Ultrabright source of entangled photon pairs.

A source of triggered entangled photon pairs is a key component in quantum information science; it is needed to implement functions such as linear quantum computation, entanglement swapping and quantum teleportation. Generation of polarization entangled photon pairs can be obtained through parametric conversion in nonlinear optical media or by making use of the radiative decay of two electron-hole pairs trapped in a semiconductor quantum dot. Today, these sources operate at a very low rate, below 0.01 photon pairs per excitation pulse, which strongly limits their applications. For systems based on parametric conversion, this low rate is intrinsically due to the Poissonian statistics of the source. Conversely, a quantum dot can emit a single pair of entangled photons with a probability near unity but suffers from a naturally very low extraction efficiency. Here we show that this drawback can be overcome by coupling an optical cavity in the form of a 'photonic molecule' to a single quantum dot. Two coupled identical pillars-the photonic molecule-were etched in a semiconductor planar microcavity, using an optical lithography method that ensures a deterministic coupling to the biexciton and exciton energy states of a pre-selected quantum dot. The Purcell effect ensures that most entangled photon pairs are emitted into two cavity modes, while improving the indistinguishability of the two optical recombination paths. A polarization entangled photon pair rate of 0.12 per excitation pulse (with a concurrence of 0.34) is collected in the first lens. Our results open the way towards the fabrication of solid state triggered sources of entangled photon pairs, with an overall (creation and collection) efficiency of 80%.

Bright Phonon-Tuned Single-Photon Source.

Bright single photon sources have recently been obtained by inserting solid-state emitters in microcavities. Accelerating the spontaneous emission via the Purcell effect allows both high brightness and increased operation frequency. However, achieving Purcell enhancement is technologically demanding because the emitter resonance must match the cavity resonance. Here, we show that this spectral matching requirement is strongly lifted by the phononic environment of the emitter. We study a single InGaAs quantum dot coupled to a micropillar cavity. The phonon assisted emission, which hardly represents a few percent of the dot emission at a given frequency in the absence of cavity, can become the main emission channel by use of the Purcell effect. A phonon-tuned single photon source with a brightness greater than 50% is demonstrated over a detuning range covering 10 cavity line widths (0.8 nm). The same concepts applied to defects in diamonds pave the way toward ultrabright single photon sources operating at room temperature.

Origin of optical losses in gallium arsenide disk whispering gallery resonators.

Whispering gallery modes in GaAs disk resonators reach half a million of optical quality factor. These high Qs remain still well below the ultimate design limit set by bending losses. Here we investigate the origin of residual optical dissipation in these devices. A Transmission Electron Microscope analysis is combined with an improved Volume Current Method to precisely quantify optical scattering losses by roughness and waviness of the structures, and gauge their importance relative to intrinsic material and radiation losses. The analysis also provides a qualitative description of the surface reconstruction layer, whose optical absorption is then revealed by comparing spectroscopy experiments in air and in different liquids. Other linear and nonlinear optical loss channels in the disks are evaluated likewise. Routes are given to further improve the performances of these miniature GaAs cavities.

Cavity-funneled generation of indistinguishable single photons from strongly dissipative quantum emitters.

We investigate theoretically the generation of indistinguishable single photons from a strongly dissipative quantum system placed inside an optical cavity. The degree of indistinguishability of photons emitted by the cavity is calculated as a function of the emitter-cavity coupling strength and the cavity linewidth. For a quantum emitter subject to strong pure dephasing, our calculations reveal that an unconventional regime of high indistinguishability can be reached for moderate emitter-cavity coupling strengths and high-quality factor cavities. In this regime, the broad spectrum of the dissipative quantum system is funneled into the narrow line shape of the cavity. The associated efficiency is found to greatly surpass spectral filtering effects. Our findings open the path towards on-chip scalable indistinguishable-photon-emitting devices operating at room temperature.

Polariton condensation in photonic molecules.

We report on polariton condensation in photonic molecules formed by two coupled micropillars. We show that the condensation process is strongly affected by the interaction with the cloud of uncondensed excitons and thus strongly depends on the exact localization of these excitons within the molecule. Under symmetric excitation conditions, condensation is triggered on both binding and antibinding polariton states of the molecule. On the opposite, when the excitonic cloud is injected in one of the two pillars, condensation on a metastable state is observed and a total transfer of the condensate into one of the micropillars can be achieved. Our results highlight the crucial role played by relaxation kinetics in the condensation process.

Interactions in confined polariton condensates.

We investigate the effect of interactions in zero-dimensional polariton condensates. The shape of the condensate wave function is shown to be modified by repulsive interactions with the reservoir of uncondensed excitons. In large micropillar cavities, when uncondensed excitons are located at the center, the condensate is ejected toward the pillar edges. The same effect results in the generation of optical traps in wire cavities. Once polariton condensates are spatially separated from the excitonic reservoir, spectral signatures of polariton-polariton interactions within the condensate are evidenced.

High frequency GaAs nano-optomechanical disk resonator.

