Hong–Ou–Mandel (HOM) interference—the bunching of indistinguishable photons at a beamsplitter—is a staple of quantum optics and lies at the heart of many quantum sensing approaches and recent optical quantum computers. Here we report a full-field, scan-free quantum imaging technique that exploits HOM interference to reconstruct the surface depth profile of transparent samples. We demonstrate the ability to retrieve images with micrometre-scale depth features with photon flux as small as seven photon pairs per frame. Using a single-photon avalanche diode camera, we measure both bunched and anti-bunched photon-pair distributions at the output of an HOM interferometer, which are combined to provide a lower-noise image of the sample. This approach demonstrates the possibility of HOM microscopy as a tool for the label-free imaging of transparent samples in the very low photon regime. Hong–Ou–Mandel interference enables depth-resolved quantum imaging at very low light levels. 

/pdf/quantum-microscopy-based-on-hong-ou-mandel-interference-1doruoxl.pdf

Quantum microscopy based on Hong–Ou–Mandel interference

Achieving fast, sensitive, and parallel measurement of a large number of quantum particles is an essential task in building large-scale quantum platforms for different quantum information processing applications such as sensing, computation, simulation, and communication. Current quantum platforms in experimental atomic and optical physics based on CMOS sensors and CCD cameras are limited by either low sensitivity or slow operational speed. Here we integrate an array of single-photon avalanche diodes with solid-state spin defects in diamond to build a fast wide-field quantum sensor, achieving a frame rate up to 100~kHz. We present the design of the experimental setup to perform spatially resolved imaging of quantum systems. A few exemplary applications, including sensing DC and AC magnetic fields, temperature, strain, local spin density, and charge dynamics, are experimentally demonstrated using an NV ensemble diamond sample. The developed photon detection array is broadly applicable to other platforms such as atom arrays trapped in optical tweezers, optical lattices, donors in silicon, and rare earth ions in solids.

pdf/fast-wide-field-quantum-sensor-based-on-solid-state-spins-2wsk5wd3.pdf

Fast Wide‐Field Quantum Sensor Based on Solid‐State Spins Integrated with a SPAD Array

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.

Historical Perspectives, State of Art and Research Trends of SPAD Arrays and Their Applications (Part II: SPAD Arrays)

The overall sensitivity of frontside-illuminated, silicon single-photon avalanche diode (SPAD) arrays has often suffered from fill factor limitations. The fill factor loss can however be recovered by employing microlenses, whereby the challenges specific to SPAD arrays are represented by large pixel pitch (> 10 µm), low native fill factor (as low as ∼10%), and large size (up to 10 mm). In this work we report on the implementation of refractive microlenses by means of photoresist masters, used to fabricate molds for imprints of UV curable hybrid polymers deposited on SPAD arrays. Replications were successfully carried out for the first time, to the best of our knowledge, at wafer reticle level on different designs in the same technology and on single large SPAD arrays with very thin residual layers (∼10 µm), as needed for better efficiency at higher numerical aperture (NA > 0.25). In general, concentration factors within 15-20% of the simulation results were obtained for the smaller arrays (32×32 and 512×1), achieving for example an effective fill factor of 75.6-83.2% for a 28.5 µm pixel pitch with a native fill factor of 28%. A concentration factor up to 4.2 was measured on large 512×512 arrays with a pixel pitch of 16.38 µm and a native fill factor of 10.5%, whereas improved simulation tools could give a better estimate of the actual concentration factor. Spectral measurements were also carried out, resulting in good and uniform transmission in the visible and NIR.

Challenges and prospects for multi-chip microlens imprints on front-side illuminated SPAD imagers.

The ability to form reconstructions beyond line-of-sight view could be transformative in a variety of fields, including search and rescue, autonomous vehicle navigation, and reconnaissance. Most existing active non-line-of-sight (NLOS) imaging methods use data collection steps in which a pulsed laser is directed at several points on a relay surface, one at a time. The prevailing approaches include raster scanning of a rectangular grid on a vertical wall opposite the volume of interest to generate a collection of confocal measurements. These and a recent method that uses a horizontal relay surface are inherently limited by the need for laser scanning. Methods that avoid laser scanning to operate in a snapshot mode are limited to treating the hidden scene of interest as one or two point targets. In this work, based on more complete optical response modeling yet still without multiple illumination positions, we demonstrate accurate reconstructions of foreground objects while also introducing the capability of mapping the stationary scenery behind moving objects. The ability to count, localize, and characterize the sizes of hidden objects, combined with mapping of the stationary hidden scene, could greatly improve indoor situational awareness in a variety of applications. 

https://www.nature.com/articles/s41467-023-39327-2.pdf

Non-line-of-sight snapshots and background mapping with an active corner camera

We present an array of $16 \times 16$ single-photon avalanche diodes (SPADs) with 16 shared 6 ps time-to-digital converters (TDCs), designed for non-line-of-sight imaging. It features a timing jitter of 60 ps (FWHM), fast-gated capabilities and up to 1.6·108 photon time-tagging measurements per second.

