Publications (190)521.2 Total impact
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ABSTRACT: We report on the state of the art of quantum walk experiments with neutral atoms in statedependent optical lattices. We demonstrate a novel statedependent transport technique enabling the control of two spinselective sublattices in a fully independent fashion. This transport technique allowed us to carry out a test of singleparticle quantum interference based on the violation of the LeggettGarg inequality and, more recently, to probe twoparticle quantum interference effects with neutral atoms cooled into the motional ground state. These experiments lay the groundwork for the study of discretetime quantum walks of strongly interacting, indistinguishable particles to demonstrate quantum cellular automata of neutral atoms.  [Show abstract] [Hide abstract]
ABSTRACT: Fiber FabryPerot cavities, formed by micromachined mirrors on the endfacets of optical fibers, are used in an increasing number of technical and scientific applications, where they typically require precise stabilization of their optical resonances. Here, we study two different approaches to construct fiber FabryPerot resonators and stabilize their length for experiments in cavity quantum electrodynamics with neutral atoms. A piezomechanically actuated cavity with feedback based on the PoundDreverHall locking technique is compared to a novel rigid cavity design that makes use of the high passive stability of a monolithic cavity spacer and employs thermal selflocking and external temperature tuning. Furthermore, we present a general analysis of the mode matching problem in fiber FabryPerot cavities, which explains the asymmetry in their reflective line shapes and has important implications for the optimal alignment of the fiber resonators. Finally, we discuss the issue of fibergenerated background photons. We expect that our results contribute towards the integration of highfinesse fiber FabryPerot cavities into compact and robust quantumenabled devices in the future. 
Article: Ideal Negative Measurements in Quantum Walks Disprove Theories Based on Classical Trajectories
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ABSTRACT: We report on a stringent test of the nonclassicality of the motion of a massive quantum particle, which propagates on a discrete lattice. Measuring temporal correlations of the position of single atoms performing a quantum walk, we observe a 6σ violation of the LeggettGarg inequality. Our results rigorously excludes (i.e., falsifies) any explanation of quantum transport based on classical, welldefined trajectories. We use socalled ideal negative measurements—an essential requisite for any genuine LeggettGarg test—to acquire information about the atom’s position, yet avoiding any direct interaction with it. The interactionfree measurement is based on a novel atom transport system, which allows us to directly probe the absence rather than the presence of atoms at a chosen lattice site. Beyond the fundamental aspect of this test, we demonstrate the application of the LeggettGarg correlation function as a witness of quantum superposition. Here, we employ the witness to discriminate different types of walks spanning from merely classical to wholly quantum dynamics.Physical Review X 01/2015; 5(1). DOI:10.1103/PhysRevX.5.011003 · 9.04 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report on the observation of cooperative radiation of exactly two neutral atoms strongly coupled to the single mode field of an optical cavity, which is close to the losslesscavity limit. Monitoring the cavity output power, we observe constructive and destructive interference of collective Rayleigh scattering for certain relative distances between the two atoms. Because of cavity backaction onto the atoms, the cavity output power for the constructive twoatom case (N=2) is almost equal to the singleemitter case (N=1), which is in contrast to freespace where one would expect an N^{2} scaling of the power. These effects are quantitatively explained by a classical model as well as by a quantum mechanical model based on Dicke states. We extract information on the relative phases of the light fields at the atom positions and employ advanced cooling to reduce the jump rate between the constructive and destructive atom configurations. Thereby we improve the control over the system to a level where the implementation of twoatom entanglement schemes involving optical cavities becomes realistic.Physical Review Letters 01/2015; 114(2):023601. DOI:10.1103/PhysRevLett.114.023601 · 7.51 Impact Factor 
Article: Decoherence Models for DiscreteTime Quantum Walks and their Application to Neutral Atom Experiments
