Determining the precise number of photons in a weak optical pulse is of crucial importance for imaging, computing or communication applications that use single photons or few photons to increase the performance of classical techniques. Single photons or pair of photons can for instance be used to enhance the resolution of optical imaging techniques or to transmit information with ultimate security using quantum cryptography. In order to maintain this performance gain in such applications, the exact number of photons must be measured and monitored. In this contribution we present a method to measure the number of photons in an optical pulse by using 7 superconducting nanowire single photon detectors. Since a single SNSPD is not sensitive to the number of incoming photons, the optical signal multiplexing is necessary to measure photon correlations among the channels and thus reconstruct the input photons statistics. The readout and counting of the photon arrivals is performed with a fast multi-channel Asynchronous-Correlated-Digital-Counter (ACDC) implemented in the Programmable Logic (PL) of a Xilinx ZYNQ-7020 SoC device. The test of the technique has been performed coupling a weak laser via a fiber multiplexer to an array of individually fiber-coupled detectors.
Determining the precise number of photons in a weak optical pulse is of crucial importance for imaging, computing or communication applications that use single photons or few photons to increase the performance of classical techniques. Single photons or pair of photons can for instance be used to enhance the resolution of optical imaging techniques or to transmit information with ultimate security using quantum cryptography. In order to maintain this performance gain in such applications, the exact number of photons must be measured and monitored. In this contribution we present a method to measure the number of photons in an optical pulse by using 7 superconducting nanowire single photon detectors. Since a single SNSPD is not sensitive to the number of incoming photons, the optical signal multiplexing is necessary to measure photon correlations among the channels and thus reconstruct the input photons statistics. The readout and counting of the photon arrivals is performed with a fast multi-channel Asynchronous-Correlated-Digital-Counter (ACDC) implemented in the Programmable Logic (PL) of a Xilinx ZYNQ-7020 SoC device. The test of the technique has been performed coupling a weak laser via a fiber multiplexer to an array of individually fiber-coupled detectors.
The paper presents a system for measuring photon statistics and photon timing in the few-photon regime down to the single-photon level. The measurement system is based on superconducting nanowire single photon detectors and a time-to-digital converter implemented into a programmable device. The combination of these devices gives high performance to the system in terms of resolution and adaptability to the actual experimental conditions. As a case of application, we present the measurement of photon statistics for coherent light states. In this measurement, we make use of 8th order single photon correlations to reconstruct with high fidelity the statistics of a coherent state with average photon number up to 4. The processing is performed by means of a tapped-delay-line time-to-digital converter architecture that also hosts an asynchronous-correlated-digital-counter implemented in a field programmable gate array device and specifically designed for performance optimization in multi-channel usage.
Standard linear optical detectors have a maximum sensitivity in the few hundreds of photons range, limited by amplifier noise. On the other hand, single photon detectors, which are the most sensitive detectors, are strongly nonlinear: One or more photons result in the same output signal. Photon number resolving (PNR) detectors, which have the ability to discriminate the number of photons in a weak optical pulse, are of great importance in the field of quantum information processing and quantum cryptography. Moreover, a PNR detector with large dynamic range can cover the gap between these two detection modes. Such detectors are greatly desirable not only in quantum information science and technology, but also in any application dealing with low light levels. In this work, we propose a novel approach to photon number resolving detectors based on spatial multiplexing of nanowire superconducting single-photon detectors. In the proposed approach, N superconducting nanowires, each connected in parallel to an integrated resistor, are connected in series. Photon absorption in a nanowire switches its bias current to the parallel resistor, forming a voltage pulse across it. The sum of these voltages, proportional to the number of absorbed photons, is measured at the output. The use of a cryogenic preamplifier with high input impedance for the read-out increases the linearity, the signal to noise ratio, and the speed. With this combination, we expect to be able to count up to few tens of photons with high fidelity, excellent timing resolution, and very high sensitivity in the telecommunication wavelength range.
Superconducting nanowire single photon detectors (SNSPDs) are the detectors of choice in quantum optics applications due to their leading performances. To exploit their performances, great care must be devoted to efficiently couple light from a single-mode fiber to their small area, ~10 μm in size, at cryogenic temperature (~3K). In addition, to further improve the SNSPDs functionality, implementing the photon number resolving capability, arrays of spatially multiplexed SNSPDs, must be realized. We will discuss our results obtained for these two approaches.
Detecting spatial and temporal information of individual photons is a crucial technology in today's quantum information science. Among the existing single-photon detectors, superconducting nanowire single-photon detectors (SNSPDs) have been demonstrated with a sub-50 ps timing jitter, near unity detection efficiency1, wide response spectrum from visible to infrared and ~10 ns reset time. However, to gain spatial sensitivity, multiple SNSPDs have to be integrated into an array, whose spatial and temporal resolutions are limited by the multiplexing circuit. Here, we add spatial sensitivity to a single nanowire while preserving the temporal resolution from an SNSPD, thereby turning an SNSPD into a superconducting nanowire single-photon imager (SNSPI). To achieve an SNSPI, we modify a nanowire's electrical behavior from a lumped inductor to a transmission line, where the signal velocity is slowed down to 0.02c (where c is the speed of light). Consequently, we are able to simultaneously read out the landing locations and arrival times of the photons from the output electrical pulses using only two connections. We have demonstrated single-photon imaging by using a 19.7 mm long SNSPI, which is meandered into an imaging area of 286 {\mu}m*193 {\mu}m. The nanowire has a temporal resolution of 50 ps for detecting 1.5 {\mu}m photons. The 2D spatial resolution is 13.0 {\mu}m in the vertical direction and 5.6 {\mu}m in the horizontal direction. The maximum number of resolvable locations (i.e., the effective number of pixels) in such a long nanowire is calculated to be 590. Rather than operating an individual detector in a scanning mode or using SPD arrays, this SNSPI gives an alternative approach to taking large-scale single-photon images and measuring temporal and spatial correlation.
A photon-number-resolving detector based on a four-element superconducting nanowire single photon detector is demonstrated to have sub-30-ps resolution in measuring the arrival time of individual photons. This detector can be used to characterise the photon statistics of non-pulsed light sources and to mitigate dead-time effects in high-speed photon counting applications. Furthermore, a 25% system detection efficiency at 1550 nm was demonstrated, making the detector useful for both low-flux source characterization and high-speed photon counting and quantum communication applications. The design, fabrication, and testing of this detector are described, and a comparison between the measured and the theoretical performance is presented.
We report a 16-pixel (4×4) superconducting nanowire single-photon detector (SNSPD) array featuring pixel-level photon-number-resolving capability. This multidimensional sensing is enabled by integrating a compact four-element series nanowire architecture into each spatial pixel, allowing direct discrimination of one to four photons. Under single-mode fiber illumination, the device achieves a total system detection efficiency of 41.5%, given the array geometrical fill factor of 53%. Across all channels, the device exhibits reliable multiphoton discrimination, maintaining minimum readout fidelities of 96%, 89%, 73%, and 77% for one- to four-photon states, respectively. Furthermore, independent channel evaluations reveal low timing jitter and negligible inter-channel crosstalk (<0.1 % ). By implementing differentiating pulse shaping for high-flux operation, a maximum single-channel −3 dB count rate of 984 MHz is achieved, establishing an aggregated array bandwidth potential of 8.9 GHz. This sensing architecture may provide a viable pathway toward high-flux single-photon imaging, offering potential applications in quantum optics and light detection and ranging (LiDAR) systems.