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.

Global, secure quantum channels will require efficient distribution of entangled photons. Long distance, low-loss interconnects can only be realized using photons as quantum information carriers. However, a quantum light source combining both high qubit fidelity and on-demand bright emission has proven elusive. Here, we show a bright photonic nanostructure generating polarization-entangled photon pairs that strongly violates Bell’s inequality. A highly symmetric InAsP quantum dot generating entangled photons is encapsulated in a tapered nanowire waveguide to ensure directional emission and efficient light extraction. We collect ~200 kHz entangled photon pairs at the first lens under 80 MHz pulsed excitation, which is a 20 times enhancement as compared to a bare quantum dot without a photonic nanostructure. The performed Bell test using the Clauser-Horne-Shimony-Holt inequality reveals a clear violation (S CHSH > 2) by up to 9.3 standard deviations. By using a novel quasi-resonant excitation scheme at the wurtzite InP nanowire resonance to reduce multi-photon emission, the entanglement fidelity (F = 0.817 ± 0.002) is further enhanced without temporal post-selection, allowing for the violation of Bell’s inequality in the rectilinear-circular basis by 25 standard deviations. Our results on nanowire-based quantum light sources highlight their potential application in secure data communication utilizing measurement-device-independent quantum key distribution and quantum repeater protocols.

Abstract-A monolithic circuit has been designed for active-quenching and active-reset of single-photon avalanche diodes (SPADs), which operate above the breakdown voltage BD for detecting single photons. To the best of our knowledge, this is the first fully integrated circuit of this kind ever reported. It can operate with any available SPAD device, since it generates pulses high enough to quench detectors biased up to 20 V above BD . The deadtime after each photon detection is adjustable; the minimum value is 50 ns, corresponding to 20 Mcounts/s maximum saturated photon-counting rate. The power dissipation is low (20-mW standby), suitable also for portable instruments. The small size and high reliability of the circuit make it possible to develop miniaturized detector modules and SPAD-array detector instruments. The circuit opens the path to new developments in many applications of photon counting, from DNA sequencing to ultrahigh-sensitivity imaging.Index Terms-Avalanche photodiodes, Geiger mode, photon counting, photodetectors, quenching circuits, single photon. Avalanche photodiodes (APDs) operate in a linear amplification mode, biased slightly below the breakdown voltage . Their gain is moderate (a few hundred at best) and affected by strong excess noise [7]; the detection of single photons is possible, but neither practical nor very efficient. Single-photon avalanche diodes (SPADs) are instead p-n junctions that operate in Geiger mode, biased well above the breakdown voltage. At such bias, the electric field in the depletion layer is so high that a single electron-hole pair generation can trigger a selfsustaining avalanche current in the milliampere range. Current keeps flowing until the avalanche is quenched by lowering the bias voltage below ; the SPAD must then be reset to the quiescent bias level, in order to detect subsequent photons. If the primary electron-hole pair is photogenerated, the avalanche onset marks the photon arrival time. The thermal generation of carriers randomly triggers the avalanche also without illumination (dark counts) and represents the internal detector noise. Silicon SPADs can be classified in two groups. Thin silicon SPADs [8] are planar devices with depletion layer of a few micrometers, low breakdown voltage (15-40 V), good detection efficiency (about 45% at 500-nm wavelength, 10% at 830 nm, and a few 0.1% at 1064 nm) and excellent time resolution (a few tens of picoseconds). Thick SPADs [9], [15] have reachthrough structure, depletion layer some tens of micrometers thick, high breakdown voltage (250-450 V), very good detection efficiency (higher than 50% in the 540-850-nm range and still some percent at 1064 nm) and moderate time resolution (typically, 350-ps full-width-at-half-maximum). I. SINGLE-PHOTON DETECTORSBy increasing the excess bias voltage, SPAD detection efficiency and the timing resolution are improved, but dark-counting rate and afterpulsing effects are increased [10] and a tradeoff must be established. The excess bias voltage employed thus ranges from a few ...

Low noise single-photon sources are a critical element for quantum technologies. We present a heralded single-photon source with an extremely low level of residual background photons, by implementing low-jitter detectors and electronics and a fast custom-made pulse generator controlling an optical shutter (a LiNbO3 waveguide optical switch) on the output of the source.\ud This source has a second-order autocorrelation g(2)(0) = 0.005, and an output noise factor (defined as the ratio of the number of noise photons to total photons at the source output channel)\ud of 0.25(1)%. These are the best performance characteristics reported to date

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