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ABSTRACT: We discuss decoherence in discretetime quantum walks in terms of a phenomenological model that distinguishes spin and spatial decoherence. We identify the dominating mechanisms that affect quantum walk experiments realized with neutral atoms walking in an optical lattice. From the measured spatial distributions, we determine with good precision the amount of decoherence per step, which provides a quantitative indication of the quality of our quantum walks. In particular, we find that spin decoherence is the main mechanism responsible for the loss of coherence in our experiment. We also find that the sole observation of ballistic instead of diffusive expansion in position space is not a good indicator for the range of coherent delocalization. We provide further physical insight by distinguishing the effects of short and long time spin dephasing mechanisms. We introduce the concept of coherence length in the discretetime quantum walk, which quantifies the range of spatial coherences. Unexpectedly, we find that quasistationary dephasing does not modify the local properties of the quantum walk, but instead affects spatial coherences. For a visual representation of decoherence phenomena in phase space, we have developed a formalism based on a discrete analogue of the Wigner function. We show that the effects of spin and spatial decoherence differ dramatically in momentum space.New Journal of Physics 09/2014; 16(12). DOI:10.1088/13672630/16/12/123052 · 3.56 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report on the observation of cooperative radiation of exactly two neutral atoms strongly coupled to the single mode field of an optical cavity in the good cavity limit. Monitoring the cavity output power, we observe super and subradiant Rayleigh scattering into the cavity mode for certain relative distances between the two atoms. Surprisingly, due to cavity backaction onto the atoms, the cavity output power for superradiant scattering is almost equal to the single emitter case. These effects are quantitatively explained by a classical model as well as by a quantum mechanical model based on Dicke states. We extract information on the relative phases of the light fields at the atom positions and employ advanced cooling to reduce the jump rate between super and subradiant configurations. Thereby we improve the control over the system to a level where the realization of twoatom entanglement schemes involving optical cavities becomes feasible.  [Show abstract] [Hide abstract]
ABSTRACT: We experimentally realize an enhanced Raman control scheme for neutral atoms that features an intrinsic suppression of the twophoton carrier transition, but retains the sidebands which couple to the external degrees of freedom of the trapped atoms. This is achieved by trapping the atom at the node of a blue detuned standing wave dipole trap. We apply this method to perform resolved sideband cooling to the twodimensional vibrational ground state and to coherently manipulate the atomic motion. The presented scheme requires minimal additional resources and can be applied to experiments with challenging optical access, as we demonstrate by our implementation for atoms strongly coupled to an optical cavity.New Journal of Physics 06/2014; 16(11). DOI:10.1088/13672630/16/11/113042 · 3.56 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: When a physical object is in a quantum superposition, interference among the possible quantum paths affects the outcomes of a measurement. Gaining knowledge about the system with an intermediate measurement smears out, if not completely destroys, the resulting interference pattern. Based on this idea, Leggett and Garg formulated an inequality relating twotime correlation measurements, which establishes a quantitative criterion for discerning quantum superposition on a fundamental ground. Originally proposed to test realism of macroscopic physical objects, the inequality is here used to prove the nonclassicality of the motion of a cesium atom, which propagates on a spacetime discrete lattice. Making use of ideal negative measurements to test the atom's motion, we obtain a 6 sigma experimental violation of the LeggettGarg inequality. From a broader perspective, our findings provide rigorous validation of the idea that quantum transport experiments cannot be interpreted in terms of classical trajectories. 
Article: Electromagneticallyinducedtransparency control of singleatom motion in an optical cavity
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ABSTRACT: We demonstrate cooling of the motion of a single neutral atom confined by a dipole trap inside a highfinesse optical resonator. Cooling of the vibrational motion results from electromagnetically induced transparency (EIT)like interference in an atomic Λtype configuration, where one transition is strongly coupled to the cavity mode and the other is driven by an external control laser. Good qualitative agreement with the theoretical predictions is found for the explored parameter ranges. Further, we demonstrate EIT cooling of atoms in the dipole trap in free space, reaching the ground state of axial motion. By means of a direct comparison with the cooling inside the resonator, the role of the cavity becomes evident by an additional cooling resonance. These results pave the way towards a controlled interaction among atomic, photonic, and mechanical degrees of freedom.Physical Review A 02/2014; 89(3). DOI:10.1103/PhysRevA.89.033404 · 2.81 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the quantum jumps of an atom interacting with a cavity field. The strong atomfield interaction makes the cavity transmission depend on the time dependent atomic state, and we present a Hidden Markov Model description of the atomic state dynamics which is conditioned in a Bayesian manner on the detected