Keldysh field theory for nonequilibrium condensation in a parametrically pumped polariton system

We develop a quantum field theory for parametrically pumped polaritons using Keldysh Green's function techniques. By considering the mean-field and Gaussian fluctuations, we find that the low energy physics of the highly non-equilibrium phase transition to the optical parametric oscillator regime is in many ways similar to equilibrium condensation. In particular, we show that this phase transition can be associated with an effective chemical potential, at which the system's bosonic distribution function diverges, and an effective temperature. As in equilibrium systems, the transition is achieved by tuning this effective chemical potential to the energy of the lowest normal mode. Since the occupations of the modes are available, we determine experimentally observable properties, such as the luminescence and absorption spectra.Comment: 16 pages, 14 figure

Non-equilibrium quantum condensation in an incoherently pumped dissipative system

We study spontaneous quantum coherence in an out of equilibrium system, coupled to multiple baths describing pumping and decay. For a range of parameters describing coupling to, and occupation of the baths, a stable steady-state condensed solution exists. The presence of pumping and decay significantly modifies the spectra of phase fluctuations, leading to correlation functions that differ both from an isolated condensate and from a laser.Comment: 5 pages, 2 eps figure

Rare quantum metastable states in the strongly dispersive Jaynes-Cummings oscillator

We present evidence of metastable rare quantum-fluctuation switching for the driven dissipative Jaynes-Cummings oscillator coupled to a zero-temperature bath in the strongly dispersive regime. We show that single-atom complex amplitude bistability is accompanied by the appearance of a low-amplitude long-lived transient state, hereinafter called `dark state', having a distribution with quasi-Poissonian statistics both for the coupled qubit and cavity mode. We find that the dark state is linked to a spontaneous flipping of the qubit state, detuning the cavity to a low-photon response. The appearance of the dark state is correlated with the participation of the two metastable states in the dispersive bistability, as evidenced by the solution of the Master Equation and single quantum trajectories.Comment: Extensively revised text, 18 revised figures (16 in main and 2 in appendix), 38(+1) references, appendi

Thermodynamics and Excitations of Condensed Polaritons in Disordered Microcavities

We study the thermodynamic condensation of microcavity polaritons using a realistic model of disorder in semiconductor quantum wells. This approach correctly describes the polariton inhomogeneous broadening in the low density limit, and treats scattering by disorder to all orders in the condensed regime. While the weak disorder changes the thermodynamic properties of the transition little, the effects of disorder in the condensed state are prominent in the excitations and can be seen in resonant Rayleigh scattering.Comment: 5 pages, 3 eps figures (published version

Searching for the Kardar-Parisi-Zhang phase in microcavity polaritons

Recent analytical work has shown that, at certain values of the external pump, the optical parametric oscillator (OPO) regime of microcavity polaritons may provide a realisation of Kardar-Parisi-Zhang (KPZ) physics in 2D. Here, we verify this by solving the full microscopic model numerically using the truncated Wigner method, and studying the first order spatial correlations. For the predicted pump strengths, these correlations decay much faster and, perpendicular to the pump, fit closely to the stretched exponential form predicted by the KPZ equation, in contrast to the usual algebraic decay. This strongly indicates the viability of observing KPZ behaviour in future polariton OPO experiments.Comment: Main text (6 pages, 5 figures) plus supplementary material (3 pages, 5 figures

Simultaneous bistability of qubit and resonator in circuit quantum electrodynamics

We explore the joint activated dynamics exhibited by two quantum degrees of freedom: a cavity mode oscillator which is strongly coupled to a superconducting qubit in the strongly coherently driven dispersive regime. Dynamical simulations and complementary measurements show a range of parameters where both the cavity and the qubit exhibit sudden simultaneous switching between two metastable states. This manifests in ensemble averaged amplitudes of both the cavity and qubit exhibiting a partial coherent cancellation. Transmission measurements of driven microwave cavities coupled to transmon qubits show detailed features which agree with the theory in the regime of simultaneous switching

Vortex and half-vortex dynamics in a spinor quantum fluid of interacting polaritons

Spinorial or multi-component Bose-Einstein condensates may sustain fractional quanta of circulation, vorticant topological excitations with half integer windings of phase and polarization. Matter-light quantum fluids, such as microcavity polaritons, represent a unique test bed for realising strongly interacting and out-of-equilibrium condensates. The direct access to the phase of their wavefunction enables us to pursue the quest of whether half vortices ---rather than full integer vortices--- are the fundamental topological excitations of a spinor polariton fluid. Here, we are able to directly generate by resonant pulsed excitations, a polariton fluid carrying either the half or full vortex states as initial condition, and to follow their coherent evolution using ultrafast holography. Surprisingly we observe a rich phenomenology that shows a stable evolution of a phase singularity in a single component as well as in the full vortex state, spiraling, splitting and branching of the initial cores under different regimes and the proliferation of many vortex anti-vortex pairs in self generated circular ripples. This allows us to devise the interplay of nonlinearity and sample disorder in shaping the fluid and driving the phase singularities dynamicsComment: New version complete with revised modelization, discussion and added material. 8 pages, 7 figures. Supplementary videos: https://drive.google.com/folderview?id=0B0QCllnLqdyBfmc2ai0yVF9fa2g2VnZodGUwemVkLThBb3BoOVRKRDJMS2dUdjlZdkRTQk

Coherently driven microcavity-polaritons and the question of superfluidity

M.H.S. acknowledges financial support from EPSRC (Grants no. EP/I028900/2 and no. EP/K003623/2) and J.K. from EPSRC program Hybrid Polaritonics (EP/M025330/1).Due to their driven-dissipative nature, photonic quantum fluids present new challenges in understanding superfluidity. Some associated effects have been observed, and notably the report of nearly dissipationless flow for coherently driven microcavity-polaritons was taken as a 'smoking gun' for superflow. Here we show that the superfluid response - the difference between responses to longitudinal and transverse forces - is zero for coherently driven polaritons. This is a direct consequence of the gapped excitation spectrum caused by external phase locking. Furthermore, while a normal component exists at finite pump momentum, the remainder forms a rigid state that does not respond to either longitudinal or transverse perturbations. Interestingly, the total response almost vanishes when the real part of the excitation spectrum has a linear dispersion at low frequency, characteristic of equilibrium bosonic superfluids, which was the regime investigated experimentally. These results suggest that the observed suppression of scattering should be interpreted as a sign of this new rigid state and not of a superfluid.Publisher PDFPeer reviewe

Adiabatic Phase Diagram of an Ultracold Atomic Fermi Gas with a Feshbach Resonance

We determine the adiabatic phase diagram of a resonantly-coupled system of Fermi atoms and Bose molecules confined in the harmonic trap by using the local density approximation. The adiabatic phase diagram shows the fermionic condensate fraction composed of condensed molecules and Cooper pair atoms. The key idea of our work is conservation of entropy through the adiabatic process, extending the study of Williams et al. [Williams et al., New J. Phys. 6, 123 (2004)] for an ideal gas mixture to include the resonant interaction in a mean-field theory. We also calculate the molecular conversion efficiency as a function of initial temperature. Our work helps to understand recent experiments on the BCS-BEC crossover, in terms of the initial temperature measured before a sweep of the magnetic field.Comment: 13 pages, 8 figures. In press, "Journal of the Physical Society of Japan", Vol.76, No.