Single-photon avalanche diode arrays for quantum-enhanced imaging and spectroscopy

The second quantum revolution describes all new technologies enabled by quantum mechanics, not only to describe the physical world, but to address, control, and detect individual quantum systems. For example, observing the behavior of individual atoms or photons enables the achievement of phenomena such as quantum superposition and entanglement. The European Union has launched the Quantum Technologies Flagship initiative to push innovation in four main directions: quantum communication, to transmit data more securely; quantum simulation, to reproduce quantum behavior in well-controlled systems; quantum computation, to speed up computations; and quantum sensing and metrology, to improve measurements performance. Developing real-world applications is quite challenging, mainly because quantum systems require control to preserve their quantum properties from noise, decoherence and alterations caused by each observation. Just to mention the field of quantum measurements, the need to sense single quantum particles, rather than a macroscopic sample, requires detectors with single-photon sensitivity. Today, the most widely used are Single-Photon Avalanche Diodes (SPADs), which provide good overall performance together with the typical advantages of microelectronics, such as reliability, robustness, and compactness. They are also well suited to the development of large-format arrays for imaging applications. Purpose of my Ph.D. research was the development of SPAD-based microchips and camera systems with high performance, targeted to: i) quantum-enhanced imaging and microscopy; ii) Raman spectroscopy; iii) quantum magnetometry. For the first two applications, I conceived the design from chip-level and then up to system level, by developing two SPAD-based ASICs in 160 nm BCD and one in 40 nm CMOS technologies. For the third application, I exploited an already available SPAD camera (developed at the SPADlab in PoliMi) to build a new optical setup for a Nitrogen-Vacancy (NV)-based measurement protocol at Massachusetts Institute of Technology (MIT). In the following, the topics of each chapter are briefly summarized. Chapter 1 introduces quantum imaging, highlighting how quantum mechanics is exploited to boost sensitivity and spatial resolution, compared to classical light. Then, the main quantum imaging techniques are presented and the requirements for high-fidelity quantum entanglement imaging detectors are reported. The operating principles of some of the available SPAD imagers are discussed, with their pros and cons. Eventually, the last part of the chapter is devoted to quantum sensing and, in particular, quantum magnetometry with single-photon emitters, to describe the application context of the work done at MIT, thanks to the “Progetto Rocca” fellowship I got. Chapter 2 presents the “Q-MIC” European Horizon 2020 FET project and the novel approach it proposed, combining visible-light polarization-entangled photon source, Lens-free Interferometric Microscope (LIM) phase imaging, and high-performance SPAD detectors to achieve a microscope with unprecedented phase resolution capabilities. The design of the new 24 × 24 SPAD array chip is presented, describing its Event-Driven (ED) architecture that acknowledges every 2-photon coincidence and directly transfers the spatial coordinates of the involved pixels. Being this approach completely innovative, the architecture and methodologies presented in this chapter resulted in a patent application (number 102021000006728). Chapter 3 deals with the design of a multi-purpose Frame-Based (FB) 24 × 24 SPAD array chip, tailored to photon coincidence detection in quantum imaging. After quickly recalling the main requirements that justify a standard timing approach, the design phase is detailed, with a focus on the very fast readout needed to avoid overhead due to useless data and to achieve specific power-saving for easy scaling toward larger format arrays. Chapter 4 includes all measurement activities I carried out in my Ph.D. research. A preliminary characterization of the new ED and FB 24 × 24 SPAD array chips is reported, together with a comparison with the state-of-the-art. Both imagers have been tested in quantum imaging protocols, demonstrating their ability to detect photon correlations at extremely low microwatt-level optical pump powers and short measurement time. Afterward, the second part of the chapter deals with the activity I carried out in the Quantum Engineering Group at MIT. First the design of a new optical setup including NV centers in diamond and the “SPC3” SPAD camera (commercialized by the PoliMi’s spin-off company Micro Photon Devices - MPD) is illustrated, followed by a report on the first obtained experimental results. Chapter 5 presents the “proID” European Horizon 2020 FET project, aiming at ultrafast Raman-based protein sequencing with single amino-acid resolution. As a preliminary test detector, a 16 × 4 SPAD array chip, operated in Time-Gated Single-Photon Counting (TG-SPC) mode, has been developed. This chapter goes through the overall chip architecture, with a focus on the dual gating modality and fast readout. Finally, results on the chip characterization are presented, in parallel to the performance of SPAD arrays already being used in Raman spectroscopy measurements.