A linear-optics quantum computer 5 requires hundreds to thousands of single-photon components including sources, detectors and interferometers, which is obviously only feasible in an integrated circuit.Even the small-scale circuits needed in quantum repeaters 2 would greatly benefit from monolithic integration in view of the improved stability and coupling efficiency attainable in a chip. A very large experimental research activity has been dedicated to the development of single-photon sources based on III-V semiconductors 10 , in view of large-scale integration, and to passive quantum circuits based on silica-onsilicon 6 and on laser-micromachined glass 8,9 , but a clear approach towards a fully integrated photonic network including sources and detectors has not been proposed. This is in large part due to the complexity of most single-photon detector technologies -for example, the complex device structures associated to avalanche photodiodes are not easily compatible with the integration of low-loss waveguides and even less of sources. Transition-edge sensors may be suited for integration 11 , but they are plagued by very slow response times (leading to maximum counting rates in the tens of kHz range) and require cooling down to <100 mK temperatures. Here we propose a platform for the full integration of quantum photonic components on the same chip. It is based on the mature III-V semiconductor technology and comprises ( Fig. 1(a) ) waveguide single-photon sources based on InAs quantum dots (QDs), GaAs/AlGaAs ridge waveguides, Mach-Zehnder interferometers using directional couplers or multimode-interference couplers, and 3 waveguide detectors based on superconducting nanowires. Efficient single-photon emission from QDs in a waveguide can be obtained by using photonic crystals (PhCs), e.g. in a cavity side-coupled to a waveguide 12 or using the slow-light regime in PhC waveguides 13 , and the photons can then be transferred to ridge waveguides using tapers. Photons emitted by distinct QDs can be made indistinguishable by using electric fields to control the exciton energy 14 . The high index contrast available in the GaAs/AlGaAs system allows circuits with short bending radii, therefore more compact than in the silica platform 6 , while the large electrooptic coefficient of GaAs enables compact modulators operating at GHz frequencies. In this letter we report the key missing component, a single-photon detector integrated with GaAs waveguides. Our waveguide single-photon detectors (WSPDs) are based on the principle of photon-induced hot-spot creation in ultranarrow superconducting NbN wires, which is also used in nanowire superconducting single-photon detectors 15 (SSPDs) and can provide ultrahigh sensitivity at telecommunication wavelengths, high counting rates, broad spectral response and high temporal resolution due to low jitter values. In our design (see Fig. 1(b)), the wires are deposited and patterned on top of a GaAs ridge waveguide, in order to sense the evanescent field on the surface. Four NbN nanowi...

We report an experimental test of the photodetection mechanism in a nanowire superconducting single photon detector. Detector tomography allows us to explore the 0.8-8 eV energy range via multiphoton excitations. High accuracy results enable a detailed comparison of the experimental data with theories for the mechanism of photon detection. We show that the temperature dependence of the efficiency of the superconducting single photon detector is determined not by the critical current but by the current associated with vortex unbinding. We find that both quasiparticle diffusion and vortices play a role in the detection event. DOI: 10.1103/PhysRevLett.112.117604 PACS numbers: 79.20.Ws, 03.65.Wj, 85.25.Oj Nanowire superconducting single photon detectors (SSPDs or SNSPDs) [1,2] are currently the most promising detection systems in the infrared, achieving detection efficiencies of up to 93% at 1550 nm [3]. Despite these technological advances, the fundamentals of the working principle of these detectors are poorly understood and under active investigation, both theoretically [4][5][6][7][8][9][10][11] and experimentally [12][13][14][15][16][17][18][19][20][21][22].A typical SSPD consists of a few nm thin film of a superconducting material such as NbN or WSi, nanofabricated into a meandering wire geometry. When biased sufficiently close to the critical current of the superconductor, the energy of one or several photons can be enough to trigger a local transition to the resistive state, resulting in a detection event. The energy of the absorbed photon is distributed through an avalanchelike process, creating a nonequlibrium population of quasiparticles. This quasiparticle population then disrupts the supercurrent flow, resulting eventually in a detection event.In this Letter, we address the nature of this disruption, which lies at the heart of the photodetection mechanism in SSPDs. At present, there are three important open questions. First, it is unknown whether the detection event occurs when the energy of the incident photon causes a cylindrical volume inside the wire to transition to the normal state [see Fig. 1(a)] [1], or whether it is enough for the superconductivity to be weakened but not destroyed by the depletion of Cooper pairs over a more extended region [see Fig. 1The second open question is whether magnetic vortices play any role in the detection mechanism. There are two varieties of vortex-based models. The first is an extension of the normal-core model, where, a vortex-antivortex pair forms at the point where the photon is absorbed [ Fig. 1(c)] [5]. In the second, the weakening of superconductivity lowers the energy barrier for either a vortex crossing [6,23] or a vortex-antivortex pair crossing [ Fig. 1(d)].The last open question pertains to the temperature dependence of the photoresponse of SSPDs. Intuitively, one would expect the SSPD to be less efficient at lower temperatures, as the detector works by breaking superconductivity and the energy gap of a superconductor decreases with increasing temp...