Fast-gated $16\times 16$ SPAD array with on-chip 6 ps TDCs for non-line-of-sight imaging

We present the design of a new fast-gated 16 x 16 silicon SPAD array developed in a 0.16 μm BCD technology with builtin 6 ps resolution TDCs (Time-to-Digital Converters), optimized for Non-Line-Of-Sight imaging. The high temporal resolution is achieved by sharing one high-performance TDC among 16 SPADs, without losing spatial resolution, thanks to an identification logic capable of detecting and rejecting collisions. In the SPAD frontend, a low-threshold comparator minimizes temporal jitter, while an active quenching circuit reduces afterpulsing. An event-driven readout scheme optimizes data transfers, however a standard frame-driven photon-counting only mode is also available. The goal is to enable quasi real-time NLOS scene reconstruction by parallelizing the acquisition across multiple spots. Thanks to its high temporal resolution, the detector can be exploited also in other scientific applications, such as clinical diagnostic (with Time-Domain Near Infrared Spectroscopy) and LiDAR (Light Detection and Ranging).

Design of a 16 x 16 fast-gated SPAD imager with 16 integrated shared picosecond TDCs for non-line-of-sight imaging

We present a compact camera module based on an array of 16 × 16 single-photon avalanche diodes (SPADs) with fastgating capabilities and hosting 16 shared time-to-digital converters (TDCs) with a least significant bit (LSB) of 6 ps. SPADs are gated with a rising-edge of less than 500 ps and show an average instrument response function (IRF) of 60 ps FWHM, including the TDCs, with less than 4 ps time-dispersion across a 30 ns gate window. Differential non-linearity (DNL) and integral non-linearity (INL) are as good as 0.04 LSB and 3.6 LSB, respectively. An event-driven readout protocol optimizes data transfer from the SPAD chip to the FPGA, handling the time-of-flight (TOF) pre-processing in order to minimize the dead-time of the TDCs, thus sustaining up to 1.6 · 108 conversions per second. TOF data can be transferred towards a PC via USB-C with a maximum throughput of about 6 Gbit/s. Our camera meets the requirements of an optimized multi-pixel solution for non-line-of-sight (NLOS) imaging, as it combines fast-gating with narrow IRF: the sub-nanosecond activation of the SPADs is exploited to reject spurious light pulses, like the first bounce one from the relay wall, and properly acquire multiply-scattered photons arriving from the hidden target, while its narrow IRF allows for centimeter-accurate NLOS reconstructions. Furthermore, while the high throughput paves the way towards real-time NLOS acquisitions at video-rates, the compact form

Event-driven SPAD camera with 60 ps IRF and up to 1.6 · 108 photon time-tagging measurements per second

: Single-photon detectors are nowadays well consolidated devices, finding employment in a multidisciplinary spectrum of both scientific and industrial applications. They are exploited in material science and biology to measure fast fluorescence decays, in rangefinders to precisely timestamp laser return echo pulses, and have been applied to the quantum optics fields to enable quantum computing and quantum communications. While originally designed as vacuum tube devices, the vast majority of single-photon detectors are today Single-Photon Avalanche Diodes (SPADs) and Superconducting Nanowire Single-Photon Detectors (SNSPDs). Even though SNSPDs can reach superior detection performance, they rely on superconducting material properties, hence require bulky cooling solutions to be operated at cryogenic temperatures. On the other hand, SPADs are compact and reliable devices that are fabricated with microelectronic technologies. Indeed, for the visible range and up to 1 µm wavelengths, silicon SPADs can be fabricated with standard CMOS processes, thus they can be monolithically integrated with dedicated electronics in large-format arrays. For detecting single photons beyond 1 µm, SPADs in different materials, like III-V compounds and germanium, can be easily bonded to Read-Out Integrated Circuits (ROICs) for developing highly compact detectors, thus making such technologies quite promising solutions for all photon counting application in the Short-Wave Infrared Range (SWIR). Beyond their ruggedness and overall good detection metrics, SPADs can be operated with moderate to none cooling solutions. Additionally, a rather unique SPADs’ feature is their ability to be time-gated: they can be swiftly driven from OFF to ON condition in just few hundreds of picoseconds, thus paving the way to new single-photon counting techniques. The goal of this Ph.D. work is to leverage SPADs’ time-gating capabilities to develop: i) a 16 × 16 SPAD camera enabling video-rate Non-Line-Of-Sight (NLOS) imaging for the first time; ii) a pair of high-throughput

Time-gated SPAD systems for visible and near-infrared photon counting

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