50 research outputs found
Exotic attractors of the non-equilibrium Rabi-Hubbard model
We explore the phase diagram of the dissipative Rabi-Hubbard model, as could
be realized by a Raman-pumping scheme applied to a coupled cavity array. There
exist various exotic attractors, including ferroelectric, antiferroelectric,
and inccomensurate fixed points, as well as regions of persistent oscillations.
Many of these features can be understood analytically by truncating to the two
lowest lying states of the Rabi model on each site. We also show that these
features survive beyond mean-field, using Matrix Product Operator simulations.Comment: 5pages, 3 figures, plus supplementary material. Final version, as
publishe
Local Integrals of Motion in Quasiperiodic Many-Body Localized Systems
Local integrals of motion play a central role in the understanding of
many-body localization in many-body quantum systems in one dimension subject to
a random external potential, but the question of how these local integrals of
motion change in a deterministic quasiperiodic potential is one that has
received significantly less attention. Here we develop a powerful new
implementation of the continuous unitary transform formalism and use this
method to directly compute both the effective Hamiltonian and the local
integrals of motion for many-body quantum systems subject to a quasiperiodic
potential. We show that the effective interactions between local integrals of
motion retain a strong fingerprint of the underlying quasiperiodic potential,
exhibiting sharp features at distances associated with the incommensurate
wavelength used to generate the potential. Furthermore, the local integrals of
motion themselves may be expressed in terms of an operator expansion which
allows us to estimate the critical strength of quasiperiodic potential required
to lead to a localization/delocalization transition, by means of a finite size
scaling analysis.Comment: 41 pages, 13 figure
On the phase transition of light in cavity QED lattices
Systems of strongly interacting atoms and photons, that can be realized
wiring up individual cavity QED systems into lattices, are perceived as a new
platform for quantum simulation. While sharing important properties with other
systems of interacting quantum particles here we argue that the nature of
light-matter interaction gives rise to unique features with no analogs in
condensed matter or atomic physics setups. By discussing the physics of a
lattice model of delocalized photons coupled locally with two-level systems
through the elementary light-matter interaction described by the Rabi model, we
argue that the inclusion of counter rotating terms, so far neglected, is
crucial to stabilize finite-density quantum phases of correlated photons out of
the vacuum, with no need for an artificially engineered chemical potential. We
show that the competition between photon delocalization and Rabi non-linearity
drives the system across a novel parity symmetry-breaking quantum
criticality between two gapped phases which shares similarities with the Dicke
transition of quantum optics and the Ising critical point of quantum magnetism.
We discuss the phase diagram as well as the low-energy excitation spectrum and
present analytic estimates for critical quantities.Comment: 5+3 pages, published versio
The Out-of-Equilibrium Time-Dependent Gutzwiller Approximation
We review the recently proposed extension of the Gutzwiller approximation, M.
Schiro' and M. Fabrizio, Phys. Rev. Lett. 105, 076401 (2010), designed to
describe the out-of-equilibrium time-evolution of a Gutzwiller-type variational
wave function for correlated electrons. The method, which is strictly
variational in the limit of infinite lattice-coordination, is quite general and
flexible, and it is applicable to generic non-equilibrium conditions, even far
beyond the linear response regime. As an application, we discuss the quench
dynamics of a single-band Hubbard model at half-filling, where the method
predicts a dynamical phase transition above a critical quench that resembles
the sharp crossover observed by time-dependent dynamical mean field theory. We
next show that one can actually define in some cases a multi-configurational
wave function combination of a whole set of mutually orthogonal Gutzwiller wave
functions. The Hamiltonian projected in that subspace can be exactly evaluated
and is equivalent to a model of auxiliary spins coupled to non-interacting
electrons, closely related to the slave-spin theories for correlated electron
models. The Gutzwiller approximation turns out to be nothing but the mean-field
approximation applied to that spin-fermion model, which displays, for any
number of bands and integer fillings, a spontaneous symmetry breaking
that can be identified as the Mott insulator-to-metal transition.Comment: 25 pages. Proceedings of the Hvar 2011 Workshop on 'New materials for
thermoelectric applications: theory and experiment
Detection of squeezed phonons in pump-probe spectroscopy
Robust engineering of phonon squeezed states in optically excited solids has
emerged as a promising tool to control and manipulate their properties.
However, in contrast to quantum optical systems, detection of phonon squeezing
is subtle and elusive, and an important question is what constitutes an
unambiguous signature of it. The state of the art involves observing
oscillations at twice the phonon frequency in time resolved measurements of the
out of equilibrium phonon fluctuation. Using Keldysh formalism we show that
such a signal is a necessary but not a sufficient signature of a squeezed
phonon, since we identify several mechanisms that do not involve squeezing and
yet which produce similar oscillations. We show that a reliable detection
requires a time and frequency resolved measurement of the phonon spectral
function
The Interspersed Spin Boson Lattice Model
We describe a family of lattice models that support a new class of quantum
magnetism characterized by correlated spin and bosonic ordering [Phys. Rev.
Lett. 112, 180405 (2014)]. We explore the full phase diagram of the model using
Matrix-Product-State methods. Guided by these numerical results, we describe a
modified variational ansatz to improve our analytic description of the
groundstate at low boson frequencies. Additionally, we introduce an
experimental protocol capable of inferring the low-energy excitations of the
system by means of Fano scattering spectroscopy. Finally, we discuss the
implementation and characterization of this model with current circuit-QED
technology.Comment: Submitted to EPJ ST issue on "Novel Quantum Phases and Mesoscopic
Physics in Quantum Gases
Mesoscopic mean-field theory for spin-boson chains in quantum optical systems
We present a theoretical description of a system of many spins strongly coupled to a bosonic chain. We rely on the use of a spin-wave theory describing the Gaussian fluctuations around the mean-field solution, and focus on spin-boson chains arising as a generalization of the Dicke Hamiltonian. Our model is motivated by experimental setups such as trapped ions, or atoms/qubits coupled to cavity arrays. This situation corresponds to the cooperative (E⊗β) Jahn-Teller distortion studied in solid-state physics. However, the ability to tune the parameters of the model in quantum optical setups opens up a variety of novel intriguing situations. The main focus of this paper is to review the spin-wave theoretical description of this problem as well as to test the validity of mean-field theory. Our main result is that deviations from mean-field effects are determined by the interplay between magnetic order and mesoscopic cooperativity effects, being the latter strongly size-dependent
Localization and Glassy Dynamics Of Many-Body Quantum Systems
When classical systems fail to explore their entire configurational space, intriguing macroscopic phenomena like aging and glass formation may emerge. Also closed quanto-mechanical systems may stop wandering freely around the whole Hilbert space, even if they are initially prepared into a macroscopically large combination of eigenstates. Here, we report numerical evidences that the dynamics of strongly interacting lattice bosons driven sufficiently far from equilibrium can be trapped into extremely long-lived inhomogeneous metastable states. The slowing down of incoherent density excitations above a threshold energy, much reminiscent of a dynamical arrest on the verge of a glass transition, is identified as the key feature of this phenomenon. We argue that the resulting long-lived inhomogeneities are responsible for the lack of thermalization observed in large systems. Such a rich phenomenology could be experimentally uncovered upon probing the out-of-equilibrium dynamics of conveniently prepared quantum states of trapped cold atoms which we hereby suggest