50 research outputs found
Casimir-Polder forces between two accelerating atoms and the Unruh effect
The Casimir-Polder force between two atoms with equal uniform acceleration
and separated by a constant distance R is considered. We show that, in the
low-acceleration limit, while the near-zone R^{-6} behavior of the interatomic
interaction energy is not changed by the acceleration of the atoms, the
far-zone interaction energy decreases as R^{-5} instead of the well-known
R^{-7} behavior for inertial atoms. Possibility of an indirect detection of the
Unruh effect through measurements of the Casimir-Polder force between the two
accelerating atoms is also suggested. We also consider a heuristic model for
calculating the Casimir-Polder potential energy between the two atoms in the
high-acceleration limit.Comment: Contribution to the Proceedings of the QFEXT09 Conference, Norman,
Oklahoma, US
Driven Markovian Quantum Criticality
We identify a new universality class in one-dimensional driven open quantum
systems with a dark state. Salient features are the persistence of both the
microscopic non-equilibrium conditions as well as the quantum coherence of
dynamics close to criticality. This provides a non-equilibrium analogue of
quantum criticality, and is sharply distinct from more generic driven systems,
where both effective thermalization as well as asymptotic decoherence ensue,
paralleling classical dynamical criticality. We quantify universality by
computing the full set of independent critical exponents within a functional
renormalization group approach.Comment: 5+3 pages, 2 figures; published version with improved presentation of
result
Pre-Thermalization in Quantum Spin Chains
The results presented in this PhD Thesis aim at understanding thermalization dynamics and, strictly speaking, pre-thermalization in both open and closed quantum spin chains driven out-of-equilibrium. In order to make analytical progress, we consider perturbations of the Quantum Ising Chain (QIC), one of the simplest integrable models, whose non-equilibrium dynamics has been studied in great details in the last few years. In Chapter 2 we introduce for the non-technical reader the main recent ideas behind quantum thermalization and relaxation in integrable and non-integrable quantum many body systems, while Chapter 3 contains an introduction to the current
understanding of pre-thermalization.
In Chapter 4 we start our study considering a QIC perturbed by a time dependent noise in the direction of the transverse magnetic field [6] [7]. We generalize the concept of pre-thermalization to a noisy quantum many body
system driven away from equilibrium simultaneously by a quantum quench of the transverse field and by the noise, highlighting the mechanism behind pre-thermalization and the subsequent relaxation towards the thermal state
in a wide class of physically relevant observables. Our main achievement is to show that pre-thermalization occurs quite generally up to time scales where the noise is dominated by the non-equilibrium dynamics induced by the quench of the transverse field, while for longer times pre-thermal plateaux disappear and the noise leaves different signatures in different observables. In Chapter 5 we consider the more challenging problem of pre-thermalization in a closed non-integrable quantum spin chain [8]. We show using a mapping of a weakly non-integrable spin chain into a hard core boson model, that pre-thermalization is successfully captured by a low-density description of the theory, which results in a integrable, bosonic model, that constitutes an excellent description for the out-of-equilibrium dynamics up to intermediate time scales. On the other hand, in order to investigate the late time
dynamics we resort to a diagrammatic approach, which serves for a two-fold scope. First, we focus on the effect of the leading inelastic processes, realizing
that they constitute only a sub-leading correction to the pre-thermal plateau, found within the effective low-energy description; and secondly, we
show from a Quantum Boltzmann equation perspective how inelastic processes
are the fundamental ingredient in order to have thermalization in the
late time dynamics.
The main technical tool used to derive the results in Chapter 4 and 5 is
the Keldysh contour technique which is reviewed in the Appendix.
This PhD Thesis is based on the following works:
1) J. Marino, A. Silva, Relaxation, pre-thermalization and diffusion in a
noisy Quantum Ising Chain, Phys. Rev. B 86, 060408 (Rapid Communications) (2012).
2) M. Marcuzzi, J. Marino, A. Gambassi, A. Silva, Pre-thermalization in
a non-integrable quantum spin chain after a quench, arXiv:1307.3738
(2013) (submitted to Phys. Rev. Lett.).
