[Show abstract][Hide abstract] ABSTRACT: We discuss decoherence in discrete-time 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
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 discrete-time quantum walk, which quantifies the range of spatial
coherences. Unexpectedly, we find that quasi-stationary dephasing does not
modify the local properties of the quantum walk, but instead affects spatial
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
New Journal of Physics 09/2014; 16(12). · 3.67 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 two-atom 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 two-photon 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 two-dimensional 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). · 3.67 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 two-time 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 non-classicality of the
motion of a cesium atom, which propagates on a space-time discrete lattice.
Making use of ideal negative measurements to test the atom's motion, we obtain
a 6 sigma experimental violation of the Leggett-Garg 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.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate cooling of the motion of a single neutral atom confined by a dipole trap inside a high-finesse optical resonator. Cooling of the vibrational motion results from electromagnetically induced transparency (EIT)-like interference in an atomic Λ-type 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.
[Show abstract][Hide abstract] ABSTRACT: We analyze the quantum jumps of an atom interacting with a cavity field. The
strong atom-field 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). · 2.99 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. Stress-induced 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. · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: form only given. We observe the quantized motion of single atoms strongly coupled to a high-finesse optical cavity and investigate dynamics of cavity-assisted atom cooling. Single caesium atoms are trapped in a blue-detuned standing-wave 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 photon-counting heterodyne spectroscopy .Measured heterodyne spectra of single atoms in the cavity are shown in Fig. 1 for two different lock-laser 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 cavity-atom detuning, and lock-laser 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 . In addition, this technique can also be used to analyze the cooling dynamics of a cavity-assisted 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 discrete-time 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.
[Show abstract][Hide abstract] ABSTRACT: Spin-dependent 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 widely-employed Raman sideband coupling method. We provide
a simplified model for sideband cooling in a spin-dependent potential, and we
discuss it in terms of the generalized Lamb-Dicke 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). · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on controlled doping of an ultracold Rb gas with single neutral Cs impurity atoms. Elastic two-body 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 s-wave collisions. Inelastic interactions are restricted to a single three-body recombination channel in a highly controlled and pure setting, which allows us to determine the Rb-Rb-Cs three-body loss rate with unprecedented precision. Our results pave the way for a coherently interacting hybrid system of individually controllable impurities in a quantum many-body system.
[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.
[Show abstract][Hide abstract] ABSTRACT: We experimentally demonstrate real-time feedback control of the joint spin-state 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 two-atom 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.
[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 quasi-particle effects in quantum many-body systems.
New Journal of Physics 07/2012; 14(7). · 3.67 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 state-dependent 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. Time-resolved 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.
[Show abstract][Hide abstract] 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 spin-dependent 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 space-time analogue of the well-known spin echo technique, yielding insight into decoherence mechanisms. We also demonstrate mesoscopic delocalization of single atoms with a separation-to-localization ratio exceeding 500; this result suggests their utilization beyond quantum logic applications as nano-resolution 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):9770-4. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ultracold gases doped with impurity atoms are promising hybrid systems
that pave the way for investigation of a series of novel and interesting
scenarios: They can be employed for studying polaron physics, the
impurity atoms can act as coherent probes for the many-body system, and
the coherent cooling of neutral atoms containing quantum information has
been proposed. Here, we immerse single and few Cs atoms into an
ultracold Rb cloud. Elastic collisions lead to rapid thermalization of
both sub-systems, while inelastic collisions lead to a loss of Cs from
the trap. When thermalized, the impurity atom is localized inside the Rb
gas. The ultracold Rb gas remains effectively unaffected by the
interaction with the Cs impurity atoms. The poster will present details
of the experimental setup, sequence and data analysis needed to extract
the interspecies scattering length and three-body loss coefficient from
the thermalization dynamics and loss rates measured.
[Show abstract][Hide abstract] ABSTRACT: We apply the Collins-Huygens integral to analytically describe propagation of a doughnut beam generated by a spiral phase plate. Measured beam profiles in free space and through an ABCD-lens system illustrate excellent agreement with theory. Applications range from the creation of optical beams with angular momentum to microscopy to trapping neutral atoms. The method extends to other beam shaping components, too.
[Show abstract][Hide abstract] ABSTRACT: We report on the controlled insertion of individual Cs atoms into an
ultracold Rb gas at about 400 nK. This requires to combine the techniques
necessary for cooling, trapping and manipulating single laser cooled atoms
around the Doppler temperature with an experiment to produce ultracold
degenerate quantum gases. In our approach, both systems are prepared in
separated traps and then combined. Our results pave the way for coherent
interaction between a quantum gas and a single or few neutral atoms of another
[Show abstract][Hide abstract] ABSTRACT: Coherent control and delocalization of individual atoms is a pivotal
challenge in quantum technologies. As a new step on this road, we
present an individual atom interferometer that is capable of splitting a
trapped Cs atom by up to 10 μm, allowing us to measure potential
gradients on the microscale. The atom is confined in a 1D optical
lattice, which is capable of performing discrete state-dependent shifts
to split the atom by the desired number of sites. We establish a high
degree of control, as the initial atom position, vibrational state and
spin state can all be prepared with above 95% fidelity. To unravel
decoherence effects and phase influences, we have explored several basic
interferometer geometries, among other things demonstrating a positional
spin echo to cancel background effects. As a test case, an inertial
force has been applied and successfully measured using the atomic phase.
This will offer us a new tool to investigate the interaction between two
atoms in a controlled model system.