44 research outputs found
From Few to Many: Observing the Formation of a Fermi Sea One Atom at a Time
Knowing when a physical system has reached sufficient size for its
macroscopic properties to be well described by many-body theory is difficult.
We investigate the crossover from few to many-body physics by studying quasi
one-dimensional systems of ultracold atoms consisting of a single impurity
interacting with an increasing number of identical fermions. We measure the
interaction energy of such a system as a function of the number of majority
atoms for different strengths of the interparticle interaction. As we increase
the number of majority atoms one by one we observe the fast convergence of the
normalized interaction energy towards a many-body limit calculated for a single
impurity immersed in a Fermi sea of majority particles.Comment: 9 pages, 5 figure
Quantum Speed Limit and Optimal Control of Many-Boson Dynamics
We extend the concept of quantum speed limit -- the minimal time needed to
perform a driven evolution -- to complex interacting many-body systems. We
investigate a prototypical many-body system, a bosonic Josephson junction, at
increasing levels of complexity: (a) within the two-mode approximation
{corresponding to} a nonlinear two-level system, (b) at the mean-field level by
solving the nonlinear Gross-Pitaevskii equation in a double well potential, and
(c) at an exact many-body level by solving the time-dependent many-body
Schr\"odinger equation. We propose a control protocol to transfer atoms from
the ground state of a well to the ground state of the neighbouring well.
Furthermore, we show that the detrimental effects of the inter-particle
repulsion can be eliminated by means of a compensating control pulse, yielding,
quite surprisingly, an enhancement of the transfer speed because of the
particle interaction -- in contrast to the self-trapping scenario. Finally, we
perform numerical optimisations of both the nonlinear and the (exact) many-body
quantum dynamics in order to further enhance the transfer efficiency close to
the quantum speed limit.Comment: 5 pages, 3 figures, and supplemental material (4 pages 1 figure
Scattering off an oscillating target: Basic mechanisms and their impact on cross sections
We investigate classical scattering off a harmonically oscillating target in
two spatial dimensions. The shape of the scatterer is assumed to have a
boundary which is locally convex at any point and does not support the presence
of any periodic orbits in the corresponding dynamics. As a simple example we
consider the scattering of a beam of non-interacting particles off a circular
hard scatterer. The performed analysis is focused on experimentally accessible
quantities, characterizing the system, like the differential cross sections in
the outgoing angle and velocity. Despite the absence of periodic orbits and
their manifolds in the dynamics, we show that the cross sections acquire rich
and multiple structure when the velocity of the particles in the beam becomes
of the same order of magnitude as the maximum velocity of the oscillating
target. The underlying dynamical pattern is uniquely determined by the phase of
the first collision between the beam particles and the scatterer and possesses
a universal profile, dictated by the manifolds of the parabolic orbits, which
can be understood both qualitatively as well as quantitatively in terms of
scattering off a hard wall. We discuss also the inverse problem concerning the
possibility to extract properties of the oscillating target from the
differential cross sections.Comment: 18 page
Bose-Hubbard model with occupation dependent parameters
We study the ground-state properties of ultracold bosons in an optical
lattice in the regime of strong interactions. The system is described by a
non-standard Bose-Hubbard model with both occupation-dependent tunneling and
on-site interaction. We find that for sufficiently strong coupling the system
features a phase-transition from a Mott insulator with one particle per site to
a superfluid of spatially extended particle pairs living on top of the Mott
background -- instead of the usual transition to a superfluid of single
particles/holes. Increasing the interaction further, a superfluid of particle
pairs localized on a single site (rather than being extended) on top of the
Mott background appears. This happens at the same interaction strength where
the Mott-insulator phase with 2 particles per site is destroyed completely by
particle-hole fluctuations for arbitrarily small tunneling. In another regime,
characterized by weak interaction, but high occupation numbers, we observe a
dynamical instability in the superfluid excitation spectrum. The new ground
state is a superfluid, forming a 2D slab, localized along one spatial direction
that is spontaneously chosen.Comment: 16 pages, 4 figure