332 research outputs found
Geometrically protected triple-point crossings in an optical lattice
We show how to realize topologically protected crossings of three energy
bands, integer-spin analogs of Weyl fermions, in three-dimensional optical
lattices. Our proposal only involves ultracold atom techniques that have
already been experimentally demonstrated and leads to isolated triple-point
crossings (TPCs) which are required to exist by a novel combination of lattice
symmetries. The symmetries also allow for a new type of topological object, the
type-II, or tilted, TPC. Our Rapid Communication shows that spin-1 Weyl points,
which have not yet been observed in the bandstructure of crystals, are within
reach of ultracold atom experiments.Comment: 5 pages, 2 figures + 3 pages, 3 figures supplemental material. Added
appendix on model symmetries, fixed typos and added references. This is the
final, published versio
Majorana Quasi-Particles Protected by Angular Momentum Conservation
We show how angular momentum conservation can stabilise a symmetry-protected
quasi-topological phase of matter supporting Majorana quasi-particles as edge
modes in one-dimensional cold atom gases. We investigate a number-conserving
four-species Hubbard model in the presence of spin-orbit coupling. The latter
reduces the global spin symmetry to an angular momentum parity symmetry, which
provides an extremely robust protection mechanism that does not rely on any
coupling to additional reservoirs. The emergence of Majorana edge modes is
elucidated using field theory techniques, and corroborated by
density-matrix-renormalization-group simulations. Our results pave the way
toward the observation of Majorana edge modes with alkaline-earth-like fermions
in optical lattices, where all basic ingredients for our recipe - spin-orbit
coupling and strong inter-orbital interactions - have been experimentally
realized over the last two years.Comment: 12 pages (6 + 6 supplementary material
Multi-band spectroscopy of inhomogeneous Mott-insulator states of ultracold bosons
In this work, we use inelastic scattering of light to study the response of
inhomogeneous Mott-insulator gases to external excitations. The experimental
setup and procedure to probe the atomic Mott states are presented in detail. We
discuss the link between the energy absorbed by the gases and accessible
experimental parameters as well as the linearity of the response to the
scattering of light. We investigate the excitations of the system in multiple
energy bands and a band-mapping technique allows us to identify band and
momentum of the excited atoms. In addition the momentum distribution in the
Mott states which is spread over the entire first Brillouin zone enables us to
reconstruct the dispersion relation in the high energy bands using a single
Bragg excitation with a fixed momentum transfer.Comment: 19 pages, 7 figure
Cold atoms: A field enabled by light
International audienceBesides being a source of energy, light can also cool gases of atoms down to the lowest temperatures ever measured, where atomic motion almost stops. The research field of cold atoms has emerged as a multidisciplinary one, highly relevant, e.g., for precision measurements, quantum gases, simulations of many-body physics, and atom optics. In this focus article, we present the field as seen in 2015, and emphasise the fundamental role in its development that has been played by mastering light. Introduction Cold atom physics has become a mature field of research , but this maturity has been achieved relatively fast. Ideas about mechanical action of light have existed for a long time, and there were preliminary experiments in the late '60s and in the '70s involving cooling and trapping of atoms with light. However, the significant experimental developments gathered momentum in the mid '80s. There are many conceptual and technological ingredients involved in this research. However, one thing that runs through everything at every stage is light. Without the ability to control and detect light with high precision, the field of cold atoms would not exist. This article about cold atoms, inspired by the International Year of Light 2015, is therefore dedicated to light
Localization of cold atoms in state-dependent optical lattices via a Rabi pulse
We propose a novel realization of Anderson localization in non-equilibrium
states of ultracold atoms trapped in state-dependent optical lattices. The
disorder potential leading to localization is generated with a Rabi pulse
transfering a fraction of the atoms into a different internal state for which
tunneling between lattice sites is suppressed. Atoms with zero tunneling create
a quantum superposition of different random potentials, localizing the mobile
atoms. We investigate the dynamics of the mobile atoms after the Rabi pulse for
non-interacting and weakly interacting bosons, and we show that the evolved
wavefunction attains a quasi-stationary profile with exponentially decaying
tails, characteristic of Anderson localization. The localization length is seen
