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...

We investigate both experimentally and theoretically a simple yet more robust and flexible alternative to Rabi oscillation-type biexciton preparation protocols traditionally used for semiconductor quantum dots. The quantum dot is excited by a strong laser pulse positively detuned from the twophoton resonance yielding an on demand initialization of the biexciton state by making use of the phonon-induced thermalization of the photon dressed states. It is shown that for excitation pulses in the picosecond range, a stable and high fidelity of up to fXX = 0.98 ± 0.01 is reached. Notably, the generated photons show similar coherence properties as measured in the resonant two-photon scheme. This protocol is a powerful tool for the control of complex solid state systems combining radiative cascades, entanglement and resonant cavity modes.

The development of multinode quantum optical circuits has attracted great attention in recent years. In particular, interfacing quantum-light sources, gates, and detectors on a single chip is highly desirable for the realization of large networks. In this context, fabrication techniques that enable the deterministic integration of preselected quantum-light emitters into nanophotonic elements play a key role when moving forward to circuits containing multiple emitters. Here, we present the deterministic integration of an InAs quantum dot into a 50/50 multimode interference beamsplitter via in situ electron beam lithography. We demonstrate the combined emitter-gate interface functionality by measuring triggered single-photon emission on-chip with g(0) = 0.13 ± 0.02. Due to its high patterning resolution as well as spectral and spatial control, in situ electron beam lithography allows for integration of preselected quantum emitters into complex photonic systems. Being a scalable single-step approach, it paves the way toward multinode, fully integrated quantum photonic chips.

Epitaxial semiconductor quantum dots are particularly promising as realistic single-photon sources for their compatibility with manufacturing techniques and possibility to be implemented in compact devices. Here, we demonstrate for the first time single-photon emission up to room temperature from an epitaxial quantum dot inserted in a nanowire, namely a CdSe slice in a ZnSe nanowire. The exciton and biexciton lines can still be resolved at room temperature and the biexciton turns out to be the most appropriate transition for single-photon emission due to a large nonradiative decay of the bright exciton to dark exciton states. With an intrinsically short radiative decay time (≈300 ps) this system is the fastest room temperature single-photon emitter, allowing potentially gigahertz repetition rates.