We present a loophole-free violation of local realism using entangled photon pairs. We ensure that all relevant events in our Bell test are spacelike separated by placing the parties far enough apart and by using fast random number generators and high-speed polarization measurements. A high-quality polarization-entangled source of photons, combined with high-efficiency, low-noise, single-photon detectors, allows us to make measurements without requiring any fair-sampling assumptions. Using a hypothesis test, we compute p-values as small as 5.9 × 10−9 for our Bell violation while maintaining the spacelike separation of our events. We estimate the degree to which a local realistic system could predict our measurement choices. Accounting for this predictability, our smallest adjusted p-value is 2.3 × 10−7. We therefore reject the hypothesis that local realism governs our experiment.

Introductory paragraphSingle-photon detectors (SPDs) 1 at near-infrared wavelengths with high system detection efficiency (> 90%), low dark count rate (< 1 counts per second, cps), low timing jitter (< 100 ps), and short reset time (< 100 ns) would enable landmark experiments in a variety of fields [2][3][4][5][6] . Although some of the existing approaches to single-photon detection fulfill one or two of the above specifications 1 , to date no detector has met all of the specifications simultaneously. Here we report on a fiber-coupled singlephoton-detection system employing superconducting nanowire single-photon detectors (SNSPDs) 7 that closely approaches the ideal performance of SPDs. Our detector system has a system detection efficiency (SDE), including optical-coupling losses, greater than 90% in the wavelength range λ = 1520 -1610 nm; device dark count rate (measured with the device shielded from room-temperature blackbody radiation) of ≈ 0.01 cps; timing jitter of ≈ 150 ps FWHM; and reset time of 40 ns.

Optical-to-electrical conversion, which is the basis of the operation of optical detectors, can be linear or nonlinear. When high sensitivities are needed, single-photon detectors are used, which operate in a strongly nonlinear mode, their response being independent of the number of detected photons. However, photon-number-resolving detectors are needed, particularly in quantum optics, where n-photon states are routinely produced. In quantum communication and quantum information processing, the photon-numberresolving functionality is key to many protocols, such as the implementation of quantum repeaters 1 and linear-optics quantum computing 2 . A linear detector with single-photon sensitivity can also be used for measuring a temporal waveform at extremely low light levels, such as in longdistance optical communications, fluorescence spectroscopy and optical time-domain reflectometry. We demonstrate here a photon-number-resolving detector based on parallel superconducting nanowires and capable of counting up to four photons at telecommunication wavelengths, with an ultralow dark count rate and high counting frequency.Among the approaches proposed so far for photon-numberresolving (PNR) detection (Table 1) are detectors based on charge integration or field-effect transistors 3-5 , which are, however, affected by long integration times, leading to bandwidths of ,1 MHz. Transition edge sensors 6 operate at 100 mK and show long response times (several microseconds). Approaches based on photomultipliers 7 and avalanche photodiodes, such as the visiblelight photon counter 8,9 , two-dimensional arrays of avalanche photodiodes 10,11 and time-multiplexed detectors 12,13 are not sensitive or are plagued by high dark count rates (DKs) and long dead times in the telecommunication spectral windows. Arrays of single-photon detectors (SPDs) also involve complex readout schemes 11 or separate contacts, amplification and discrimination 14. The parallel nanowire detector (PND) presented here significantly outperforms these approaches in terms of simplicity, sensitivity, speed and multiplication noise.The basic structure of the PND comprises the parallel connection of N superconducting nanowires, each connected in series to a resistor R 0 (Fig.

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.