450 research outputs found
Non-equilibrium Berezinskii-Kosterlitz-Thouless Transition in a Driven Open Quantum System
The Berezinskii-Kosterlitz-Thouless mechanism, in which a phase transition is
mediated by the proliferation of topological defects, governs the critical
behaviour of a wide range of equilibrium two-dimensional systems with a
continuous symmetry, ranging from superconducting thin films to two-dimensional
Bose fluids, such as liquid helium and ultracold atoms. We show here that this
phenomenon is not restricted to thermal equilibrium, rather it survives more
generally in a dissipative highly non-equilibrium system driven into a
steady-state. By considering a light-matter superfluid of polaritons, in the
so-called optical parametric oscillator regime, we demonstrate that it indeed
undergoes a vortex binding-unbinding phase transition. Yet, the exponent of the
power-law decay of the first order correlation function in the (algebraically)
ordered phase can exceed the equilibrium upper limit -- a surprising
occurrence, which has also been observed in a recent experiment. Thus we
demonstrate that the ordered phase is somehow more robust against the quantum
fluctuations of driven systems than thermal ones in equilibrium.Comment: 11 pages, 9 figure
Full and fractional defects across the Berezinskii-Kosterlitz-Thouless transition in a driven-dissipative spinor quantum fluid
We investigate the properties of a two-dimensional \emph{spinor} microcavity
polariton system driven by a linearly polarised continuous pump. In particular,
we establish the role of the elementary excitations, namely the so-called
half-vortices and full-vortices; these objects carry a quantum rotation only in
one of the two, or both, spin components respectively. Our numerical analysis
of the steady-state shows that it is only the half-vortices that are present in
the vortex-antivortex pairing/dissociation responsible for the
Berezinskii-Kosterlitz-Thouless transition. These are the relevant elementary
excitations close to the critical point. However, by exploring the
phase-ordering dynamics following a sudden quench across the transition we
prove that full-vortices become the relevant excitations away from the critical
point in a deep quasi-ordered state at late times. The time-scales for
half-vortices binding into full vortices are much faster than the
vortex-antivortex annihilations.Comment: 6 pages, 3 figure
Kibble-Zurek mechanism in driven-dissipative systems crossing a non-equilibrium phase transition
The Kibble-Zurek mechanism constitutes one of the most fascinating and
universal phenomena in the physics of critical systems. It describes the
formation of domains and the spontaneous nucleation of topological defects when
a system is driven across a phase transition exhibiting spontaneous symmetry
breaking. While a characteristic dependence of the defect density on the speed
at which the transition is crossed was observed in a vast range of equilibrium
condensed matter systems, its extension to intrinsically driven-dissipative
systems is a matter of ongoing research. In this work we numerically confirm
the Kibble-Zurek mechanism in a paradigmatic family of driven-dissipative
quantum systems, namely exciton-polaritons in microcavities. Our findings show
how the concepts of universality and critical dynamics extend to
driven-dissipative systems that do not conserve energy or particle number nor
satisfy a detailed balance condition
Critical slowing down in circuit quantum electrodynamics
Critical slowing down of the time it takes a system to reach equilibrium is a key signature of bistability in dissipative first-order phase transitions. Understanding and characterizing this process can shed light on the underlying many-body dynamics that occur close to such a transition. Here, we explore the rich quantum activation dynamics and the appearance of critical slowing down in an engineered superconducting quantum circuit. Specifically, we investigate the intermediate bistable regime of the generalized Jaynes-Cummings Hamiltonian (GJC), realized by a circuit quantum electrodynamics (cQED) system consisting of a transmon qubit coupled to a microwave cavity. We find a previously unidentified regime of quantum activation in which the critical slowing down reaches saturation and, by comparing our experimental results with a range of models, we shed light on the fundamental role played by the qubit in this regime
Excitons in T-shaped quantum wires
We calculate energies, oscillator strengths for radiative recombination, and
two-particle wave functions for the ground state exciton and around 100 excited
states in a T-shaped quantum wire. We include the single-particle potential and
the Coulomb interaction between the electron and hole on an equal footing, and
perform exact diagonalisation of the two-particle problem within a finite basis
set. We calculate spectra for all of the experimentally studied cases of
T-shaped wires including symmetric and asymmetric GaAs/AlGaAs and
InGaAs/AlGaAs structures. We study in detail the
shape of the wave functions to gain insight into the nature of the various
states for selected symmetric and asymmetric wires in which laser emission has
been experimentally observed. We also calculate the binding energy of the
ground state exciton and the confinement energy of the 1D quantum-wire-exciton
state with respect to the 2D quantum-well exciton for a wide range of
structures, varying the well width and the Al molar fraction . We find that
the largest binding energy of any wire constructed to date is 16.5 meV. We also
notice that in asymmetric structures, the confinement energy is enhanced with
respect to the symmetric forms with comparable parameters but the binding
energy of the exciton is then lower than in the symmetric structures. For
GaAs/AlGaAs wires we obtain an upper limit for the binding energy
of around 25 meV in a 10 {\AA} wide GaAs/AlAs structure which suggests that
other materials must be explored in order to achieve room temperature
applications. There are some indications that
InGaAs/AlGaAs might be a good candidate.Comment: 20 pages, 10 figures, uses RevTeX and psfig, submitted to Physical
Review
Properties of the signal mode in the polariton optical parametric oscillator regime
Theoretical analyses of the polariton optical parametric oscillator (OPO) regime often rely on a mean-field approach based on the complex Gross-Pitaevskii equations in a three-mode approximation, where only three momentum states, the signal, pump, and idler, are assumed to be significantly occupied. This approximation, however, lacks a constraint to uniquely determine the signal and idler momenta. In contrast, multimode numerical simulations and experiments show a unique momentum structure for the OPO states. In this work we show that an estimate for the signal momentum chosen by the system can be found from a simple analysis of the pump-only configuration. We use this estimate to investigate how the chosen signal momentum depends on the properties of the drive
Semiconductor nanostructures engineering: Pyramidal quantum dots
Pyramidal quantum dots (QDs) grown in inverted recesses have demonstrated
over the years an extraordinary uniformity, high spectral purity and strong
design versatility. We discuss recent results, also in view of the
Stranski-Krastanow competition and give evidence for strong perspectives in
quantum information applications for this system. We examine the possibility of
generating entangled and indistinguishable photons, together with the need for
the implementation of a, regrettably still missing, strategy for electrical
control
Dynamical Critical Exponents in Driven-Dissipative Quantum Systems
We study the phase ordering of parametrically and incoherently driven microcavity polaritons after an
infinitely rapid quench across the critical region. We confirm that the system, despite its driven-dissipative
nature, satisfies the dynamical scaling hypothesis for both driving schemes by exhibiting self-similar
patterns for the two-point correlator at late times of the phase ordering. We show that polaritons are
characterized by the dynamical critical exponent z ≈ 2 with topological defects playing a fundamental role
in the dynamics, giving logarithmic corrections both to the power-law decay of the number of vortices and
to the associated growth of the characteristic length scale
Many-body physics of a quantum fluid of exciton-polaritons in a semiconductor microcavity
Some recent results concerning nonlinear optics in semiconductor
microcavities are reviewed from the point of view of the many-body physics of
an interacting photon gas. Analogies with systems of cold atoms at thermal
equilibrium are drawn, and the peculiar behaviours due to the non-equilibrium
regime pointed out. The richness of the predicted behaviours shows the
potentialities of optical systems for the study of the physics of quantum
fluids.Comment: Proceedings of QFS2006 conference to appear on JLT
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