Optomechanical coupling between a mechanical oscillator and light trapped in a cavity increases when the coupling takes place in a reduced volume. Here we demonstrate a GaAs semiconductor optomechanical disk system where both optical and mechanical energy can be confined in a subwavelength scale interaction volume. We observe a giant optomechanical coupling rate up to 100 GHz/nm involving picogram mass mechanical modes with a frequency between 100 MHz and 1 GHz. The mechanical modes are singled-out measuring their dispersion as a function of disk geometry. Their Brownian motion is optically resolved with a sensitivity of 10(-17) m/√Hz] at room temperature and pressure, approaching the quantum limit imprecision.

Extreme multiexciton emission from deterministically assembled single-emitter subwavelength plasmonic patch antennas.

Coupling nano-emitters to plasmonic antennas is a key milestone for the development of nanoscale quantum light sources. One challenge, however, is the precise nanoscale positioning of the emitter in the structure. Here, we present a laser etching protocol that deterministically positions a single colloidal CdSe/CdS core/shell quantum dot emitter inside a subwavelength plasmonic patch antenna with three-dimensional nanoscale control. By exploiting the properties of metal-insulator-metal structures at the nanoscale, the fabricated single-emitter antenna exhibits a very high-Purcell factor (>72) and a brightness enhancement of a factor of 70. Due to the unprecedented quenching of Auger processes and the strong acceleration of the multiexciton emission, more than 4 photons per pulse can be emitted by a single quantum dot, thus increasing the device yield. Our technology can be applied to a wide range of photonic nanostructures and emitters, paving the way for scalable and reliable fabrication of ultra-compact light sources.

High-performance semiconductor quantum-dot single-photon sources.

Single photons are a fundamental element of most quantum optical technologies. The ideal single-photon source is an on-demand, deterministic, single-photon source delivering light pulses in a well-defined polarization and spatiotemporal mode, and containing exactly one photon. In addition, for many applications, there is a quantum advantage if the single photons are indistinguishable in all their degrees of freedom. Single-photon sources based on parametric down-conversion are currently used, and while excellent in many ways, scaling to large quantum optical systems remains challenging. In 2000, semiconductor quantum dots were shown to emit single photons, opening a path towards integrated single-photon sources. Here, we review the progress achieved in the past few years, and discuss remaining challenges. The latest quantum dot-based single-photon sources are edging closer to the ideal single-photon source, and have opened new possibilities for quantum technologies.

A solid-state single-photon filter.

A strong limitation of linear optical quantum computing is the probabilistic operation of two-quantum-bit gates based on the coalescence of indistinguishable photons. A route to deterministic operation is to exploit the single-photon nonlinearity of an atomic transition. Through engineering of the atom-photon interaction, phase shifters, photon filters and photon-photon gates have been demonstrated with natural atoms. Proofs of concept have been reported with semiconductor quantum dots, yet limited by inefficient atom-photon interfaces and dephasing. Here, we report a highly efficient single-photon filter based on a large optical nonlinearity at the single-photon level, in a near-optimal quantum-dot cavity interface. When probed with coherent light wavepackets, the device shows a record nonlinearity threshold around 0.3 ± 0.1 incident photons. We demonstrate that 80% of the directly reflected light intensity consists of a single-photon Fock state and that the two- and three-photon components are strongly suppressed compared with the single-photon one.

Macroscopic rotation of photon polarization induced by a single spin.

Entangling a single spin to the polarization of a single incoming photon, generated by an external source, would open new paradigms in quantum optics such as delayed-photon entanglement, deterministic logic gates or fault-tolerant quantum computing. These perspectives rely on the possibility that a single spin induces a macroscopic rotation of a photon polarization. Such polarization rotations induced by single spins were recently observed, yet limited to a few 10(-3) degrees due to poor spin-photon coupling. Here we report the enhancement by three orders of magnitude of the spin-photon interaction, using a cavity quantum electrodynamics device. A single hole spin in a semiconductor quantum dot is deterministically coupled to a micropillar cavity. The cavity-enhanced coupling between the incoming photons and the solid-state spin results in a polarization rotation by ± 6° when the spin is optically initialized in the up or down state. These results open the way towards a spin-based quantum network.

Optical bistability in a GaAs-based polariton diode.

We report on a new type of optical nonlinearity in a polariton p-i-n microcavity. Abrupt switching between the strong and weak coupling regime is induced by controlling the electric field within the cavity. As a consequence, bistable cycles are observed for low optical powers (2-3 orders of magnitude less than for Kerr induced bistability). Signatures of switching fronts propagating through the whole 300 x 300 microm2 mesa surface are evidenced.

Polariton laser using single micropillar GaAs-GaAlAs semiconductor cavities.

Polariton lasing is demonstrated on the zero-dimensional states of single GaAs/GaAlAs micropillar cavities. Under nonresonant excitation, the measured polariton ground-state occupancy is found as large as 10(4). Changing the spatial excitation conditions, competition between several polariton lasing modes is observed, ruling out Bose-Einstein condensation. When the polariton state occupancy increases, the emission blueshift is the signature of self-interaction within the half-light half-matter polariton lasing mode.