112 research outputs found
Nondispersive two-electron wave packets in driven helium
We provide a detailed quantum treatment of the spectral characteristics and
of the dynamics of nondispersive two-electron wave packets along the
periodically driven, collinear frozen planet configuration of helium. These
highly correlated, long-lived wave packets arise as a quantum manifestation of
regular islands in a mixed classical phase space, which are induced by
nonlinear resonances between the external driving and the unperturbed dynamics
of the frozen-planet configuration. Particular emphasis is given to the
dependence of the ionization rates of the wave packet states on the driving
field parameters and on the quantum mechanical phase space resolution, preceded
by a comparison of 1D and 3D life times of the unperturbed frozen planet.
Furthermore, we study the effect of a superimposed static electric field
component, which, on the grounds of classical considerations, is expected to
stabilize the real 3D dynamics against large (and possibly ionizing) deviations
from collinearity.Comment: 31 pages, 18 figures, submitted to European Physical Journal
Elastic and inelastic transmission in guided atom lasers: a truncated Wigner approach
We study the transport properties of an ultracold gas of Bose-Einstein
condensate that is coupled from a magnetic trap into a one-dimensional
waveguide. Our theoretical approach to tackle this problem is based on the
truncated Wigner method for which we assume the system to consist of two
semi-infinite non-interacting leads and a finite interacting scattering region
with two constrictions modelling an atomic quantum dot. The transmission is
computed in the steady-state regime and we find a good agreement between
truncated Wigner and Matrix-Product State calculations. We also identify clear
signatures of inelastic resonant scattering by analyzing the distribution of
energy in the transmitted atomic matter wave beam.Comment: 10 pages, 5 figures, submitted to Phys. Rev.
Effects of short-range interactions on transport through quantum point contacts: A numerical approach
We study electronic transport through a quantum point contact, where the
interaction between the electrons is approximated by a contact potential. Our
numerical approach is based on the non-equilibrium Green function technique
which is evaluated at Hartree-Fock level. We show that this approach allows us
to reproduce relevant features of the so-called "0.7 anomaly" observed in the
conductance at low temperatures, including the characteristic features in
recent shot noise measurements. This is consistent with a spin-splitting
interpretation of the process, and indicates that the "0.7 anomaly" should also
be observable in transport experiments with ultracold fermionic atoms.Comment: 12 pages, 10 figure
Waveguides for walking droplets
When gently placing a droplet onto a vertically vibrated bath, a drop can
bounce without coalescing. Upon increasing the forcing acceleration, the
droplet is propelled by the wave it generates and becomes a walker with a well
defined speed. We investigate the confinement of a walker in different
rectangular cavities, used as waveguides for the Faraday waves emitted by
successive droplet bounces. By studying the walker velocities, we discover that
1d confinement is optimal for narrow channels of width of . We also propose an analogy with waveguide models based on the
observation of the Faraday instability within the channels.Comment: 8 pages, 6 figure
NOON states with ultracold bosonic atoms via resonance- and chaos-assisted tunneling
We theoretically investigate the generation of microscopic atomic NOON
states, corresponding to the coherent |N,0> + |0,N> superposition with N ~ 5
particles, via collective tunneling of interacting ultracold bosonic atoms
within a symmetric double-well potential in the self-trapping regime. We show
that a periodic driving of the double well with suitably tuned amplitude and
frequency parameters allows one to substantially boost this tunneling process
without altering its collective character. The time scale to generate the NOON
superposition, which corresponds to half the tunneling time and would be
prohibitively large in the undriven double well for the considered atomic
populations, can thereby be drastically reduced, which renders the realization
of NOON states through this protocol experimentally feasible. Resonance- and
chaos-assisted tunneling are identified as key mechanisms in this context. A
quantitative semiclassical evaluation of their impact onto the collective
tunneling process allows one to determine the optimal choice for the driving
parameters in order to generate those NOON states as fast as possible.Comment: 10 pages, 6 figure
Many-Body Spin Echo
We predict a universal echo phenomenon present in the time evolution of
many-body states of interacting quantum systems described by Fermi-Hubbard
models. It consists of the coherent revival of transition probabilities echoing
