Photonic-integrated circuits have emerged as a scalable platform for complex quantum systems. A central goal is to integrate single-photon detectors to reduce optical losses, latency and wiring complexity associated with off-chip detectors. Superconducting nanowire single-photon detectors (SNSPDs) are particularly attractive because of high detection efficiency, sub-50-ps jitter and nanosecond-scale reset time. However, while single detectors have been incorporated into individual waveguides, the system detection efficiency of multiple SNSPDs in one photonic circuit—required for scalable quantum photonic circuits—has been limited to <0.2%. Here we introduce a micrometer-scale flip-chip process that enables scalable integration of SNSPDs on a range of photonic circuits. Ten low-jitter detectors are integrated on one circuit with 100% device yield. With an average system detection efficiency beyond 10%, and estimated on-chip detection efficiency of 14–52% for four detectors operated simultaneously, we demonstrate, to the best of our knowledge, the first on-chip photon correlation measurements of non-classical light.

We report efficient single-photon detection (η = 20% at 1550 nm wavelength) with ultranarrow (20 and 30 nm wide) superconducting nanowires, which were shown to be more robust to constrictions and more responsive to 1550 nm wavelength photons than standard superconducting nanowire single-photon detectors, based on 90 nm wide nanowires. We also improved our understanding of the physics of superconducting nanowire avalanche photodetectors, which we used to increase the signal-to-noise ratio of ultranarrow-nanowire detectors by a factor of 4, thus relaxing the requirements on the read-out circuitry and making the devices suitable for a broader range of applications.

We report on superconducting nanowire single photon detectors (SNSPDs) based on 30 nm wide nanowires with detection efficiency η ∼ 2.6-5.5% in the wavelength range λ = 0.5-5 μm. We compared the sensitivity of 30 nm wide SNSPDs with the sensitivity of SNSPDs based on wider (85 and 50 nm wide) nanowires for λ = 0.5-5 μm. The detection efficiency of the detectors based on the wider nanowires became negligible at shorter wavelengths than the 30 nm wide SNSPDs. Our 30 nm wide SNSPDs showed 2 orders of magnitude higher detection efficiency (η ∼ 2%) up to longer wavelength (λ = 5 μm) than previously reported. On the basis of our simulations, we expect that by changing the optical coupling scheme and by integrating the detectors in an optical cavity, the detection efficiency of our detectors could be increased by a factor of ∼6.

Detecting spatial and temporal information of individual photons by using singlephoton-detector (SPD) arrays is critical to applications in spectroscopy, communication, biological imaging, astronomical observation, and quantum-information processing. Among the current SPDs 1 , detectors based on superconducting nanowires have outstanding performance 2 , but are limited in their ability to be integrated into large scale arrays due to the engineering difficulty of high-bandwidth cryogenic electronic readout [3][4][5][6][7][8] . Here, we address this problem by demonstrating a scalable single-photon imager using a single continuous photon-sensitive superconducting nanowire microwave-plasmon transmission line. By appropriately designing the nanowire's local electromagnetic environment so that the nanowire guides microwave plasmons, the propagating voltages signals generated by a photon-detection event were slowed down to ~ 2% of the speed of light. As a result, the time difference between arrivals of the signals at the two 2 ends of the nanowire naturally encoded the position and time of absorption of the photon. Thus, with only two readout lines, we demonstrated that a 19.7-mm-long nanowire meandered across an area of 286 μm × 193 μm was capable of resolving ~ 590 effective pixels while simultaneously recording the arrival times of photons with a temporal resolution of 50 ps. The nanowire imager presents a scalable approach to realizing high-resolution photon imaging in time and space. Main Text:Quantum and classical optics are currently limited by our ability to efficiently sense and process information about single photons. For example, to enhance the information-carrying capacity of a quantum channel 9 and improve security in quantum key distribution 10,11 , information is typically encoded in the position and arrival time of individual photons.Determining the spatial and temporal information of photons is currently accomplished by single-photon detector (SPD) arrays. Among existing SPD array technologies, the transition edge sensor (TES) and the microwave kinetic inductance detector (MKID) provide moderate spectral information but less impressive temporal resolution (e.g., the timing jitter is measured in nanoseconds for TESs 12 and microseconds for MKIDs 13 ). Photomultiplier tubes and singlephoton avalanche diodes have sub-1-ns timing jitter in the visible domain, but their detection performance deteriorates in the infrared, and scaling these technologies to large spatial arrays is challenging 1 . Improved timing performance of sub-20-ps timing jitter 14 and sub-10-ns recovery time 15 is possible with superconducting-nanowire single-photon detectors (SNSPDs), which also have been demonstrated to have near-unity detection efficiency 2 , less than 1 dark-count per second (cps) 16 , a wide spectral response from the visible to infrared 17 and greater than 100 cps 3 counting rate 18 . However, attempts to create arrays of SNSPDs have had limited success 3-8 .Traditional row-column rectangular pixel arra...

Superconducting nanowire single photon detectors (SNSPDs) promise to combine near-unity quantum efficiency with >100 megacounts per second rates, picosecond timing jitter, and sensitivity ranging from x-ray to mid-infrared wavelengths. However, this promise is not yet fulfilled, as superior performance in all metrics is yet to be combined into one device. The highest single-pixel detection efficiency and the widest bias windows for saturated quantum efficiency have been achieved in SNSPDs based on amorphous materials, while the lowest timing jitter and highest counting rates were demonstrated in devices made from polycrystalline materials. Broadly speaking, the amorphous superconductors that have been used to make SNSPDs have higher resistivities and lower critical temperature (T c ) values than typical polycrystalline materials. Here, we demonstrate a method of preparing niobium nitride (NbN) that has lower-than-typical superconducting transition temperature and higher-than-typical resistivity. As we will show, NbN deposited onto unheated SiO 2 has a low T c and high resistivity but is too rough for fabricating unconstricted nanowires, and T c is too low to yield SNSPDs that can operate well at liquid helium temperatures. By adding a 50 W RF bias to the substrate holder during sputtering, the T c of the unheated NbN films was increased by up to 73%, and the roughness was substantially reduced. After optimizing the deposition for nitrogen flow rates, we obtained 5 nm thick NbN films with a T c of 7.8 K and a resistivity of 253 lX cm. We used this bias sputtered room temperature NbN to fabricate SNSPDs. Measurements were performed at 2.5 K using 1550 nm light. Photon count rates appeared to saturate at bias currents approaching the critical current, indicating that the device's quantum efficiency was approaching unity. We measured a single-ended timing jitter of 38 ps. The optical coupling to these devices was not optimized; however, integration with front-side optical structures to improve absorption should be straightforward. This material preparation was further used to fabricate nanocryotrons and a large-area imager device, reported elsewhere. The simplicity of the preparation and promising device performance should enable future high-performance devices.