We present a quantum key distribution system with a 2.5 GHz repetition rate using a threestate time-bin protocol combined with a one-decoy approach. Taking advantage of superconducting single-photon detectors optimized for quantum key distribution and ultra low-loss fiber, we can distribute secret keys at a maximum distance of 421 km and obtain secret key rates of 6.5 bps over 405 km.

Recent progress in the development of superconducting nanowire single-photon detectors (SNSPDs) made of amorphous material has delivered excellent performances, and has had a great impact on a range of research fields. Despite showing the highest system detection efficiency (SDE) ever reported with SNSPDs, amorphous materials typically lead to lower critical currents, which impacts on their jitter performance. Combining a very low jitter and a high SDE remains a challenge. Here, we report on highly efficient superconducting nanowire single-photon detectors based on amorphous MoSi, combining system jitters as low as 26 ps and a SDE of 80% at 1550 nm. We also report detailed observations on the jitter behaviour, which hints at intrinsic limitations and leads to practical implications for SNSPD performance.

We demonstrate the violation of an EPR steering inequality developed for single photon path entanglement with displacement-based detection. We use a high-rate source of heralded single-photon path-entangled states, combined with high-efficiency superconducting-based detectors, in a scheme that is free of any post-selection and thus immune to the detection loophole. This result conclusively demonstrates single-photon entanglement in a one-sided device-independent scenario, and opens the way towards implementations of device-independent quantum technologies within the paradigm of path entanglement.Single-photon entanglement is not only one of the simplest forms of entanglement to generate, it is both fundamentally fascinating and potentially practical. At times its mere existence was debated [1], however, today it lies at the heart of key quantum information protocols, such as quantum repeaters [2]. Path entanglement is generated when a single photon is delocalized over several modes, or paths, e.g. via a 50/50 beam splitter, where it produces a state of the formwhere A and B denote the two entangled output modes. , and is the underlying resource for heralded photon amplification [7]. Another direction of interest is to use single-photon entanglement for demonstrations of quantum nonlocality and related device-independent applications. In particular, the combination of single-photon entanglement with weak displacement-based local measurements [8,9] was recently proposed as a practical platform for demonstrating loophole-free Bell-inequality violations [10,11] and device-independent protocols for quantum information processing [12]. Notably, this approach offers a promising alternative to standard setups based on two-photon entanglement, with clear practical advantages, such as the possibility of heralding the entanglement creation (at high rates, e.g. compared to atomic systems [12][13][14][15]) and scalability to networks involving more parties [16].Here we report the observation of EPR steering via local weak displacements performed on single-photon entanglement, as sketched in FIG. 1 box while the other is trusted [19]. In the following, we first theoretically develop a steering test (a so-called steering inequality [20]) tailored to our setup. We then present an experimental violation of our steering inequality by 4 standard deviations, using a heralded single photon source and an all-fiber displacement-based measurement scheme featuring high-efficiency superconducting nanowire singlephoton detectors. As our setup is inherently free of any post-selection, it is immune to the detection loophole [21]. Our experiment thus provides the conclusive demonstration of single-photon path entanglement in a one-sided DI scenario. Moreover, our approach is directly extensible to a loophole-free Bell-inequality test, and thus to the implementation of fully DI protocols [12,22].Steering. In a generic steering experiment, as in FIG. 1, two separate parties (Alice and Bob) perform local measurements on a shared entangled stat...

We experimentally investigate the detection mechanism in a meandered molybdenum silicide (MoSi) superconducting nanowire single-photon detector by characterising the detection probability as a function of bias current in the wavelength range of 750 to 2050 nm. Contrary to some previous observations on niobium nitride (NbN) or tungsten silicide (WSi) detectors, we find that the energycurrent relation is nonlinear in this range. Furthermore, thanks to the presence of a saturated detection efficiency over the whole range of wavelengths, we precisely quantify the shape of the curves. This allows a detailed study of their features, which are indicative of both Fano fluctuations and position-dependent effects.

Recent progress in the development of superconducting nanowire single-photon detectors (SNSPD) has delivered excellent performance, and their increased adoption has had a great impact on a range of applications. One of the key characteristic of SNSPDs is their detection rate, which is typically higher than other types of free-running single-photon detectors. The maximum achievable rate is limited by the detector recovery time after a detection, which itself is linked to the superconducting material properties and to the geometry of the meandered SNSPD. Arrays of detectors biased individually can be used to solve this issue, but this approach significantly increases both the thermal load in the cryostat and the need for time processing of the many signals, and this scales unfavorably with a large number of detectors. One potential scalable approach to increase the detection rate of individual detectors further is based on parallelizing smaller meander sections. In this way, a single detection temporarily disables only one subsection of the whole active area, thereby leaving the overall detection efficiency mostly unaffected. In practice however, cross-talk between parallel nanowires typically leads to latching, which prevents high detection rates. Here we show how this problem can be avoided through a careful design of the whole SNSPD structure. Using the same electronic readout as with conventional SNSPDs and a single coaxial line, we demonstrate detection rates over 200 MHz without any latching, and a fibre-coupled SDE as high as 77%, and more than 50% average SDE per photon at 50 MHz detection rate under continuous wave illumination.