[en] A system of cascaded qubits interacting via the one-way exchange of photons is studied. While for general operating conditions the system evolves to a superposition of Bell states (a dark state) in the long-time limit, under a particular resonance condition no steady state is reached within a finite time. We analyze the conditional quantum evolution (quantum trajectories) to characterize the asymptotic behavior under this resonance condition. A distinct bimodality is observed: for perfect qubit coupling, the system either evolves to a maximally entangled Bell state without emitting photons (the dark state) or executes a sustained entangled-state cycle--random switching between a pair of Bell states while emitting a continuous photon stream; for imperfect coupling, two entangled-state cycles coexist, between which a random selection is made from one quantum trajectory to another

[en] We calculate the weak-driving transmission of a linearly polarized cavity mode strongly coupled to the D2 transition of a single cesium atom. Results are relevant to future experiments with microtoroid cavities, where the single-photon Rabi frequency g exceeds the excited-state hyperfine splittings, and photonic band-gap resonators, where g is greater than both the excited- and ground-state splitting

[en] We study a generalized Dicke model, as recently realized in an atomic quantum gas experiment, describing the collective interaction of N two-level atoms with a single cavity mode. The model takes account of dissipation of the cavity field, and includes a nonlinear atom–photon coupling, not present in the conventional Dicke model. We extend previous theoretical investigations of a semiclassical model by including all quantum effects and considering finite atom number N. Our results show good agreement between quantum expectation values and the semiclassical model as N is increased, but also show exotic behaviour for the corresponding quantum state as the nonlinear atom–photon coupling is varied. We consider the cavity-field Wigner function, the atomic Q-function for the collective spin, and the entanglement between the cavity mode and the spin. (paper)

[en] We consider two different collective spin systems subjected to strong dissipation-on the same scale as interaction strengths and external fields-and show that either continuous or discontinuous dissipative quantum phase transitions can occur as the dissipation strength is varied. First, we consider a well-known model of cooperative resonance fluorescence that can exhibit a second-order quantum phase transition, and analyse the entanglement properties near the critical point. Next, we examine a dissipative version of the Lipkin-Meshkov-Glick interacting collective spin model, where we find that either first- or second-order quantum phase transitions can occur, depending only on the ratio of the interaction and external field parameters. We give detailed results and interpretation for the steady-state entanglement in the vicinity of the critical point, where it reaches a maximum. For the first-order transition we find that the semiclassical steady states exhibit a region of bistability. (fast track communication)

[en] We propose a time-delayed feedback control scheme for open quantum systems that can dramatically reduce the time to reach steady state. No measurement is performed in the feedback loop, and we suggest a simple all-optical implementation for a cavity QED system. We demonstrate the potential of the scheme by applying it to a driven and dissipative Dicke model, as recently realized in a quantum gas experiment. The time to reach steady state can be reduced by two orders of magnitude for the parameters taken from the experiment, making previously inaccessible long time attractors reachable within typical experimental run times. The scheme also offers the possibility of slowing down the dynamics, as well as qualitatively changing the phase diagram of the system. (paper)

[en] We present experimental observations of diffusion resonances for the quantum kicked rotor with weak decoherence. Cold caesium atoms are subject to a pulsed standing wave of near-resonant light, with spontaneous emission providing environmental coupling. The mean energy as a function of the pulse period is determined during the late-time diffusion period for a constant probability of spontaneous emission. Structure in the late-time energy is seen to increase with physical kicking strength. The observed structure is related to Shepelyansky's predictions for the initial quantum diffusion rates