nottingham

We consider the general problem of estimating an unknown control parameter of an open quantum system. We establish a direct relation between the evolution of both system and environment and the precision with which the parameter can be estimated. We show that when the open quantum system undergoes a first-order dynamical phase transition the quantum Fisher information (QFI), which gives the upper bound on the achievable precision of any measurement of the system and environment, becomes quadratic in observation time (cf. “Heisenberg scaling”). In fact, the QFI is identical to the variance of the dynamical observable that characterises the phases that coexist at the transition, and enhanced scaling is a consequence of the divergence of the variance of this observable at the transition point. For the systems near a transition, i.e., those displaying metastability, the QFI is quadratic in time for times shorter than the correlation time of the dynamics. A proper definition of metastable phases and their coexistence can be constructed mathematically from the eigenvectors of the master operator governing the system dynamics. In the regime of enhanced scaling the optimal measurement whose precision is given by the QFI involves measuring both system and output. As a particular realisation of these ideas, we describe a theoretical scheme for quantum enhanced estimation of an optical phase-shift using the photons being emitted from a quantum system near the coexistence of dynamical phases with distinct photon emission rates.

May 4
14:15 in Fellows' Aud.
Quantum Emulators - Simulating few- and many-body physics with Rydberg atoms
David Petrosyan, AIAS Fellow (abstract)
March 13
9:25 in AIAS Aud.
Elucidating the dynamics of single photosynthetic light-harvesting complexes
Gabriela S. Schlau-Cohen (external speaker), MIT
March 12
15:00 in AIAS Aud.
The challenge of achieving quantum supremacy in quantum simulators
Matthias Troyer (external speaker), ETH Zürich
March 11
15:30 in AIAS Aud.
Status and open problems of lattice field theories
Karl Jansen (external speaker), DESY
January 29-30

Retreat for the full QUSCOPE Centre to Bramslev Gaard (program, photos)

2014

December 11
12:30 in 1525-626
Scientific Meeting of Aarhus teams
Select members of QUSCOPE Aarhus
November 18
10:15 in 1520-732
Efficient Characterization and Optimal Control of Open Quantum Systems
Daniel Reich, University of Kassel, Germany
Since no system can ever be completely isolated from its environment the study of open quantum systems is pivotal to reliably and accurately control complex quantum systems. In practice, reliability of the control needs to be confirmed via certification of the target evolution while accuracy requires the derivation of high-fidelity control schemes in the presence of decoherence. We present frameworks for highly efficient characterization of unitary transformations and their application to gate certification and Optimal Control Theory. Furthermore, we devise control techniques that work not only against but also with the environment to fulfill certain quantum control tasks. In particular, we will demonstrate the power of Optimal Control Theory in the presence of environmental effects for vibrational cooling of molecules and superconducting quantum gate implementations.
October 13

14:15 in 1520-616
Qubit interference at avoided crossings: The role of driving shape and bath coupling
Ralf Blattmann, University of Augsburg, Germany
Sweeping through an avoided crossing with a periodic, large amplitude driving leads to a sequence of transition, where the phase accumulation between transition may result in constructive or destructive interference. This leads to so called Landau-Zener-Stueckelberg-Majorana (LZSM) interference patterns. We derive the structure of the LZSM patterns for a qubit that experiences quantum dissipation and is additionally subjected to time-periodic but otherwise general driving. Numerical as well as analytical results predict a peak structure that depends sensitively on the details of the qubit-bath coupling. The Fourier transforms of the LZSM patterns exhibit arc structures which reflect the shape of the driving, and provide an opportunity to characterize the qubit coherence. We derive an analytical description for the observed structures and numerically determine their decay as a function of dissipation strength and temperature.

August 14-15

Retreat for the full QUSCOPE Centre to Bramslev Gaard (program, photos)

June 13
10:15 in 1520-732

Experimental investigations of resonant dipole-dipole interaction between cold atoms
Antoine Browaeys, Institut d’Optique, Palaiseau, France

This talk will present our on-going effort to understand and manipulate resonant dipole-dipole interaction between cold atoms. This interaction results from the non-radiative energy exchange between two-level systems, mediated by the vacuum field between atoms. It is long-range, with scaling between 1/R3to 1/R, with R the distance between atoms.

We are working on two different systems of cold atoms where this interaction plays an important role. The first one is a dense ensemble of cold atoms confined in a volume on the order of the wavelength of an optical transition. Here the interaction results into the collective scattering of a near-resonant laser by the ensemble, described by a collection of eigen-modes. In particular the scattering is strongly suppressed with respect to the single atom case. The presence of this interaction leads to open question in the theory of the optical response of an ensemble of scatterers.

In our second systems, we manipulate individual atoms in arrays of optical tweezers separated by few micrometers. There we control the interaction between atoms with microwave and DC electric fields. We observe in particular the coherent energy exchange between two atoms. This control of the interaction has application in quantum state engineering.

June 12
15:15 in 1525-323
Quantum digital signatures
Erika Andersson, Heriot-Watt University, Edinburgh, Scotland
Digital signatures ensure that messages cannot be forged or tampered with. They are widely used to provide security for electronic communications, for example in financial transactions and electronic mail. Importantly, signed messages are also transferrable, meaning that if one recipient accepts a message as genuine, then she is guaranteed that others will also accept the same message if it is forwarded. Digital signatures are different from encryption, which guarantees the privacy of a message. Currently used classical digital signature schemes, however, only offer security relying on unproven computational assumptions. In contrast, quantum digital signatures (QDS), similar to quantum key distribution (QKD), offer information-theoretic security based on principles of quantum mechanics. A serious drawback of previous QDS schemes is however that they require long-term quantum memory, making them unfeasible. We present protocols which do not need quantum memory and which use only standard linear optical components and photodetectors. With this, it seems that QDS and QKD are similar in terms of experimental requirements. Important work remains in investigating which QDS schemes are most suited for real applications, and in completing full security proofs.

March 12
10:15 in 1520-616

Controlling the rotational dynamics of molecules using combined laser pulses and static electric fields (abstract)
Juan Jose Omiste, Department of Atomic, Molecular and Nuclear Physics, University of Granada, Spain