146 research outputs found
Observation of many-body localization in a one-dimensional system with single-particle mobility edge
We experimentally study many-body localization (MBL) with ultracold atoms in
a weak one-dimensional quasiperiodic potential, which in the noninteracting
limit exhibits an intermediate phase that is characterized by a mobility edge.
We measure the time evolution of an initial charge density wave after a quench
and analyze the corresponding relaxation exponents. We find clear signatures of
MBL, when the corresponding noninteracting model is deep in the localized
phase. We also critically compare and contrast our results with those from a
tight-binding Aubry-Andr\'{e} model, which does not exhibit a single-particle
intermediate phase, in order to identify signatures of a potential many-body
intermediate phase
Observing non-ergodicity due to kinetic constraints in tilted Fermi-Hubbard chains
The thermalization of isolated quantum many-body systems is deeply related to
fundamental questions of quantum information theory. While integrable or
many-body localized systems display non-ergodic behavior due to extensively
many conserved quantities, recent theoretical studies have identified a rich
variety of more exotic phenomena in between these two extreme limits. The
tilted one-dimensional Fermi-Hubbard model, which is readily accessible in
experiments with ultracold atoms, emerged as an intriguing playground to study
non-ergodic behavior in a clean disorder-free system. While non-ergodic
behavior was established theoretically in certain limiting cases, there is no
complete understanding of the complex thermalization properties of this model.
In this work, we experimentally study the relaxation of an initial
charge-density wave and find a remarkably long-lived initial-state memory over
a wide range of parameters. Our observations are well reproduced by numerical
simulations of a clean system. Using analytical calculations we further provide
a detailed microscopic understanding of this behavior, which can be attributed
to emergent kinetic constraints.Comment: accepted in Nature Communication
Non-Equilibrium Mass Transport in the 1D Fermi-Hubbard Model
We experimentally and numerically investigate the sudden expansion of
fermions in a homogeneous one-dimensional optical lattice. For initial states
with an appreciable amount of doublons, we observe a dynamical phase separation
between rapidly expanding singlons and slow doublons remaining in the trap
center, realizing the key aspect of fermionic quantum distillation in the
strongly-interacting limit. For initial states without doublons, we find a
reduced interaction dependence of the asymptotic expansion speed compared to
bosons, which is explained by the interaction energy produced in the quench
Nonequilibrium Mass Transport in the 1D Fermi-Hubbard Model.
We experimentally and numerically investigate the sudden expansion of fermions in a homogeneous one-dimensional optical lattice. For initial states with an appreciable amount of doublons, we observe a dynamical phase separation between rapidly expanding singlons and slow doublons remaining in the trap center, realizing the key aspect of fermionic quantum distillation in the strongly interacting limit. For initial states without doublons, we find a reduced interaction dependence of the asymptotic expansion speed compared to bosons, which is explained by the interaction energy produced in the quench
Optical lattice quantum simulator for QED in strong external fields: spontaneous pair creation and the Sauter-Schwinger effect
Spontaneous creation of electron-positron pairs out of the vacuum due to a
strong electric field is a spectacular manifestation of the relativistic
energy-momentum relation for the Dirac fermions. This fundamental prediction of
Quantum Electrodynamics (QED) has not yet been confirmed experimentally as the
generation of a sufficiently strong electric field extending over a large
enough space-time volume still presents a challenge. Surprisingly, distant
areas of physics may help us to circumvent this difficulty. In condensed matter
and solid state physics (areas commonly considered as low energy physics), one
usually deals with quasi-particles instead of real electrons and positrons.
