Single-photon detectors based on superconducting nanowires (SSPDs or SNSPDs) have rapidly emerged as a highly promising photon counting technology for infrared wavelengths. These devices offer high efficiency, low dark counts and excellent timing resolution. In this Review, we consider the basic SNSPD operating principle and models of device behaviour. We give an overview of the evolution of SNSPD device design and the improvements in performance which have been achieved. We also evaluate device limitations and noise mechanisms. We survey practical refrigeration technologies and optical coupling schemes for SNSPDs. Finally we summarise promising application areas, ranging from quantum cryptography to remote sensing. Our goal is to capture a detailed snapshot of an emerging superconducting detector technology on the threshold of maturity.

Integrated quantum optics promises to enhance the scale and functionality of quantum technologies, and has become a leading platform for the development of complex and stable quantum photonic circuits. Here, we report path-entangled photon-pair generation from two distinct waveguide sources, the manipulation of these pairs, and their resulting high-visibility quantum interference, all on a single photonic chip. Degenerate and non-degenerate photon pairs were created via the spontaneous four-wave mixing process in the silicon-on-insulator waveguides of the device. We manipulated these pairs to exhibit on-chip quantum interference with visibility as high as 100.0 ± 0.4%. Additionally, the device can serve as a two-spatial-mode source of photon-pairs: we measured Hong-Ou-Mandel interference, off-chip, with visibility up to 95 ± 4%. Our results herald the next generation of monolithic quantum photonic circuits with integrated sources, and the new levels of complexity they will offer.

Improvement in secure transmission of information is an urgent need for governments, corporations and individuals. Quantum key distribution (QKD) promises security based on the laws of physics and has rapidly grown from proof-of-concept to robust demonstrations and deployment of commercial systems. Despite these advances, QKD has not been widely adopted, and large-scale deployment will likely require chip-based devices for improved performance, miniaturization and enhanced functionality. Here we report low error rate, GHz clocked QKD operation of an indium phosphide transmitter chip and a silicon oxynitride receiver chip—monolithically integrated devices using components and manufacturing processes from the telecommunications industry. We use the reconfigurability of these devices to demonstrate three prominent QKD protocols—BB84, Coherent One Way and Differential Phase Shift—with performance comparable to state-of-the-art. These devices, when combined with integrated single photon detectors, pave the way for successfully integrating QKD into future telecommunications networks.

This paper highlights a significant advance in time-of-flight depth imaging: by using a scanning transceiver which incorporated a freerunning, low noise superconducting nanowire single-photon detector, we were able to obtain centimeter resolution depth images of low-signature objects in daylight at stand-off distances of the order of one kilometer at the relatively eye-safe wavelength of 1560 nm. The detector used had an efficiency of 18% at 1 kHz dark count rate, and the overall system jitter was ~100 ps. The depth images were acquired by illuminating the scene with an optical output power level of less than 250 µW average, and using per-pixel dwell times in the millisecond regime.

Photon sources are fundamental components for any quantum photonic technology. The ability to generate high count-rate and low-noise correlated photon pairs via spontaneous parametric down-conversion using bulk crystals has been the cornerstone of modern quantum optics. However, future practical quantum technologies will require a scalable integration approach, and waveguide-based photon sources with high-count rate and low-noise characteristics will be an essential part of chip-based quantum technologies. Here, we demonstrate photon pair generation through spontaneous four-wave mixing in a silicon micro-ring resonator, reporting separately a maximum coincidence-to-accidental (CAR) ratio of 602 ± 37 (for a generation rate of 827kHz), and a maximum photon pair generation rate of 123 MHz ± 11 kHz (with a CAR value of 37). To overcome freecarrier related performance degradations we have investigated reverse biased p-i-n structures, demonstrating an improvement in the pair generation rate by a factor of up to 2 with negligible impact on CAR.