3) J. Marino, A. Silva, Non-Equilibrium Dynamics of a Noisy Quantum
Ising Chain: statistics of the work and prethermalization after a sudden
quench of the transverse field, arXiv:1309.7595 (2013) (submitted to
Phys. Rev. B)
Casimir forces and quantum friction from Ginzburg radiation in atomic BECs
We theoretically propose an experimentally viable scheme to use an impurity
atom in an atomic Bose-Einstein condensate, in order to realize
condensed-matter analogs of quantum vacuum effects. In a suitable atomic level
configuration, the collisional interaction between the impurity atom and the
density fluctuations in the condensate can be tailored to closely reproduce the
electric-dipole coupling of quantum electrodynamics. By virtue of this analogy,
we recover and extend the paradigm of electromagnetic vacuum forces to the
domain of cold atoms, showing in particular the emergence, at supersonic atomic
speeds, of a novel power-law scaling of the Casimir force felt by the atomic
impurity, as well as the occurrence of a quantum frictional force, accompanied
by the Ginzburg emis- sion of Bogoliubov quanta. Observable consequences of
these quantum vacuum effects in realistic spectroscopic experiments are
discussed.Comment: 5 pages, 2 figures. Revised version accepted in PR
Exploring many-body localization in quantum systems coupled to an environment via Wegner-Wilson flows
Inspired by recent experiments on many-body localized systems coupled to an
environment, we apply a Flow Equation method to study the problem of a disorder
chain of spinless fermions, coupled via density-density interactions to a
second clean chain of spinless fermions. In particular, we focus on the
conditions for the onset of a many-body localized phase in the clean sector of
our model by proximity to the dirty one. We find that a many-body localization
proximity effect in the clean component is established when the density of
dirty fermions exceeds a threshold value. From the flow equation method we find
that, similar to many-body localization in a single chain, the many-body
localization proximity effect is also described by an extensive set of local
integrals of motion. Furthermore, by tuning the geometry of the inter-chain
couplings, we show that the dynamics of the model is ruled, on intermediate
time scales, by an emergent set of quasi-conserved charges.Comment: 22 pages, 7 figure
Entanglement entropy in a periodically driven quantum Ising chain
We numerically study the dynamics of entanglement entropy, induced by an
oscillating time periodic driving of the transverse field, h(t), of a
one-dimensional quantum Ising chain. We consider several realizations of h(t),
and we find a number of results in analogy with entanglement entropy dynamics
induced by a sudden quantum quench. After short-time relaxation, the dynamics
of entanglement entropy synchronises with h(t), displaying an oscillatory
behaviour at the frequency of the driving. Synchronisation in the dynamics of
entanglement entropy, is spoiled by the appearance of quasi-revivals which fade
out in the thermodynamic limit, and which we interpret using a quasi-particle
picture adapted to periodic drivings. Taking the time-average of the
entanglement entropy in the synchronised regime, we find that it obeys a volume
law scaling with the subsystem's size. Such result is reminiscent of a thermal
state or of a Generalised Gibbs ensemble of a quenched Ising chain, although
the system does not heat up towards infinite temperature as a consequence of
the integrability of the model.Comment: 6 pages, 3 figure
Relaxation, pre-thermalization and diffusion in a noisy Quantum Ising Chain
We study the dynamics of thermalization resulting from a time-dependent noise
in a Quantum Ising Chain subject to a sudden quench of the transverse magnetic
field. For weak noise the dynamics shows a pre-thermalized state at
intermediate time scales, eventually drifting towards an asymptotic infinite
temperature steady state characterized by diffusive behavior. By computing
analytically the density of kinks, as well as the transverse and longitudinal
magnetic field correlators, we characterize these two regimes, their
observability and their signatures in the various physical quantities.Comment: 5 pages, 2 figures. Accepted for publication in PRB Rapid
Communication
Effects of a uniform acceleration on atom-field interactions
We review some quantum electrodynamical effects related to the uniform
acceleration of atoms in vacuum. After discussing the energy level shifts of a
uniformly accelerated atom in vacuum, we investigate the atom-wall
Casimir-Polder force for accelerated atoms, and the van der
Waals/Casimir-Polder interaction between two accelerated atoms. The possibility
of detecting the Unruh effect through these phenomena is also discussed in
detail.Comment: 6 pages. Special Issue: 20th Central European Workshop on Quantum
Optics - Stockholm - June 201