51 research outputs found
Collective modes across the soliton-droplet crossover in binary Bose mixtures
We study the collective modes of a binary Bose mixture across the soliton to
droplet crossover in a quasi one dimensional waveguide with a beyond-mean-field
equation of state and a variational Gaussian ansatz for the scalar bosonic
field of the corresponding effective action. We observe a sharp difference in
the collective modes in the two regimes. Within the soliton regime modes vary
smoothly upon the variation of particle number or interaction strength. On the
droplet side collective modes are inhibited by the emission of particles. This
mechanism turns out to be dominant for a wide range of particle numbers and
interactions. In a small window of particle number range and for intermediate
interactions we find that monopole frequency is likely to be observed. In the
last part we focus on the spin-dipole modes for the case of equal intraspecies
interactions and equal equilibrium particle numbers in the presence of a weak
longitudinal confinement. We found that such modes might be unobservable in the
real-time dynamics close to the equilibrium as their frequency is higher than
the particle emission spectrum by at least one order of magnitude in the
droplet phase. Our results are relevant for experiments with two-component BECs
for which we provide realistic parameters.Comment: Accepted for Publication in PR
Thermalization of the Lipkin-Meshkov-Glick model in blackbody radiation
In a recent work, we have derived simple Lindblad-based equations for the
thermalization of systems in contact with a thermal reservoir. Here, we apply
these equations to the Lipkin-Meshkov-Glick model (LMG) in contact with a
blackbody radiation and analyze the dipole matrix elements involved in the
thermalization process. We find that the thermalization can be complete only if
the density is sufficiently high, while, in the limit of low density, the
system thermalizes partially, namely, within the Hilbert subspaces where the
total spin has a fixed value. In this regime, and in the isotropic case, we
evaluate the characteristic thermalization time analytically, and show that it
diverges with the system size in correspondence of the critical points and
inside the ferromagnetic region. Quite interestingly, at zero temperature the
thermalization time diverges only quadratically with the system size, whereas
quantum adiabatic algorithms, aimed at finding the ground state of same system,
imply a cubic divergence of the required adiabatic time.Comment: 13 pages, 3 figure
Thermal and Quantum Fluctuation Effects in Quasiperiodic Systems in External Potentials
We analyze the many-body phases of an ensemble of particles interacting via a
Lifshitz--Petrich--Gaussian pair potential in a harmonic confinement. We focus
on specific parameter regimes where we expect decagonal quasiperiodic cluster
arrangements. Performing classical Monte Carlo as well as path integral quantum
Monte Carlo methods, we numerically simulate systems of a few thousand
particles including thermal and quantum fluctuations. Our findings indicate
that the competition between the intrinsic length scale of the harmonic
oscillator and the wavelengths associated to the minima of the pair potential
generically lead to a destruction of the quasicrystalline pattern. Extensions
of this work are also discussed.Comment: 8 pages, 4 figure
Superfluid filaments of dipolar bosons in free space
We systematically investigate the zero temperature phase diagram of bosons
interacting via dipolar interactions in three dimensions in free space via path
integral Monte Carlo simulations with few hundreds of particles and periodic
boundary conditions based on the worm algorithm. Upon increasing the strength
of the dipolar interaction and at sufficiently high densities we find a wide
region where filaments are stabilized along the direction of the external
field. Most interestingly by computing the superfluid fraction we conclude that
superfluidity is anisotropic and is greatly suppressed along the orthogonal
plane. Finally we perform simulations at finite temperature confirming the
stability of filaments against thermal fluctuations and provide an estimate of
the superfluid fraction in the weak coupling limit in the framework of the
Landau two-fluid model.Comment: 5 pages, 4 figures (supplemental materials: 5 pages, 6 figures).
Revised version, to appear in Physical Review Letter
Equation of state and self-bound droplet in Rabi-coupled Bose mixtures
Laser induced transitions between internal states of atoms have been playing
a fundamental role to manipulate atomic clouds for many decades. In absence of
interactions each atom behaves independently and their coherent quantum
dynamics is described by the Rabi model. Since the experimental observation of
Bose condensation in dilute gases, static and dynamical properties of
multicomponent quantum gases have been extensively investigated. Moreover, at
very low temperatures quantum fluctuations crucially affect the equation of
state of many-body systems. Here we study the effects of quantum fluctuations
on a Rabi-coupled two-component Bose gas of interacting alkali atoms. The
divergent zero-point energy of gapless and gapped elementary excitations of the
uniform system is properly regularized obtaining a meaningful analytical
expression for the beyond-mean-field equation of state. In the case of
attractive inter-particle interaction we show that the quantum pressure arising
from Gaussian fluctuations can prevent the collapse of the mixture with the
creation of a self-bound droplet. We characterize the droplet phase and
discover an energetic instability above a critical Rabi frequency provoking the
evaporation of the droplet. Finally, we suggest an experiment to observe such
quantum droplets using Rabi-coupled internal states of K atoms.Comment: to be published in Scientific Report
Microscopy of a scalable superatom
Strong interactions can amplify quantum effects such that they become
important on macroscopic scales. Controlling these coherently on a single
particle level is essential for the tailored preparation of strongly correlated
quantum systems and opens up new prospects for quantum technologies. Rydberg
atoms offer such strong interactions which lead to extreme nonlinearities in
laser coupled atomic ensembles. As a result, multiple excitation of a
Micrometer sized cloud can be blocked while the light-matter coupling becomes
collectively enhanced. The resulting two-level system, often called
"superatom", is a valuable resource for quantum information, providing a
collective Qubit. Here we report on the preparation of two orders of magnitude
scalable superatoms utilizing the large interaction strength provided by
Rydberg atoms combined with precise control of an ensemble of ultracold atoms
in an optical lattice. The latter is achieved with sub shot noise precision by
local manipulation of a two-dimensional Mott insulator. We microscopically
confirm the superatom picture by in-situ detection of the Rydberg excitations
and observe the characteristic square root scaling of the optical coupling with
the number of atoms. Furthermore, we verify the presence of entanglement in the
prepared states and demonstrate the coherent manipulation of the superatom.
Finally, we investigate the breakdown of the superatom picture when two Rydberg
excitations are present in the system, which leads to dephasing and a loss of
coherence.Comment: 7 pages, 5 figure
- …