signal. We suggest that small variations in the observed signal may be due to spatial motion of the atom within the cavity, and we represent the atomic system by a number of hidden states to account for both the small variations and the internal state jump dynamics. In our theory, the atomic state is determined in a Bayesian manner from the measurement data, and we present an iterative protocol, which determines both the atomic state and the model parameters. As a new element in the treatment of observed quantum systems, we employ a Bayesian approach that conditions the atomic state at time t on the data acquired both before and after t and we show that the state assignment by this approach is more decisive than the usual conditional quantum states, based on only earlier measurement data.Physical Review A 12/2013; 89(4). DOI:10.1103/PhysRevA.89.043839 · 2.81 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present an in situ method to measure the birefringence of a single vacuum window by means of microwave spectroscopy on an ensemble of cold atoms. Stressinduced birefringence can cause an ellipticity in the polarization of an initially linearly polarized laser beam. The amount of ellipticity can be reconstructed by measuring the differential vector light shift of an atomic hyperfine transition. Measuring the ellipticity as a function of the linear polarization angle allows us to infer the amount of birefringence Δn at the level of 10(8) and identify the orientation of the optical axes. The key benefit of this method is the ability to separately characterize each vacuum window, allowing the birefringence to be precisely compensated in existing vacuum apparatuses.The Review of scientific instruments 12/2013; 84(12):126103. DOI:10.1063/1.4847075 · 1.61 Impact Factor 
Conference Paper: Detecting The Motional State Of Single Atoms In A HighFinesse Optical Cavity By Heterodyne Spectroscopy
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ABSTRACT: form only given. We observe the quantized motion of single atoms strongly coupled to a highfinesse optical cavity and investigate dynamics of cavityassisted atom cooling. Single caesium atoms are trapped in a bluedetuned standingwave intracavity dipole trap, formed by a lock laser which is used to stabilize the cavity resonance frequency. A probe laser is coupled into the cavity mode and its transmission spectrum is monitored by means of photoncounting heterodyne spectroscopy [2].Measured heterodyne spectra of single atoms in the cavity are shown in Fig. 1 for two different locklaser intensities and compared with theoretical models. The motional Raman sidebands are offset from the carrier beat signal at 1 MHz by the atomic vibrational frequency (±ν). They show asymmetric lineshapes due to the anharmonicity of the dipole potential.We investigate the dependence of the positions and lineshapes of the motional Raman sidebands on experimantal parameters such as the cavityatom detuning, and locklaser intensity. Information on the atomic temperature, cooling and heating rates, as well as the atomic position in the cavity with respect to an antinode of the cavity probe field are found by comparing the observed spectra with theoretical predictions [1]. In addition, this technique can also be used to analyze the cooling dynamics of a cavityassisted EIT cooling scheme which has been recently studied and demonstrated in [3,4].International Quantum Electronics Conference; 05/2013  [Show abstract] [Hide abstract]
ABSTRACT: We report on the experimental realization of electric quantum walks, which mimic the effect of an electric field on a charged particle in a lattice. Starting from a textbook implementation of discretetime quantum walks, we introduce an extra operation in each step to implement the effect of the field. The recorded dynamics of such a quantum particle exhibits features closely related to Bloch oscillations and interband tunneling. In particular, we explore the regime of strong fields, demonstrating contrasting quantum behaviors: quantum resonances versus dynamical localization depending on whether the accumulated Bloch phase is a rational or irrational fraction of 2π.Physical Review Letters 05/2013; 110(19):190601. DOI:10.1103/PhysRevLett.110.190601 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Spindependent optical potentials allow us to use microwave radiation to manipulate the motional state of trapped neutral atoms (F\"orster et al. 2009 Phys. Rev. Lett. 103, 233001). Here, we discuss this method in greater detail, comparing it to the widelyemployed Raman sideband coupling method. We provide a simplified model for sideband cooling in a spindependent potential, and we discuss it in terms of the generalized LambDicke parameter. Using a master equation formalism, we present a quantitative analysis of the cooling performance for our experiment, which can be generalized to other experimental settings. We additionally use microwave sideband transitions to engineer motional Fock states and coherent states, and we devise a technique for measuring the population distribution of the prepared states.Journal of Physics B Atomic Molecular and Optical Physics 02/2013; 46(10). DOI:10.1088/09534075/46/10/104006 · 1.98 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report on controlled