An on-demand source of indistinguishable and entangled photon pairs is a fundamental component for different quantum information applications such as optical quantum computing, quantum repeaters, quantum teleportation and quantum communication 1 . Parametric downconversion 2, 3 and four-wave mixing sources 4 of entangled photons have shown high degrees of entanglement and indistinguishability but the probabilistic nature of their generation process also creates zero or multiple photon pairs following a Poissonian distribution. This limits their use in complex algorithms where many qubits and gate operations are required. Here we show simultaneously ultrahigh purity (g (2) (0) < 0.004), high entanglement fidelity (0.81 ± 0.02), high two-photon interference non-post selective visibilities (0.86 ± 0.03 and 0.71 ± 0.04) and on-demand generation of polarization-entangled photon pairs from a single semiconductor quantum dot (QD). Through a twophoton resonant excitation scheme, the biexciton population is deterministically prepared by a π-pulse. Applied on a quantum dot showing no exciton fine structure splitting, this results in the deterministic generation of indistinguishable entangled photon pairs.To date, spontaneous parametric down-conversion (SPDC) and four wave mixing sources have been mostly used for the generation of entangled photon pairs to realize quantum communication protocols and to demonstrate basic quantum logic experiments 5 . However, the photon pair statistics of these sources is described by a Poissonian distribution which implies also the generation of zero and multiple pairs. This leads to errors in the quantum algorithm protocols 6 which effectively limit their usefulness for deterministic quantum technologies. Radiative cascades in single quantum emitters, such as atoms 7 or quantum dots 8 , can in principle emit on demand single pairs of polarization-entangled photons with high generation efficiencies 9 . After optical excitation of two electron-hole pairs (biexciton, called |XX state) in a quantum dot, the biexciton decays through a two-photon cascade ( Fig. 1a). If the fine structure splitting between the intermediate states (excitons called |X ) is smaller than the radiative linewidth, the two decay paths are indistinguishable and the two photons are polarization-entangled which results in a two-photon Bell state |ψ + = 1 √ 2 (|HXX |HX + |VXX |VX ). To ensure the emission of a single pair of entangled photons per excitation pulse the biexcitonic state has to be pumped into saturation. So far, non-resonant pulsed pumping schemes have been successfully applied for entangled photon generation 10, 11 but no simultaneous information on indistinguishability has been provided. Anyhow, it is well known that non-resonant pumping schemes limit the coherence and indistinguishability of the emitted photons making them unfeasible for many quantum information applications. In a recent study, Stevenson and co-workers reported interference and entanglement properties of photons emitted by a QD embedded wi...

True on-demand high-repetition-rate single-photon sources are highly sought after for quantum information processing applications. However, any coherently driven two-level quantum system suffers from a finite re-excitation probability under pulsed excitation, causing undesirable multi-photon emission. Here, we present a solid-state source of on-demand single photons yielding a raw second-order coherence of g (2) (0) = (7.5 ± 1.6) × 10 −5 without any background subtraction nor data processing. To this date, this is the lowest value of g (2) (0) reported for any single-photon source even compared to the previously best background subtracted values. We achieve this result on GaAs/AlGaAs quantum dots embedded in a low-Q planar cavity by employing (i) a two-photon excitation process and (ii) a filtering and detection setup featuring two superconducting single-photon detectors with ultralow dark-count rates of (0.0056 ± 0.0007) s −1 and (0.017 ± 0.001) s −1 , respectively. Re-excitation processes are dramatically suppressed by (i), while (ii) removes false coincidences resulting in a negligibly low noise floor.

A bright photon source that combines high-fidelity entanglement, on-demand generation, high extraction efficiency, directional and coherent emission, as well as position control at the nanoscale is required for implementing ambitious schemes in quantum information processing, such as that of a quantum repeater. Still, all of these properties have not yet been achieved in a single device. Semiconductor quantum dots embedded in nanowire waveguides potentially satisfy all of these requirements; however, although theoretically predicted, entanglement has not yet been demonstrated for a nanowire quantum dot. Here, we demonstrate a bright and coherent source of strongly entangled photon pairs from a position-controlled nanowire quantum dot with a fidelity as high as 0.859±0.006 and concurrence of 0.80±0.02. The two-photon quantum state is modified via the nanowire shape. Our new nanoscale entangled photon source can be integrated at desired positions in a quantum photonic circuit, single-electron devices and light-emitting diodes.

A major step toward fully integrated quantum optics is the deterministic incorporation of high quality single photon sources in on-chip optical circuits. We show a novel hybrid approach in which preselected III-V single quantum dots in nanowires are transferred and integrated in silicon based photonic circuits. The quantum emitters maintain their high optical quality after integration as verified by measuring a low multiphoton probability of 0.07 ± 0.07 and emission line width as narrow as 3.45 ± 0.48 GHz. Our approach allows for optimum alignment of the quantum dot light emission to the fundamental waveguide mode resulting in very high coupling efficiencies. We estimate a coupling efficiency of 24.3 ± 1.7% from the studied single-photon source to the photonic channel and further show by finite-difference time-domain simulations that for an optimized choice of material and design the efficiency can exceed 90%.