42 research outputs found
Observation of Phase Defects in Quasi-2D Bose-Einstein Condensates
We have observed phase defects in quasi-2D Bose-Einstein condensates close to
the condensation temperature. Either a single or several equally spaced
condensates are produced by selectively evaporating the sites of a 1D optical
lattice. When several clouds are released from the lattice and allowed to
overlap, dislocation lines in the interference patterns reveal nontrivial phase
defects
The atomic Bose gas in Flatland
We describe a recent experiment performed with rubidium atoms (Rb),
aiming at studying the coherence properties of a two-dimensional gas of bosonic
particles at low temperature. We have observed in particular a
Berezinskii--Kosterlitz--Thouless (BKT) type crossover in the system, using a
matter wave heterodyning technique. At low temperatures, the gas is
quasi-coherent on the length scale set by the system size. As the temperature
is increased, the loss of long-range coherence coincides with the onset of the
proliferation of free vortices, in agreement with the microscopic BKT theory.Comment: To appear in "ATOMIC PHYSICS 20" Proceedings of the XX International
Conference on Atomic Physics (ICAP
Propagation front of correlations in an interacting Bose gas
We analyze the quench dynamics of a one-dimensional bosonic Mott insulator
and focus on the time evolution of density correlations. For these we identify
a pronounced propagation front, the velocity of which, once correctly
extrapolated at large distances, can serve as a quantitative characteristic of
the many-body Hamiltonian. In particular, the velocity allows the weakly
interacting regime, which is qualitatively well described by free bosons, to be
distinguished from the strongly interacting one, in which pairs of distinct
quasiparticles dominate the dynamics. In order to describe the latter case
analytically, we introduce a general approximation to solve the Bose-Hubbard
Hamiltonian based on the Jordan-Wigner fermionization of auxiliary particles.
This approach can also be used to determine the ground-state properties. As a
complement to the fermionization approach, we derive explicitly the
time-dependent many-body state in the noninteracting limit and compare our
results to numerical simulations in the whole range of interactions of the
Bose-Hubbard model.Comment: 16 pages, 7 figure
An atomic Hong-Ou-Mandel experiment
The celebrated Hong, Ou and Mandel (HOM) effect is one of the simplest
illustrations of two-particle interference, and is unique to the quantum realm.
In the original experiment, two photons arriving simultaneously in the input
channels of a beam-splitter were observed to always emerge together in one of
the output channels. Here, we report on the realisation of a closely analogous
experiment with atoms instead of photons. This opens the prospect of testing
Bell's inequalities involving mechanical observables of massive particles, such
as momentum, using methods inspired by quantum optics, with an eye on theories
of the quantum-to-classical transition. Our work also demonstrates a new way to
produce and benchmark twin-atom pairs that may be of interest for quantum
information processing and quantum simulation
Practical scheme for a light-induced gauge field in an atomic Bose gas
We propose a scheme to generate an Abelian gauge field in an atomic gas using
two crossed laser beams. If the internal atomic state follows adiabatically the
eigenstates of the atom-laser interaction, Berry's phase gives rise to a vector
potential that can nucleate vortices in a Bose gas. The present scheme operates
even for a large detuning with respect to the atomic resonance, making it
applicable to alkali-metal atoms without significant heating due to spontaneous
emission. We test the validity of the adiabatic approximation by integrating
the set of coupled Gross-Pitaevskii equations associated with the various
internal atomic states, and we show that the steady state of the interacting
gas indeed exhibits a vortex lattice, as expected from the adiabatic gauge
field.Comment: 4 pages, 3 figure
Coherent light scattering from a two-dimensional Mott insulator
We experimentally demonstrate coherent light scattering from an atomic Mott
insulator in a two-dimensional lattice. The far-field diffraction pattern of
small clouds of a few hundred atoms was imaged while simultaneously laser
cooling the atoms with the probe beams. We describe the position of the
diffraction peaks and the scaling of the peak parameters by a simple analytic
model. In contrast to Bragg scattering, scattering from a single plane yields
diffraction peaks for any incidence angle. We demonstrate the feasibility of
detecting spin correlations via light scattering by artificially creating a
one-dimensional antiferromagnetic order as a density wave and observing the