doping of an ultracold Rb gas with single neutral Cs impurity atoms. Elastic twobody collisions lead to a rapid thermalization of the impurity inside the Rb gas, representing the first realization of an ultracold gas doped with a precisely known number of impurity atoms interacting via swave collisions. Inelastic interactions are restricted to a single threebody recombination channel in a highly controlled and pure setting, which allows to determine the RbRbCs threebody loss rate with unprecedented precision. Our results pave the way for a coherently interacting hybrid system of individually controllable impurities in a quantum manybody system.Physical Review Letters 12/2012; 109(23):235301. DOI:10.1103/PhysRevLett.109.235301 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Den diesjährigen Physik‐Nobelpreis erhielten zu gleichen Teilen Serge Haroche und David Wineland für ihre bahnbrechenden experimentellen Methoden, die es ermöglichen, individuelle Quantensysteme zu messen und zu manipulieren.Physik in unserer Zeit 11/2012; 43(6):272273. DOI:10.1002/piuz.201290099  [Show abstract] [Hide abstract]
ABSTRACT: We experimentally demonstrate realtime feedback control of the joint spinstate of two neutral cesium atoms inside a high finesse optical cavity. The quantum states are discriminated by their different cavity transmission levels. A Bayesian update formalism is used to estimate state occupation probabilities as well as transition rates. We stabilize the balanced twoatom mixed state, which is deterministically inaccessible, via feedback control and find very good agreement with Monte Carlo simulations. On average, the feedback loop achieves near optimal conditions by steering the system to the target state marginally exceeding the time to retrieve information about its state.Physical Review Letters 10/2012; 109(17):173601. DOI:10.1103/PhysRevLett.109.173601 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that the presence of an interaction in the quantum walk of two atoms leads to the formation of a stable compound, a molecular state. The wave function of the molecule decays exponentially in the relative position of the two atoms; hence it constitutes a true bound state. Furthermore, for a certain class of interactions, we develop an effective theory and find that the dynamics of the molecule is described by a quantum walk in its own right. We propose a setup for the experimental realization as well as sketch the possibility to observe quasiparticle effects in quantum manybody systems.New Journal of Physics 07/2012; 14(7). DOI:10.1088/13672630/14/7/073050 · 3.56 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The internal state of organic photochromic spiropyran molecules adsorbed on optical microfibres is optically controlled and measured by statedependent light absorption. Repeated switching between the states is achieved by exposure to the evanescent field of a few nanowatts of light guided in the microfibre. By adjusting the microfibre evanescent field strength the dynamic equilibrium state of the molecules is controlled. Timeresolved photoswitching dynamics are measured and modelled with a rate equation model. We also study how many times the photochromic system can be switched before undergoing significant photochemical degradation.Optics Express 06/2012; 20(12):1271020. DOI:10.1364/OE.20.012710 · 3.49 Impact Factor 
Article: Digital atom interferometer with single particle control on a discretized spacetime geometry
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ABSTRACT: Engineering quantum particle systems, such as quantum simulators and quantum cellular automata, relies on full coherent control of quantum paths at the single particle level. Here we present an atom interferometer operating with single trapped atoms, where single particle wave packets are controlled through spindependent potentials. The interferometer is constructed from a sequence of discrete operations based on a set of elementary building blocks, which permit composing arbitrary interferometer geometries in a digital manner. We use this modularity to devise a spacetime analogue of the wellknown spin echo technique, yielding insight into decoherence mechanisms. We also demonstrate mesoscopic delocalization of single atoms with a separationtolocalization ratio exceeding 500; this result suggests their utilization beyond quantum logic applications as nanoresolution quantum probes in precision measurements, being able to measure potential gradients with precision 5 x 10(4) in units of gravitational acceleration g.Proceedings of the National Academy of Sciences 06/2012; 109(25):97704. DOI:10.1073/pnas.1204285109 · 9.67 Impact Factor
Publication Stats
5k  Citations  
521.20  Total Impact Points  
Top Journals
 Physical Review A (24)
 Applied Physics B (18)
 Physical Review Letters (11)
 Physical Review Letters (8)
 New Journal of Physics (8)
Institutions

19962015

University of Bonn
 Institute for Applied Physics
Bonn, North RhineWestphalia, Germany


2014

Universität des Saarlandes
 Physikalische und Theoretische Chemie
Saarbrücken, Saarland, Germany


19851992

Max Planck Institute of Quantum Optics
 Division of Laser Spectroscopy
Arching, Bavaria, Germany


19861988

Yale University
 Department of Physics
New Haven, CT, United States