We probe the local detection efficiency in a nanowire superconducting single-photon detector along the cross-section of the wire with a spatial resolution of 10 nm. We experimentally find a strong variation in the local detection efficiency of the device. We demonstrate that this effect explains previously observed variations in NbN detector efficiency as function of device geometry.Nanowire superconducting single-photon detectors (SSPDs) consist of a superconducting wire of nanoscale cross-section [1], typically 4 nm by 100 nm. Photon detection occurs when a single quantum of light is absorbed and triggers a transition from the superconducting to the normal state. SSPDs have high efficiency, low jitter, low dark count rate and fast reset time [2], and are therefore a key technology for, among others, quantum key distribution Although progress has been made recently, the underlying physical mechanism responsible for photon detection on the nanoscale is still under active investigation. A combination of theory [6,7], experiments [8][9][10] and simulations [11,12] on NbN SSPDs indicates that the absorption of a photon destroys Cooper pairs in the superconductor and creates a localized cloud of quasiparticles that diverts current across the wire. This makes the wire susceptible to the entry of a superconducting vortex from the edge of the wire. Energy dissipation by this moving vortex drives the system to the normal state.An important and unexpected implication of this detection model is a nanoscale position variation in the photodetection properties of the device. The conditions at the entry point of the vortex determine the energy required for it to cross the wire. This causes photons absorbed close to the edge to have a local detection efficiency (LDE) [13] compared to photons absorbed in the center of the wire [12]. This effect has practical implications for the operation of SSPDs, since it represents a potential limitation of the detection efficiency. In addition, SSPDs have been proposed for nanoscale sensing, either in a near-field optical microscope configuration [14] or as a subwavelength multiphoton probe [15], where this effect would be of major importance for the properties of such a microscope. While this effect has been predicted theoretically, clear experimental evidence is missing. * renema@physics.leidenuniv.nlIn this work, we experimentally explore the nanoscale variations in the intrinsic response of the detector. We spatially resolve the LDE with a resolution of approximately 10 nm, better than λ/50, using far-field illumination only. We find that our results are qualitatively consistent with numerical simulations [11,12]. Our results provide excellent quantitative agreement with experiments that indicate a polarization dependence in the LDE that was hitherto not understood [16].The key technique used in this work is a differential polarization measurement that probes the IDE of the detector (see Figure 1). The technique is based on the fact that polarized light is absorbed preferentially in dif...

We demonstrate efficient nanowire superconducting single photon detectors (SSPDs) based on NbN thin films grown on GaAs. NbN films ranging from 3 to 5 nm in thickness have been deposited by dc magnetron sputtering on GaAs substrates at 350 °C. These films show superconducting properties comparable to similar films grown on sapphire and MgO. In order to demonstrate the potential for monolithic integration, SSPDs were fabricated and measured on GaAs/AlAs Bragg mirrors, showing a clear cavity enhancement, with a peak quantum efficiency of 18.3% at λ=1300 nm and T=4.2 K.

We present the experimental demonstration of a superconducting photon number resolving detector. It is based on the series connection of N superconducting nanowires, each connected in parallel to an integrated resistor. The device provides a single voltage readout, proportional to the number of photons absorbed in distinct nanowires. Clearly separated output levels corresponding to the detection of n=1-4 photons are observed in a 4-element detector fabricated from an NbN film on GaAs substrate, with a single-photon system quantum efficiency of 2.6% at =1.3m. The series-nanowire structure is promising in view of its scalability to large photon numbers and high efficiencies.Conventional optical detectors generate an electrical signal proportional to the intensity of the incident light. However their sensitivity is limited by the electrical noise in the amplification circuit. On the other hand, single photon detectors (SPDs), which are extremely sensitive devices, usually show a strongly nonlinear response, i.e. their output signal level is independent of the number of photons that simultaneously hit the detector. The gap between these two detection regimes can be filled with a photon-number-resolving (PNR) detector, a device as sensitive as an SPD, and with a capability of precisely determining the number of photons that

Quantum photonic integration circuits are a promising approach to scalable quantum processing with photons. Waveguide single-photon-detectors (WSPDs) based on superconducting nanowires have been recently shown to be compatible with single-photon sources for a monolithic integration. While standard WSPDs offer single-photon sensitivity, more complex superconducting nanowire structures can be configured to have photon-number-resolving capability. In this work, we present waveguide photonnumber-resolving detectors (WPNRDs) on GaAs/Al 0.75 Ga 0.25 As ridge waveguides based on a series connection of nanowires. The detection of 0-4 photons has been demonstrated with a four-wire WPNRD, having a single electrical read-out. A device quantum efficiency ~24 % is reported at 1310 nm for the TE polarization.

Quantum photonic integration is one of the leading approaches for enabling the implementation of quantum simulation and computing at the scale of tens to hundreds of photons. Quantum photonic integrated circuits (QPICs) require the monolithic integration of single-photon sources and passive circuit elements, such as waveguides and couplers, with singlephoton detectors. A promising approach for on-chip singlephoton detection is the use of superconducting nanowires on top of semiconductor waveguides. Here we present state-of-the-art NbN films on GaAs for the realization of waveguide superconducting single-photon detectors, suitable for integration with sources and linear optical circuits. Based on the measured optical properties, we propose a new design which allows high absorptance for short nanowires in order to increase the integration density in a quantum photonic chip. Finally, we

A superconducting thin film with uniform properties is the key to realize nanowire superconducting single-photon detectors (SSPDs) with high performance and high yield. To investigate the uniformity of NbN films, we introduce and characterize simple detectors consisting of short nanowires with length ranging from 100nm to 15μm. Our nanowires, contrary to meander SSPDs, allow probing the homogeneity of NbN at the nanoscale. Experimental results, endorsed by a microscopic model, show the strongly inhomogeneous nature of NbN films on the sub-100nm scale.