103 research outputs found
Non-thermal quantum phase transitions
We report a kind of quantum phase transition which takes place in isolated
quantum systems with non-thermal equilibrium states and an extra symmetry that
commutes with the Hamiltonian for any values of the system parameters. A
critical energy separates two different phases, one in which the symmetry is
broken. This critical behavior is ruled out as soon as the system is put in
contact with a thermal bath. The critical point is crossed when a sufficent
amount of work is performed on the system, keeping it isolated from the
environment. Different phases are identified by means of an order parameter,
which is only different from zero in the symmetry-breaking phase. The behavior
of the system near the critical point is determined by a set of critical
exponents. We illustrate this phenomenon by means of numerical calculations in
three different two-level systems
Irreversible processes without energy dissipation in an isolated Lipkin-Meshkov-Glick model
For a certain class of isolated quantum systems, we report the existence of
irreversible processes in which the energy is not dissipated. After a closed
cycle in which the initial energy distribution is fully recovered, the
expectation value of a symmetry-breaking observable changes from a value
different from zero in the initial state, to zero in the final state. This
entails the unavoidable loss of a certain amount of information, and
constitutes a source of irreversibility. We show that the von Neumann entropy
of time-averaged equilibrium states increases in the same magnitude as a
consequence of the process. We support this result by means of numerical
calculations in an experimentally feasible system, the Lipkin-Meshkov-Glick
model.Comment: 10 pages, 7 figure
Quantum state engineering by shortcuts-to-adiabaticity in interacting spin-boson systems
We present a fast and robust framework to prepare non-classical states of a
bosonic mode exploiting a coherent exchange of excitations with a two-level
system ruled by a Jaynes-Cummings interaction mechanism. Our protocol, which is
built on shortcuts to adiabaticity, allows for the generation of arbitrary Fock
states of the bosonic mode, as well as coherent quantum superpositions of a
Schr\"odinger cat-like form. In addition, we show how to obtain a class of
photon-shifted states where the vacuum population is removed, a result akin to
photon addition, but displaying more non-classicality than standard
photon-added states. Owing to the ubiquity of the spin-boson interaction that
we consider, our proposal is amenable for implementations in state-of-the-art
experiments.Comment: 11 pages, 10 figure
Quantum simulation of multiphoton and nonlinear dissipative spin-boson models
We present a framework for the realization of dissipative evolutions of
spin-boson models, including multiphoton exchange dynamics, as well as
nonlinear transition rates. Our approach is based on the implementation of a
generalized version of a dissipative linear quantum Rabi model. The latter
comprises a linearly coupled spin-boson term, spin rotations, and standard
dissipators. We provide numerical simulations of illustrative cases supporting
the good performance of our method. Our work allows for the simulation of a
large class of fundamentally different quantum models where the effect of
distinct dissipative processes can be easily investigated.Comment: 14 pages, 6 figs. Comments are welcome
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