3,262 research outputs found
Small rare gas clusters in soft X-ray pulses
We develop a microscopic model for the interaction of small rare gas clusters
with soft X-ray radiation. It is shown that, while the overall charging of the
clusters is rather low, unexpectedly high atomic charge states can arise due to
charge imbalances inside the cluster. The mechanism does not require unusually
high absorption rates, and the heating can be described by standard inverse
bremsstrahlung formulae.Comment: 4 pages, 4 figure
A Levinson theorem for scattering from a Bose-Einstein condensate
A relation between the number of bound collective excitations of an atomic
Bose-Einstein condensate and the phase shift of elastically scattered atoms is
derived. Within the Bogoliubov model of a weakly interacting Bose gas this
relation is exact and generalises Levinson's theorem. Specific features of the
Bogoliubov model such as complex-energy and continuum bound states are
discussed and a numerical example is given.Comment: 4 pages, 3 figure
Influence of electron-ion collisions on Coulomb crystallization of ultracold neutral plasmas
While ion heating by elastic electron-ion collisions may be neglected for a
description of the evolution of freely expanding ultracold neutral plasmas, the
situation is different in scenarios where the ions are laser-cooled during the
system evolution. We show that electron-ion collisions in laser-cooled plasmas
influence the ionic temperature, decreasing the degree of correlation
obtainable in such systems. However, taking into account the collisions
increases the ion temperature much less than what would be estimated based on
static plasma clouds neglecting the plasma expansion. The latter leads to both
adiabatic cooling of the ions as well as, more importantly, a rapid decrease of
the collisional heating rate
Switching Exciton Pulses Through Conical Intersections
Exciton pulses transport excitation and entanglement adiabatically through
Rydberg aggregates, assemblies of highly excited light atoms, which are set
into directed motion by resonant dipole-dipole interaction. Here, we
demonstrate the coherent splitting of such pulses as well as the spatial
segregation of electronic excitation and atomic motion. Both mechanisms exploit
local nonadiabatic effects at a conical intersection, turning them from a
decoherence source into an asset. The intersection provides a sensitive knob
controlling the propagation direction and coherence properties of exciton
pulses. The fundamental ideas discussed here have general implications for
excitons on a dynamic network.Comment: Letter with 4 pages and 4 figures. Supplemental material with 4 pages
and 4 figure
Relaxation to non-equilibrium in expanding ultracold neutral plasmas
We investigate the strongly correlated ion dynamics and the degree of
coupling achievable in the evolution of freely expanding ultracold neutral
plasmas. We demonstrate that the ionic Coulomb coupling parameter increases considerably in later stages of the expansion, reaching the
strongly coupled regime despite the well-known initial drop of
to order unity due to disorder-induced heating. Furthermore, we formulate a
suitable measure of correlation and show th at calculated from
the ionic temperature and density reflects the degree of order in the system if
it is sufficiently close to a quasisteady state. At later times, however, the
expansion of the plasma cloud becomes faster than the relaxation of
correlations, and the system does not reach thermodynamic equilibrium anymore
Friction as Contrast Mechanism in Heterodyne Force Microscopy
The nondestructive imaging of subsurface structures on the nanometer scale
has been a long-standing desire in both science and industry. A few impressive
images were published so far that demonstrate the general feasibility by
combining ultrasound with an Atomic Force Microscope. From different excitation
schemes, Heterodyne Force Microscopy seems to be the most promising candidate
delivering the highest contrast and resolution. However, the physical contrast
mechanism is unknown, thereby preventing any quantitative analysis of samples.
Here we show that friction at material boundaries within the sample is
responsible for the contrast formation. This result is obtained by performing a
full quantitative analysis, in which we compare our experimentally observed
contrasts with simulations and calculations. Surprisingly, we can rule out all
other generally believed responsible mechanisms, like Rayleigh scattering,
sample (visco)elasticity, damping of the ultrasonic tip motion, and ultrasound
attenuation. Our analytical description paves the way for quantitative
SubSurface-AFM imaging.Comment: 7 pages main paper + 11 pages supplementary material
On-chip quantum tomography of mechanical nano-scale oscillators with guided Rydberg atoms
Nano-mechanical oscillators as well as Rydberg-atomic waveguides hosted on
micro-fabricated chip surfaces hold promise to become pillars of future quantum
technologies. In a hybrid platform with both, we show that beams of Rydberg
atoms in waveguides can quantum-coherently interrogate and manipulate
nanomechanical elements, allowing full quantum state tomography. Central to the
tomography are quantum non-demolition measurements using the Rydberg atoms as
probes. Quantum coherent displacement of the oscillator is also made possible,
by driving the atoms with external fields while they interact with the
oscillator. We numerically demonstrate the feasibility of this fully integrated
on-chip control and read-out suite for quantum nano-mechanics, taking into
account noise and error sources.Comment: 11 pages, 5 figures, 1 tabl
Inelastic semiclassical Coulomb scattering
We present a semiclassical S-matrix study of inelastic collinear
electron-hydrogen scattering. A simple way to extract all necessary information
from the deflection function alone without having to compute the stability
matrix is described. This includes the determination of the relevant Maslov
indices. Results of singlet and triplet cross sections for excitation and
ionization are reported. The different levels of approximation -- classical,
semiclassical, and uniform semiclassical -- are compared among each other and
to the full quantum result.Comment: 9 figure
Quantum dynamics of long-range interacting systems using the positive-P and gauge-P representations
We provide the necessary framework for carrying out stochastic positive-P and
gauge-P simulations of bosonic systems with long range interactions. In these
approaches, the quantum evolution is sampled by trajectories in phase space,
allowing calculation of correlations without truncation of the Hilbert space or
other approximations to the quantum state. The main drawback is that the
simulation time is limited by noise arising from interactions.
We show that the long-range character of these interactions does not further
increase the limitations of these methods, in contrast to the situation for
alternatives such as the density matrix renormalisation group. Furthermore,
stochastic gauge techniques can also successfully extend simulation times in
the long-range-interaction case, by making using of parameters that affect the
noise properties of trajectories, without affecting physical observables.
We derive essential results that significantly aid the use of these methods:
estimates of the available simulation time, optimized stochastic gauges, a
general form of the characteristic stochastic variance and adaptations for very
large systems. Testing the performance of particular drift and diffusion gauges
for nonlocal interactions, we find that, for small to medium systems, drift
gauges are beneficial, whereas for sufficiently large systems, it is optimal to
use only a diffusion gauge.
The methods are illustrated with direct numerical simulations of interaction
quenches in extended Bose-Hubbard lattice systems and the excitation of Rydberg
states in a Bose-Einstein condensate, also without the need for the typical
frozen gas approximation. We demonstrate that gauges can indeed lengthen the
useful simulation time.Comment: 19 pages, 11 appendix, 3 figure
- …