to increase with increasing disorder and interaction strength, oppositely to
what is expected for equilibrium localization.Comment: 4 pages, 4 figure
Momentum-resolved study of an array of 1D strongly phase-fluctuating Bose gases
We investigate the coherence properties of an array of one-dimensional Bose
gases with short-scale phase fluctuations. The momentum distribution is
measured using Bragg spectroscopy and an effective coherence length of the
whole ensemble is defined. In addition, we propose and demonstrate that
time-of-flight absorption imaging can be used as a simple probe to directly
measure the coherence-length of 1D gases in the regime where phase-fluctuations
are strong. This method is suitable for future studies such as investigating
the effect of disorder on the phase coherence.Comment: 4 pages, 4 figure
Matter-wave localization in a random potential
By numerical and variational solution of the Gross-Pitaevskii equation, we
studied the localization of a noninteracting and weakly-interacting
Bose-Einstein condensate (BEC) in a disordered cold atom lattice and a speckle
potential. In the case of a single BEC fragment, the variational analysis
produced good results. For a weakly disordered potential, the localized BECs
are found to have an exponential tail as in weak Anderson localization. We also
investigated the expansion of a noninteracting BEC in these potential. We find
that the BEC will be locked in an appropriate localized state after an initial
expansion and will execute breathing oscillation around a mean shape when a BEC
at equilibrium in a harmonic trap is suddenly released into a disorder
potential
Localization in momentum space of ultracold atoms in incommensurate lattices
We characterize the disorder induced localization in momentum space for
ultracold atoms in one-dimensional incommensurate lattices, according to the
dual Aubry-Andr\'e model. For low disorder the system is localized in momentum
space, and the momentum distribution exhibits time-periodic oscillations of the
relative intensity of its components. The behavior of these oscillations is
explained by means of a simple three-mode approximation. We predict their
frequency and visibility by using typical parameters of feasible experiments.
Above the transition the system diffuses in momentum space, and the
oscillations vanish when averaged over different realizations, offering a clear
signature of the transition
A strongly interacting gas of two-electron fermions at an orbital Feshbach resonance
We report on the experimental observation of a strongly interacting gas of
ultracold two-electron fermions with orbital degree of freedom and magnetically
tunable interactions. This realization has been enabled by the demonstration of
a novel kind of Feshbach resonance occurring in the scattering of two 173Yb
atoms in different nuclear and electronic states. The strongly interacting
regime at resonance is evidenced by the observation of anisotropic hydrodynamic
expansion of the two-orbital Fermi gas. These results pave the way towards the
realization of new quantum states of matter with strongly correlated fermions
with orbital degree of freedom.Comment: 5 pages, 4 figure
Coherent Manipulation of Orbital Feshbach Molecules of Two-Electron Atoms
Ultracold molecules have experienced increasing attention in recent years.
Compared to ultracold atoms, they possess several unique properties that make
them perfect candidates for the implementation of new quantum-technological
applications in several fields, from quantum simulation to quantum sensing and
metrology. In particular, ultracold molecules of two-electron atoms (such as
strontium or ytterbium) also inherit the peculiar properties of these atomic
species, above all the possibility to access metastable electronic states via
direct excitation on optical clock transitions with ultimate sensitivity and
accuracy. In this paper we report on the production and coherent manipulation
of molecular bound states of two fermionic Yb atoms in different
electronic (orbital) states S and P in proximity of a
scattering resonance involving atoms in different spin and electronic states,
called orbital Feshbach resonance. We demonstrate that orbital molecules can be
coherently photoassociated starting from a gas of ground-state atoms in a
three-dimensional optical lattices by observing several photoassociation and
photodissociation cycles. We also show the possibility to coherently control
the molecular internal state by using Raman-assisted transfer to swap the
nuclear spin of one of the atoms forming the molecule, thus demonstrating a
powerful manipulation and detection tool of these molecular bound states.
Finally, by exploiting this peculiar detection technique we provide first
information on the lifetime of the molecular states in a many-body setting,
paving the way towards future investigations of strongly interacting Fermi
gases in a still unexplored regime.Comment: 11 pages, 8 figure
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