Cavity quantum electrodynamics (QED) studies the interaction between a quantum emitter and a single radiation-field mode. When an atom is strongly coupled to a cavity mode, it is possible to realize important quantum information processing tasks, such as controlled coherent coupling and entanglement of distinguishable quantum systems. Realizing these tasks in the solid state is clearly desirable, and coupling semiconductor self-assembled quantum dots to monolithic optical cavities is a promising route to this end. However, validating the efficacy of quantum dots in quantum information applications requires confirmation of the quantum nature of the quantum-dot-cavity system in the strong-coupling regime. Here we find such confirmation by observing quantum correlations in photoluminescence from a photonic crystal nanocavity interacting with one, and only one, quantum dot located precisely at the cavity electric field maximum. When off-resonance, photon emission from the cavity mode and quantum-dot excitons is anticorrelated at the level of single quanta, proving that the mode is driven solely by the quantum dot despite an energy mismatch between cavity and excitons. When tuned to resonance, the exciton and cavity enter the strong-coupling regime of cavity QED and the quantum-dot exciton lifetime reduces by a factor of 145. The generated photon stream becomes antibunched, proving that the strongly coupled exciton/photon system is in the quantum regime. Our observations unequivocally show that quantum information tasks are achievable in solid-state cavity QED.

We theoretically investigate the optical response of a one-dimensional array of strongly nonlinear optical microcavities. When the optical nonlinearity is much larger than both losses and intercavity tunnel coupling, the nonequilibrium steady state of the system is reminiscent of a strongly correlated Tonks-Girardeau gas of impenetrable bosons. Signatures of strong correlations are identified in the transmission spectrum of the system, as well as in the intensity correlations of the transmitted light. Possible experimental implementations in state-of-the-art solid-state devices are discussed.

We present a theory of the quantum vacuum radiation that is generated by a fast modulation of the vacuum Rabi frequency of a single two-level system strongly coupled to a single cavity mode. The dissipative dynamics of the Jaynes-Cummings model in the presence of anti-rotating wave terms is described by a generalized master equation including non-Markovian terms. Peculiar spectral properties and significant extracavity quantum vacuum radiation output are predicted for state-of-the-art circuit cavity quantum electrodynamics systems with superconducting qubits

According to a recent proposal [S. Takayama et al., Appl. Phys. Lett. 87, 061107 (2005)], the triangular lattice of triangular air holes may allow to achieve a complete photonic band gap in two-dimensional photonic crystal slabs. In this work we present a systematic theoretical study of this photonic lattice in a high-index membrane, and a comparison with the conventional triangular lattice of circular holes, by means of the guided-mode expansion method whose detailed formulation is described here. Photonic mode dispersion below and above the light line, gap maps, and intrinsic diffraction losses of quasi-guided modes are calculated for the periodic lattice as well as for line-and point-defects defined therein. The main results are summarized as follows: (i) the triangular lattice of triangular holes does indeed have a complete photonic band gap for the fundamental guided mode, but the useful region is generally limited by the presence of second-order waveguide modes; (ii) the lattice may support the usual photonic band gap for even modes (quasi-TE polarization) and several band gaps for odd modes (quasi-TM polarization), which could be tuned in order to achieve doubly-resonant frequency conversion between an even mode at the fundamental frequency and an odd mode at the second-harmonic frequency; (iii) diffraction losses of quasi-guided modes in the triangular lattices with circular and triangular holes, and in line-defect waveguides or pointdefect cavities based on these geometries, are comparable. The results point to the interest of the triangular lattice of triangular holes for nonlinear optics, and show the usefulness of the guidedmode expansion method for calculating photonic band dispersion and diffraction losses, especially for higher-lying photonic modes.

We show that photons in two tunnel-coupled microwave resonators each containing a single superconducting qubit undergo a sharp non-equilibrium delocalization-localization (self-trapping) transition due to strong photon-qubit coupling. We find, that self-trapping of photons in one of the resonators (spatial localization) forces the qubit in the opposite resonator to remain in its initial state (energetic localization). This allows for an easy experimental observation of the transition by local read-out of the qubit state. Dissipation of photons and decoherence of the qubit favor the self-trapped regime.

