27 research outputs found
Quantum Zeno suppression of dipole-dipole forces
We consider inter-atomic forces due to resonant dipole-dipole interactions
within a dimer of highly excited Rydberg atoms, embedded in an ultra-cold gas.
These forces rely on a coherent superposition of two-atom electronic states,
which is destroyed by continuous monitoring of the dimer state through a
detection scheme utilizing controllable interactions with the background gas
atoms. We show that this intrinsic decoherence of the molecular energy surface
can gradually deteriorate a repulsive dimer state, causing a mixing of
attractive and repulsive character. For sufficiently strong decoherence, a
Zeno-like effect causes a complete arrest of interatomic forces. We finally
show how short decohering pulses can controllably redistribute population
between the different molecular energy surfaces.Comment: 4+3 pages, 4+1 figure
Multi-Excitons in Flexible Rydberg Aggregates
Flexible Rydberg aggregates, assemblies of few Rydberg atoms coherently
sharing electronic excitations while undergoing directed atomic motion, show
great promise as quantum simulation platform for nuclear motional dynamics in
molecules or quantum energy transport. Here we study additional features that
are enabled by the presence of more than a single electronic excitation, thus
considering multi-exciton states. We describe cases where these can be
decomposed into underlying single exciton states and then present dynamical
scenarios with atomic motion that illustrate exciton-exciton collisions,
exciton routing, and strong non-adiabatic effects in simple one-dimensional
settings.Comment: 7 pages, 8 figure
Van-der-Waals stabilized Rydberg aggregates
Assemblies of Rydberg atoms subject to resonant dipole-dipole interactions
form Frenkel excitons. We show that van-der-Waals shifts can significantly
modify the exciton wave function, whenever atoms approach each other closely.
As a result, attractive excitons and repulsive van-der-Waals interactions can
be combined to form stable one-dimensional atom chains, akin to bound
aggregates. Here the van-der-Waals shifts ensure a stronger homogeneous
delocalisation of a single excitation over the whole chain, enabling it to bind
up to six atoms. When brought into unstable configurations, such Rydberg
aggregates allow the direct monitoring of their dissociation dynamics.Comment: 6 pages, 6 figure
Break-up of Rydberg superatoms via dipole-dipole interactions
We investigate resonant dipole-dipole interactions between two "superatoms"
of different angular momentum, consisting of two Rydberg-blockaded atom clouds
where each of them carries initially a coherently shared single excitation. We
demonstrate that the dipole-dipole interaction breaks up the superatoms by
removing the excitations from the clouds. The dynamics is akin to an ensemble
average over systems where only one atom per cloud participates in entangled
motion and excitation transfer. Our findings should thus facilitate the
experimental realization of adiabatic exciton transport in Rydberg systems by
replacing single sites with atom clouds.Comment: 10 pages, 5 figure
Solitons explore the quantum classical boundary
It is an open fundamental question how the classical appearance of our
environment arises from the underlying quantum many-body theory. We propose
that the quantum-classical boundary can be probed in collisions of bright
solitons in Bose-Einstein condensates, where thousands of atoms form a large
compound object at ultra cold temperatures. We show that these collisions
exhibit intricate many-body quantum behavior, invalidating mean field theory.
Prior to collision, solitons can loose their well defined quantum phase
relation through phase diffusion, essentially caused by atom number
fluctuations. This dephasing should typically render the subsequent dynamics
more classical. Instead, we find that it opens the door for a tremendous
proliferation of mesoscopic entanglement: After collision the two solitons find
themselves in a superposition state of various constituent atom numbers,
positions and velocities, in which all these quantities are entangled with
those of the collision partner.
As the solitons appear to traverse the quantum-classical boundary back and
forth during their scattering process, they emerge as natural probe of
mesoscopic quantum coherence and decoherence phenomena.Comment: 6 pages, 4 figure
Hyper-entangling mesoscopic bound states
We predict hyper-entanglement generation during binary scattering of
mesoscopic bound states, solitary waves in Bose-Einstein condensates containing
thousands of identical Bosons. The underlying many-body Hamiltonian must not be
integrable, and the pre-collision quantum state of the solitons fragmented.
Under these conditions, we show with pure state quantum field simulations that
the post-collision state will be hyper-entangled in spatial degrees of freedom
and atom number within solitons, for realistic parameters. The effect links
aspects of non-linear systems and quantum-coherence and the entangled
post-collision state challenges present entanglement criteria for identical
particles. Our results are based on simulations of colliding quantum solitons
in a quintic interaction model beyond the mean-field, using the truncated
Wigner approximation.Comment: 6 figure
The disordered Dicke model
We introduce and study the disordered Dicke model in which the spin-boson
couplings are drawn from a random distribution with some finite width.
Regarding the quantum phase transition we show that when the standard deviation
of the coupling strength gradually increases, the critical value of
the mean coupling strength gradually decreases and after a certain
there is no quantum phase transition at all; the system always lies in
the super-radiant phase. We derive an approximate expression for the quantum
phase transition in the presence of disorder in terms of and ,
which we numerically verify. Studying the thermal phase transition in the
disordered Dicke model, we obtain an analytical expression for the critical
temperature in terms of the mean and standard deviation of the coupling
strength. We observe that even when the mean of the coupling strength is zero,
there is a finite temperature transition if the standard deviation of the
coupling is sufficiently high. Disordered couplings in the Dicke model will
exist in quantum dot superlattices, and we also sketch how they can be
engineered and controlled with ultracold atoms or molecules in a cavity.Comment: 11 pages, 6 figure
Trapping and binding by dephasing
The binding and trapping of particles usually rely on conservative forces,
described by unitary quantum dynamics. We show that both can also arise solely
from spatially dependent dephasing, the simplest type of decoherence. This can
be based on continuous weak position measurements in only selected regions of
space, for which we propose a practical realisation. For a single particle, we
demonstrate a quantum particle-in-the-box based on dephasing. For two
particles, we demonstrate their binding despite repulsive interactions, if
their molecular states are dephased at large separations only. Both mechanisms
are experimentally accessible, as we show for an example with Rydberg atoms in
a cold gas background.Comment: 9 pages, 8 figure