Since their mass gap can often be freely tuned, it is much easier to create
these light quasi-particles by an analogue of the Sauter-Schwinger effect. This
motivates our proposal of a quantum simulator in which excitations of
ultra-cold atoms moving in a bichromatic optical lattice represent particles
and antiparticles (holes) satisfying a discretized version of the Dirac
equation together with fermionic anti-commutation relations. Using the language
of second quantization, we are able to construct an analogue of the spontaneous
pair creation which can be realized in an (almost) table-top experiment.Comment: 21 pages, 10 figure
Superconductivity in a Topological Lattice Model with Strong Repulsion
The highly tunable nature of synthetic quantum materials -- both in the
solid-state and cold atom contexts -- invites examining which microscopic
ingredients aid in the realization of correlated phases of matter such as
superconductors. Recent experimental advances in moir\'e materials suggest that
unifying the features of the Fermi-Hubbard model and quantum Hall systems
creates a fertile ground for the emergence of such phases. Here, we introduce a
minimal 2D lattice model that incorporates exactly these features:
time-reversal symmetry, band topology, and strong repulsive interactions. By
using infinite cylinder density matrix renormalization group methods (cylinder
iDMRG), we investigate the ground state phase diagram of this model. We find
that it hosts an interaction-induced quantum spin Hall (QSH) insulator and
demonstrate that weakly hole-doping this state gives rise to a superconductor
at a finite circumference, with indications that this behavior persists on
larger cylinders. At the aforementioned circumference, the superconducting
phase is surprisingly robust to perturbations including additional repulsive
interactions in the pairing channel. By developing a technique to probe the
superconducting gap function in iDMRG, we phenomenologically characterize the
superconductor. Namely, we demonstrate that it is formed from the weak pairing
of holes atop the QSH insulator. Furthermore, we determine the pairing symmetry
of the superconductor, finding it to be -wave -- reminiscent of the
unconventional superconductivity reported in experiments on twisted bilayer
graphene (TBG). Motivated by this, we elucidate structural similarities and
differences between our model and those of TBG in its chiral limit. Finally, to
provide a more direct experimental realization, we detail an implementation of
our Hamiltonian in a system of cold fermionic alkaline-earth atoms in an
optical lattice.Comment: 27 pages (with 8 figures) + 35 pages supplementary (with 14 figures
Quantum transport in ultracold atoms
Ultracold atoms confined by engineered magnetic or optical potentials are
ideal systems for studying phenomena otherwise difficult to realize or probe in
the solid state because their atomic interaction strength, number of species,
density, and geometry can be independently controlled. This review focuses on
quantum transport phenomena in atomic gases that mirror and oftentimes either
better elucidate or show fundamental differences with those observed in
mesoscopic and nanoscopic systems. We discuss significant progress in
performing transport experiments in atomic gases, contrast similarities and
differences between transport in cold atoms and in condensed matter systems,
and survey inspiring theoretical predictions that are difficult to verify in
conventional setups. These results further demonstrate the versatility offered
by atomic systems in the study of nonequilibrium phenomena and their promise
for novel applications.Comment: 24 pages, 7 figures. A revie
Identifying topological edge states in 2D optical lattices using light scattering
We recently proposed in a Letter [Physical Review Letters 108 255303] a novel
scheme to detect topological edge states in an optical lattice, based on a
generalization of Bragg spectroscopy. The scope of the present article is to
provide a more detailed and pedagogical description of the system - the
Hofstadter optical lattice - and probing method. We first show the existence of
topological edge states, in an ultra-cold gas trapped in a 2D optical lattice
and subjected to a synthetic magnetic field. The remarkable robustness of the
edge states is verified for a variety of external confining potentials. Then,
we describe a specific laser probe, made from two lasers in Laguerre-Gaussian
modes, which captures unambiguous signatures of these edge states. In
particular, the resulting Bragg spectra provide the dispersion relation of the
edge states, establishing their chiral nature. In order to make the Bragg
signal experimentally detectable, we introduce a "shelving method", which
simultaneously transfers angular momentum and changes the internal atomic
state. This scheme allows to directly visualize the selected edge states on a
dark background, offering an instructive view on topological insulating phases,
not accessible in solid-state experiments.Comment: 17 pages, 10 figures. Revised and extended version, to appear in EJP
Special Topic for the special issue on "Novel Quantum Phases and Mesoscopic
Physics in Quantum Gases". Extended version of arXiv:1203.124
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