The photon-blockade effect, where nonlinearities at the single-photon level alter the quantum statistics of light emitted from a cavity 1 , has been observed in cavity quantum electrodynamics experiments with atomic 2,3 and solid-state systems 4-8 . Motivated by the success of single-cavity quantum electrodynamics experiments, the focus has recently shifted to the exploration of the rich physics promised by strongly correlated quantum-optical systems in multicavity and extended photonic media 9-14 . Even though most cavity quantum electrodynamics structures are inherently dissipative, most of the early work on strongly correlated photonic systems has assumed cavity structures where losses are essentially negligible. Here we investigate a dissipative quantum-optical system that consists of two coherently driven linear optical cavities connected through a central cavity with a single-photon nonlinearity (an optical analogue of the Josephson interferometer). The interplay of tunnelling and interactions is analysed in the steady state of the system, when a dynamical equilibrium between driving and losses is established. Strong photonic correlations can be identified through the suppression of Josephson-like oscillations of the light emitted from the central cavity as the nonlinearity is increased. In the limit of a single nonlinear cavity coupled to two linear waveguides, we show that photon-correlation measurements would provide a unique probe of the crossover to the strongly correlated regime.We investigate an optical analogue of the superconducting Josephson interferometer, which we name the quantum-optical Josephson interferometer, revealing new features due to the genuine non-equilibrium interplay of coherent tunnelling and on-site interactions. We consider two variants of the proposed device with a central nonlinear cavity coupled to two external driving lasers through either two side cavities (Fig. 1a,b) or two waveguides (Fig. 1d). The three-cavity system can be generalized to an N -cavity system with a central nonlinear one 14 (Fig. 1c), and in the limiting case of very large N this reduces to the single cavity coupled to two side waveguides (Fig. 1d). In both cases, the coupling to the side cavities (or waveguides) is a consequence of photon tunnelling. We assume the central cavity to have a sizable single-photon nonlinearity, for example due to some radiation-matter interaction, be it Jaynes-Cummings-type interaction (with a single atom or quantum dot in the central cavity) 15 , giant Kerr nonlinearity 1 or confined polariton interaction (for example with a quantum well embedded in the cavity) 16 . The model discussed here is fairly general and can be realized in a variety of quantum-optical systems. In the following we show that the light emitted from the central cavity reflects the interplay of two competing effects, tunnelling and interactions. As the relative magnitude of the interaction parameter is varied with respect to the tunnelling strength, the system shows a crossover between a coherent and a ...

We report an experimental study of superfluid hydrodynamic effects in a one-dimensional polariton fluid flowing along a laterally patterned semiconductor microcavity and hitting a micron-sized engineered defect. At high excitation power, superfluid propagation effects are observed in the polariton dynamics; in particular, a sharp acoustic horizon is formed at the defect position, separating regions of sub- and supersonic flow. Our experimental findings are quantitatively reproduced by theoretical calculations based on a generalized Gross-Pitaevskii equation. Promising perspectives to observe Hawking radiation via photon correlation measurements are illustrated.

Artificial neural networks are the heart of machine learning algorithms and artificial intelligence protocols. Historically, the simplest implementation of an artificial neuron traces back to the classical Rosenblatt's "perceptron", but its long term practical applications may be hindered by the fast scaling up of computational complexity, especially relevant for the training of multilayered perceptron networks. Here we introduce a quantum information-based algorithm implementing the quantum computer version of a perceptron, which shows exponential advantage in encoding resources over alternative realizations. We experimentally test a few qubits version of this model on an actual small-scale quantum processor, which gives remarkably good answers against the expected results. We show that this quantum model of a perceptron can be used as an elementary nonlinear classifier of simple patterns, as a first step towards practical training of artificial quantum neural networks to be efficiently implemented on near-term quantum processing hardware.