a sudden flip of the spins that, contrary to its single-particle (Hahn)
version, is not dephased by interactions or spin-orbit coupling. The many-body
spin echo signal has a universal shape independent of the interaction strength,
and an amplitude and sign depending only on combinatorial relations between the
number of particles and the number of applied spin flips. Our analytical
predictions, based on semiclassical interfering amplitudes in Fock space
associated with chaotic mean-field solutions, are tested against extensive
numerical simulations confirming that the coherent origin of the echo lies in
the existence of anti-unitary symmetries.Comment: 5 pages, 4 figure
Intensity distribution of non-linear scattering states
We investigate the interplay between coherent effects characteristic of the
propagation of linear waves, the non-linear effects due to interactions, and
the quantum manifestations of classical chaos due to geometrical confinement,
as they arise in the context of the transport of Bose-Einstein condensates. We
specifically show that, extending standard methods for non-interacting systems,
the body of the statistical distribution of intensities for scattering states
solving the Gross-Pitaevskii equation is very well described by a local
Gaussian ansatz with a position-dependent variance. We propose a semiclassical
approach based on interfering classical paths to fix the single parameter
describing the universal deviations from a global Gaussian distribution. Being
tail effects, rare events like rogue waves characteristic of non-linear field
equations do not affect our results.Comment: 18 pages, 7 figures, submitted to Proceedings MARIBOR 201
Nonlinear dynamical tunneling of optical whispering gallery modes in the presence of a Kerr nonlinearity
The effect of a Kerr nonlinearity on dynamical tunneling is studied, using
coupled whispering gallery modes in an optical microcavity. The model system
that we have chosen is the 'add-drop filter', which comprises an optical
microcavity and two waveguide coupled to the cavity. Due to the evanescent
field's scattering on the waveguide, the whispering gallery modes in the
microcavity form doublets, which manifest themselves as splittings in the
spectrum. As these doublets can be regarded as a spectral feature of dynamical
tunneling between two different dynamical states with a spatial overlap, the
effect of a Kerr nonlinearity on the doublets is numerically investigated in
the more general context of the relationship between cubic nonlinearity and
dynamical tunneling. Within the numerical realization of the model system, it
is observed that the doublets shows a bistable transition in its transmission
curve as the Kerr-nonlinearity in the cavity is increased. At the same time,
one rotational mode gets dominant over the other one in the transmission, since
the two states in the doublet have uneven linewidths. By using coupled mode
theory, the underlying mode dynamics of the phenomena is theoretically modelled
and clarified.Comment: 7 pages, 5 figure
Faraday instability and subthreshold Faraday waves: surface waves emitted by walkers
A walker is a fluid entity comprising a bouncing droplet coupled to the waves
that it generates at the surface of a vibrated bath. Thanks to this coupling,
walkers exhibit a series of wave-particle features formerly thought to be
exclusive to the quantum realm. In this paper, we derive a model of the Faraday
surface waves generated by an impact upon a vertically vibrated liquid surface.
We then particularise this theoretical framework to the case of forcing
slightly below the Faraday instability threshold. Among others, this theory
yields a rationale for the dependence of the wave amplitude to the phase of
impact, as well as the characteristic timescale and length scale of viscous
damping. The theory is validated with experiments of bead impact on a vibrated
bath. We finally discuss implications of these results for the analogy between
walkers and quantum particles
Influence of classical resonances on chaotic tunnelling
Dynamical tunnelling between symmetry-related stable modes is studied in the
periodically driven pendulum. We present strong evidence that the tunnelling
process is governed by nonlinear resonances that manifest within the regular
phase-space islands on which the stable modes are localized. By means of a
quantitative numerical study of the corresponding Floquet problem, we identify
the trace of such resonances not only in the level splittings between
near-degenerate quantum states, where they lead to prominent plateau
structures, but also in overlap matrix elements of the Floquet eigenstates,
which reveal characteristic sequences of avoided crossings in the Floquet
spectrum. The semiclassical theory of resonance-assisted tunnelling yields good
overall agreement with the quantum-tunnelling rates, and indicates that partial
barriers within the chaos might play a prominent role
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