Integrated photonics has enabled much progress towards quantum technologies. Many applications, e.g., quantum communication, sensing, and distributed cloud quantum computing, require coherent photonic interconnection between separate on--chip subsystems. Large--scale quantum computing architectures and systems may ultimately require quantum interconnects to enable scaling beyond the limits of a single wafer, and towards multi--chip systems. However, coherently connecting separate chips remains a challenge, due to the fragility of entangled quantum states. The distribution and manipulation of entanglement between multiple integrated devices is one of the strictest requirements of these systems. Here, we report the first quantum photonic interconnect, demonstrating high--fidelity entanglement distribution and manipulation between two separate photonic chips, implemented using state--of--the--art silicon photonics. Path--entangled states are generated on one chip, and distributed to another chip by interconverting between path and polarization degrees of freedom, via a two--dimensional grating coupler on each chip. This path--to--polarization conversion allows entangled quantum states to be coherently distributed. We use integrated state analyzers to confirm a Bell--type violation of S=2.638±0.039 between the two chips. With further improvements in loss, this quantum photonic interconnect will provide new levels of flexibility in quantum systems and architectures.

Integrated quantum photonics is a promising approach for future practical and large-scale quantum information processing technologies, with the prospect of on-chip generation, manipulation and measurement of complex quantum states of light. The gallium arsenide (GaAs) material system is a promising technology platform, and has already successfully demonstrated key components including waveguide integrated single-photon sources and integrated single-photon detectors. However, quantum circuits capable of manipulating quantum states of light have so far not been investigated in this material system. Here, we report GaAs photonic circuits for the manipulation of single-photon and two-photon states. Two-photon quantum interference with a visibility of 94.9±1.3% was observed in GaAs directional couplers. Classical and quantum interference fringes with visibilities of 98.6±1.3% and 84.4±1.5% respectively were demonstrated in Mach-Zehnder interferometers exploiting the electro-optic Pockels effect. This work paves the way for a fully integrated quantum technology platform based on the GaAs material system.

Conventional imaging systems rely upon illumination light that is scattered or transmitted by the object and subsequently imaged. Ghost-imaging systems based on parametric downconversion use twin beams of position-correlated signal and idler photons. One beam illuminates an object while the image information is recovered from a second beam that has never interacted with the object. In this Letter, we report on a camera-based ghost imaging system where the correlated photons have significantly different wavelengths. Infrared photons at 1550 nm wavelength illuminate the object and are detected by an InGaAs/InP single-photon avalanche diode. The image data are recorded from the coincidently detected, position-correlated, visible photons at a wavelength of 460 nm using a highly efficient, low-noise, photon-counting camera. The efficient transfer of the image information from infrared illumination to visible detection wavelengths and the ability to count single photons allows the acquisition of an image while illuminating the object with an optical power density of only 100 pJ cm −2 s −1 . This wavelengthtransforming ghost-imaging technique has potential for the imaging of light-sensitive specimens or where covert operation is desired. Low-light-level imaging at infrared wavelengths has many applications within both the technological and biological sectors. These applications span covert security systems, the imaging of light-sensitive biological samples, and imaging within semiconductor devices. However, given that the majority of single-photon-sensitive, large-format detector arrays are siliconbased and therefore ineffective at wavelengths greater than 1 μm, the technological difficulties with such applications are readily apparent: crafting a camera with high quantum efficiency and low noise at infrared wavelengths is difficult and expensive.In this Letter, we circumvent the lack of infrared cameras that combine low-noise with single-photon sensitivity by performing the imaging using the so-called "ghost imaging" method. This method utilizes the spatial correlations between photons in the two output beams, signal and idler, generated through the spontaneous parametric down-conversion (SPDC) process [1].In the 1990s, it was shown how the correlations between photons generated through SPDC could be utilized to create imaging systems [2,3]. These ghost-imaging systems rely on the strong position correlations between the beams of signal and idler photons that are produced by the SPDC process [4]. In a ghost-imaging system a transmissive object is placed in the idler beam and the transmitted photons are measured using a single-element, heralding detector. The use of a single-element detector means that measurements of photons that probe the object reveal no spatial information. In parallel to these measurements of the idler photons, a scanning single-element detector measures the corresponding signal photons-but since these signal photons do not interact with the object, again no image is formed. However, although the ...