Quantum technologies provide new base capabilities which open up new frontiers in sensing, networking, and computation. In all cases, working with systems at the limits set by nature requires high degrees of integration of complex systems to realize practical results. Jake will discuss the promise quantum systems in diverse areas from particle physics to drug discovery, and highlight the many challenges to be overcome and the ways in which the nascent field of quantum engineering can tackle these challenges.

Single Photon Avalanche Diodes (SPADs) are the enabling device for different kind of applications in which low noise, high photon detection efficiency, and compactness are required. They are capable of providing high photon count rate and picosecond timing precision. For these reasons, SPADs are the sensors of choice in many applications such as LiDAR, TCSPC and quantum key distribution (QKD). Whether the SPAD is implemented in a custom technology, allowing detector tailoring on specific application constraints, or in a CMOS process, with great benefits in terms of large-scale integration and compactness, a quenching circuit is always required, and it sets the ultimate performance that can be extracted from this sensor. In this work, we present a fully-integrated active quenching circuit capable of driving external custom SPADs up to 250 Mcps. The circuit has been fabricated exploiting a 150nm high voltage technology and extensively tested with a custom SPAD.

The latest advances of Quanta Image Sensors (QIS) with CMOS photon-counting pixels are discussed in this paper, featuring two commercially available QIS devices with 16MPixel and 4MPixel resolution and 1.1µm and 2.2µm pixel sizes, respectively. These two breakthrough sensors both achieve reliable photon number resolving with deep sub-electron read noise under room temperature with full-speed operation. The low-noise readout is combined with a sizable full well capacity to realize high dynamic range (HDR). The new QIS devices provide unprecedented capability and value under low-light and HDR imaging conditions for security, defense, scientific, and consumer imaging applications.

Quantum key distribution is a well-established approach for sharing a classical key between two parties. In most cases, the parties are at fixed locations and use an optical fiber or free-space channel with fixed telescopes. To enable flexible and mobile quantum key distribution applications, we are developing a drone-based system. Here, the entire quantum transmitter and receiver system must have low size, weight, and power so that it can fly on medium-scale drones (Group 2, 21-55 lb gross takeoff weight). We will describe these key technologies and our current progress on achieving secure key exchange between drones.

We present the latest developments carried on in Quandela, where solid-state based quantum light sources with increased performances and usability were realized, allowing for plug-and-play operation, portability and integration in commercial infrastructure.

Single-photon direct detectors in the submillimeter, far-IR and mid-IR range are highly desirable for reaching the sensitivity permitted by the low background noise of cryogenic-optics space telescopes, such as envisioned in NASA’s Origins Space Telescope concept. When combined with arrays of integrated spectrometers they can enable revolutionary advances in astrophysical studies such as fast spectroscopic surveys of the high redshift universe from space. Coherent receivers, which are fundamentally limited by quantum vacuum noise are critical for high-spectral-resolution studies of molecular and atomic emission lines and can reveal the origins of galaxies, stars, and even life. They can also enable future space-based VLBI missions, such as the Photon Ring Telescope concept, to study black holes. I will review recent advances in key superconducting technologies being developed for the above areas, including single-photon far/IR Kinetic Inductance Detectors, integrated submillimeter spectrometers, and quantum-limited kinetic inductance amplifiers for microwave readout of detectors and for mm/sub-mm coherent receivers.

Diffuse correlation spectroscopy (DCS) is an emerging near infrared spectroscopy modality able to measure cerebral blood flow (CBF) non-invasively and continuously in humans. We have reported a limited applicability in adults due to the significant extracerebral tissue thickness and the low signal-to-noise ratio (SNR) of the measurements. Improvements to DCS brain sensitivity and SNR can be achieved by operating DCS at 1064 and using superconducting nanowire single-photon detectors (SNSPDs). Initial human results show a 16-fold improvement in SNR and 20% improvement in depth sensitivity. This allows us to resolve changes in CBF in adult subjects more robustly and accurately than was previously achievable.

Satellites can act as trusted nodes to link up QKD networks that are far apart. This requires the satellite to be able to communicate with ground receivers that are located near the end-users, typically in an urban environment. Such environments lower the signal-to-noise ratio achievable and can reduce the effectiveness of satellite-based communication. I will discuss the variables that must be considered when discussing satellite-based QKD; these variables are also expected to affect other types of free-space quantum communication. In particular, I will use Singapore as an example of a sea-level dense urban environment, with high humidity and high probability of cloud cover, to discuss the challenges that need to be overcome.

Single-photon detectors play a prominent role in quantum photonics. In this field, superconducting nanowire single-photon detectors (SNSPDs) excel in terms of performance, but their application is often limited by the necessity of cryogenic temperatures. Single-photon avalanche diodes (SPADs) represent an alternative in all these situations. Here, we focus on the progress made on silicon SPADs, whose performance in the visible range provide a valid option for several quantum applications, and, after that, we review the novel solutions that are blossoming for the telecom optical bands. In the end, we conclude with our vision of SPADs in quantum photonics applications.

Timing measurements triggered by photo-detection are widely used in several different fields, such as Time-Correlated Single Photon Counting (TCSPC), Quantum Key Distribution (QKD) or Light Detection and Ranging (LiDAR) systems. All these applications have in common one essential element, i.e. the timing electronics, which aims at measuring the time interval between two instants and whose requirements strictly depend on the application-specific goal. In this work, we present a versatile and fully-integrated timing chip hosting eight high-performance Time-to-Amplitude Converters (TACs) integrated with a smart logic, providing to the end user a unique flexibility to select the most suitable configuration for its specific requirements.

Sub-pixel micro-scanning is used to increase the sampling of an imaging system, by taking multiple images of a scene from different sub-pixel locations and combining them into a single composite image using an advanced image processing algorithm. We have applied this method to a single-photon light detection and ranging system based on the time-correlated single-photon counting technique, operating at a wavelength of 1550 nm. Using a 32 × 32 single-photon detector array, the sub-pixel micro-scanning method allowed for composite depth maps and intensity profiles of up to 512 × 512 pixels for targets at distance of 325 meters. We will discuss the effect of micro-scanning on image enhancement.

We present the design of an innovative time-gated 32×32 InP/InGaAs-based Single Photon Avalanche Diode (SPAD) array with sub-nanosecond gating capabilities operating up to 10MHz repetition rate specifically designed for time-domain diffuse correlation spectroscopy at 1064nm. We present the detector design, experimental characterization and preliminary validations on a liquid phantom. This testing is informing us for a revision of the photodetector which will allow to reach up to 192 optical channels towards functional blood flow changes measurements with full head coverage with improved brain sensitivity thanks to early-photons rejection.

We have demonstrated the first heterogeneous integration of InGaAs/InAlAs single-photon avalanche diodes with Si photonics through a low-temperature die-to-die bonding technique. A triple-mesa structure has been adopted in InGaAs/InAlAs SPADs. The triple-mesa structure avoids surface exposure to the high electric field to suppress the surface effect induced by the surface defects. This structure can also alleviate the electric field crowding at the mesa edges to eliminate the premature breakdown. The bonding process is done using SU-8 as the adhesion layer which has high transmission efficiency at the SWIR regime and a low-curing temperature. Our integrated SPADs exhibit high single-photon detection efficiency of ~22% and a relatively low dark count rate of 8.6 ×10^5 Hz, which are among the best performance reported for InGaAs/InAlAs SPADs, and are approaching that of InGaAs/InP SPADs. High device yield and performance uniformity are also maintained after the bonding process.