Hyperentanglement and SPAD array camera enable wide-field supersensitive quantum imaging.

The ability to detect single photons is becoming an enabling key capability in an increasing number of fields. Indeed, its scope is not limited to applications that specifically rely on single photons, such as quantum imaging, but extends to applications where a low signal is overwhelmed by background light, such as laser ranging, or in which faint excitation light is required not to damage the sample or harm the patient. In the last decades, SPADs gained popularity with respect to other single-photon detectors thanks to their small size, possibility to be integrated in complementary metal-oxide semiconductor processes, room temperature operability, low power supply and, above all, the possibility to be fast gated (to time filter the incoming signal) and to precisely timestamp the detected photons. The development of large digital arrays that integrates the detectors and circuits has allowed the implementation of complex functionality on-chip, tailoring the detectors to suit the need of specific applications. This review proposes a complete overview of silicon SPADs characteristics and applications. In the previous Part I, starting with the working principle, simulation models and required frontend, the paper moves to the most common parameters adopted in literature for characterizing SPAD performance and describes single pixels applications and their performance. In this Part II, the focus is posed on the development of SPAD arrays, presenting some of the most notable examples found in literature. The actual exploitation of these designs in real applications (e.g., automotive, bioimaging and radiation detectors) is then discussed.

In this paper, we present the architecture and the experimental characterization of an improved version of a previously developed 32 × 32 Single Photon Avalanche Diodes (SPADs) and Time to Digital Converters (TDCs) array, and two new arrays (with 8 × 8 and 128 × 1 pixels) with the additional capability of actively gating the detectors with subnanosecond rise time. The arrays include high performance SPADs (0.04 cps/µm 2 , 50% peak PDE) and provide down to 410 ps Full-Width at Half-Maximum (FWHM) single shot precision and excellent linearity. We developed a camera to exploit these imagers in timeresolved, single-photon applications.

Single-photon avalanche diode (SPAD) exploitation in high-flux applications is often hindered by the trade-off between the SPAD dead-time and afterpulsing probability. In this paper, we present the architecture and the experimental characterization of two chips including a novel SPAD sensing, and readout scheme designed to minimize dead-time (1.78 ns and 0.93 ns respectively) and afterpulsing probability (0.14% maximum). We have coupled this architecture with high-performance SPADs obtaining an extremely stable dead-time (6.44 ps rms jitter) that can be easily regulated through an external voltage. Thanks to its compact size, this novel pixel architecture can be easily integrated within high-resolution SPAD arrays for GHz applications.

We present a novel architecture for multi-channel Time-to-Digital Converters to be implemented into low-cost FPGAs, achieving 10 ps LSB, 164 µs full-scale range, and good linearity both in terms of DNL and INL. The conceived architecture is based on the carry chain delay line model and wave union A method: the positions of both rising and falling edges that propagate in multiple parallel carry chains are recorded each time there is a HIT input. This technique effectively sub-divides the ultra-wide bins improving the measurement precision, and, combined with the sliding-scale technique and continuous code density calibration, improves the TDC linearity. Employing the proposed architecture, we have implemented in a Xilinx Artix-7 FPGA a TDC with 20 timestamp units and validated the device in a Time-Correlated Single Photon Counting setup, when connected to an array chip with 5 × 5 Single Photon Avalanche Diodes.