appearance of additional diffraction peaks.Comment: 4 pages, 4 figure
The 2nd order coherence of superradiance from a Bose--Einstein condensate
We have measured the 2-particle correlation function of atoms from a
Bose--Einstein condensate participating in a superradiance process, which
directly reflects the 2nd order coherence of the emitted light. We compare this
correlation function with that of atoms undergoing stimulated emission. Whereas
the stimulated process produces correlations resembling those of a coherent
state, we find that superradiance, even in the presence of strong gain, shows a
correlation function close to that of a thermal state, just as for ordinary
spontaneous emission
Single-site- and single-atom-resolved measurement of correlation functions
Correlation functions play an important role for the theoretical and
experimental characterization of many-body systems. In solid-state systems,
they are usually determined through scattering experiments whereas in
cold-gases systems, time-of-flight and in-situ absorption imaging are the
standard observation techniques. However, none of these methods allow the
in-situ detection of spatially resolved correlation functions at the
single-particle level. Here we give a more detailed account of recent advances
in the detection of correlation functions using in-situ fluorescence imaging of
ultracold bosonic atoms in an optical lattice. This method yields single-site
and single-atom-resolved images of the lattice gas in a single experimental
run, thus gaining direct access to fluctuations in the many-body system. As a
consequence, the detection of correlation functions between an arbitrary set of
lattice sites is possible. This enables not only the detection of two-site
correlation functions but also the evaluation of non-local correlations, which
originate from an extended region of the system and are used for the
characterization of quantum phases that do not possess (quasi-)long-range order
in the traditional sense.Comment: extended version of M. Endres et al., Science 334, 200-203 (2011)
[arXiv:1108.3317
Light-cone-like spreading of correlations in a quantum many-body system
How fast can correlations spread in a quantum many-body system? Based on the
seminal work by Lieb and Robinson, it has recently been shown that several
interacting many-body systems exhibit an effective light cone that bounds the
propagation speed of correlations. The existence of such a "speed of light" has
profound implications for condensed matter physics and quantum information, but
has never been observed experimentally. Here we report on the time-resolved
detection of propagating correlations in an interacting quantum many-body
system. By quenching a one-dimensional quantum gas in an optical lattice, we
reveal how quasiparticle pairs transport correlations with a finite velocity
across the system, resulting in an effective light cone for the quantum
dynamics. Our results open important perspectives for understanding relaxation
of closed quantum systems far from equilibrium as well as for engineering
efficient quantum channels necessary for fast quantum computations.Comment: 7 pages, 5 figures, 2 table
Single-Atom Resolved Fluorescence Imaging of an Atomic Mott Insulator
The reliable detection of single quantum particles has revolutionized the
field of quantum optics and quantum information processing. For several years,
researchers have aspired to extend such detection possibilities to larger scale
strongly correlated quantum systems, in order to record in-situ images of a
quantum fluid in which each underlying quantum particle is detected. Here we
report on fluorescence imaging of strongly interacting bosonic Mott insulators
in an optical lattice with single-atom and single-site resolution. From our
images, we fully reconstruct the atom distribution on the lattice and identify
individual excitations with high fidelity. A comparison of the radial density
and variance distributions with theory provides a precise in-situ temperature
and entropy measurement from single images. We observe Mott-insulating plateaus
with near zero entropy and clearly resolve the high entropy rings separating
them although their width is of the order of only a single lattice site.
Furthermore, we show how a Mott insulator melts for increasing temperatures due
to a proliferation of local defects. Our experiments open a new avenue for the
manipulation and analysis of strongly interacting quantum gases on a lattice,
as well as for quantum information processing with ultracold atoms. Using the
high spatial resolution, it is now possible to directly address individual
lattice sites. One could, e.g., introduce local perturbations or access regions
of high entropy, a crucial requirement for the implementation of novel cooling
schemes for atoms on a lattice