During this thesis work, two approaches to single-photon detection at telecom wavelengths have been investigated. The first one focuses on improving the performance of commercially available semiconductor single photon avalanche diodes (SPADs) to meet quantum communication requirements, and the second one describes the development and characterization of large active-area superconducting nanowire single-photon detectors (SNSPDs). On the application side, quantum random number generation (QRNG) was implemented using two schemes based on two different types of single photon detectors activated by a LED.
During this thesis work, two approaches to single-photon detection at telecom wavelengths have been investigated. The first one focuses on improving the performance of commercially available semiconductor single photon avalanche diodes (SPADs) to meet quantum communication requirements, and the second one describes the development and characterization of large active-area superconducting nanowire single-photon detectors (SNSPDs). On the application side, quantum random number generation (QRNG) was implemented using two schemes based on two different types of single photon detectors activated by a LED.
Two approaches to single-photon detection at telecom wavelengths have been investigated, using semiconductor and superconductor technologies. The low temperature operation of free-running InGaAs/InP negative feedback avalanche diodes (NFADs) was studied, demonstrating that they can operate with an extremely low dark count rate, which opens up a host of new applications. Secondly, superconducting nanowire single-photon detectors (SNSPDs) based on amorphous materials were developed, fabricated and characterized. These detectors are highly attractive because they can achieve excellent attributes simultaneously, such as high efficiency, low noise, good temporal resolution, no afterpulsing and high count rates. Finally, it was demonstrated that the use of NFADs can enable record distance quantum key distribution (QKD) over 307 km of optical fiber. A next generation QKD system has been proposed and numerically analyzed, aiming at extending the record distance by an additional 100 km and increasing the state-of-the-art secret key rate by over an order of magnitude.
The important development of quantum technologies during the past decade brought the need for efficient, fast and precise single-photon detection. Superconducting Nanowire Single-Photon Detectors (SNSPDs) are currently considered as the detection method of choice when extreme performances are required. This thesis aimed at investigating this technology, with the objective of improving existing detectors for high-rate applications in quantum communication. This document presents the work and result achieved over four years dedicated to the subject. During this thesis, single-photon detection based on superconducting nanowire has been investigated, with a particular focus on specific requirements for high speed quantum communication applications. The work achieved ranges from the design and nano-fabrication of the structures, to the characterization and the understanding of the physics, detection models and fundamental limits of the devices.
This dissertation is the result of research which, although electrical and computer engineering in nature, also aims to improve the performance of many systems in the field of quantum information. For example, random number generators are used in almost all areas of science, and the initial portion of this work details the theory, design, and characterization of two photon-arrival-time quantum random number generators (QRNGs). After the QRNGs were completed, it was realized that their performance was severely limited both by the maximum detection rate of the single-photon detectors used, and the precision at which the arrival times could be resolved. The single-photon detectors used for both QRNGs are single-photon avalanche photodiodes (SPADs), devices which when operated below their breakdown voltage can create a macroscopic amount of current (an avalanche) in response to a single incident photon. Some of this charge can become trapped in defects or impurities; if this trapped charge is released when the SPAD is active, a secondary ‘false’ detection event, or ‘afterpulse’ can occur. To lower the afterpulse probability to reasonable levels (< 1%), we attempted to reduce the amount of avalanche charge by halting its growth promptly with high-speed electronics, so that defects have a lower probability of becoming populated in the first place. Initial results show reductions in afterpulse probability by up to a factor of 12, corresponding to a ~20% decrease in dead time, a value that could be improved further. We developed an FPGA-based time-to-digital converter system for use specifically with SPADs, achieving a time-bin resolution of 100 ps, with lower dead time and higher maximum detection rate than all currently available detection systems. This further allowed for the creation of a new higher-order SPAD characterization technique, which was identified previously unknown subtleties to SPAD operation. Finally, we developed an ultra-low-latency QRNG, which was used in one of the recent loophole-free demonstrations of quantum nonlocality. The final latency was below 2.5 ns, to our knowledge the lowest latency QRNG to date. Of special interest, however, is our subsequent exploration into the characterization of its bit-probability drift using atomic clock stability techniques. By employing the Allan deviation and implementing precision feedback, the additional frequency drift caused by environmental fluctuations is reduced such that the resulting bit stream can pass cryptographic random number tests for sample sizes up to 5 Gb. This system is currently intended for the NIST random-number beacon, a world-wide trusted source of random bits.
Photon detectors connect optical and electrical systems by converting light to electrical signals. Single photon detectors detect single photons which measure ultra-weak light. Traditional semiconductor single-photon detectors suffer from high dark count rates, low infrared detection efficiency, and poor temporal resolution. Superconducting-nanowire single-photon detectors (SNSPDs) overcome these limitations using the hotspot effect. SNSPDs have created a solid foundation for optical technologies and become key in quantum information, LiDARs and Deep-Space Communications. This paper reviews working principles and performance metrics: system detection efficiency, dark count rate, recovery time, and timing jitter. The study summarizes applications in quantum information (quantum key distribution, optical quantum computing), single-photon LiDAR, and deep-space optical communication, replacing most single-photon avalanche diodes (SPADs). Recent experiments show that SNSPD-based systems can achieve meter-scale detection of sea fog, interplanetary laser communication over billions of kilometers, and boson sampling with 20 photons. Ongoing research is expected to improve performance in maximum count rate, system integration, and cryogenic operation, as well as to find applications in emerging fields such as environmental monitoring.
The use of quantum technologies has risen drastically in the last decades, offering an enhancement in terms of the technological capabilities available nowadays. Found in this field of research, quantum communications based on the transmission of single photons offer new approaches to redefine the use of protocols and devices for specialised applications. As a final result, this leads to the accomplishment of long-range quantum networks with terrestrial and satellite-based communications set to establish a new scale of Telecommunication applications. Within these kinds of applications, this project focuses on the assembly of a single-photon detector using a photodiode of type Single-Photon Avalanche Diode (SPAD), essential device to define the level of performance of any quantum communication system. To carry this out, the system is assembled around its centre piece, an Excelitas APD operated in Geiger mode above its breakdown voltage. The electronic circuit that drives the photodiode is characterised to work with its adjusted bias voltage, its dedicated temperature management system and its active quenching circuit to detect pulses with certainty and precision. The final version of the project is adjusted to work in the 1300nm and 1500nm operating wavelengths as the photodiode used is formed by an InGaAs/InP semiconductor structure. Working on top of the original circuit provided initially, the experimental measurements obtained in the course of the project prove that the design modifications implemented have improved the functioning of the whole system. The main achievements accomplished encompass the generation of pulses for photon detection at 20kHz, the reduction of electromagnetic interferences by a factor 40, the enhancement of the manipulated signals in the circuit by a factor of tens, and the correct management of the SPAD temperature to maintain a fixed value of dark counts for a long time.
Superconducting Nanowire Single-Photon Detectors (SNSPDs) are the premier single-photon counting technology in the near-infrared regime. Operating via a photon-induced phase transition from the superconducting to the normal state, SNSPDs act as highly sensitive digital detectors that now play a critical role in quantum communication, deep-space optical communications, and direct dark matter detection. As the demand for these detectors scales, there is a pressing need to extend their performance capabilities toward longer wavelengths, larger active areas, and higher operating temperatures. However, advancing these metrics requires navigating complex physical trade-offs inherent to the detection mechanism. This thesis explores the material optimizations, novel detector architectures, and fundamental device physics required to push SNSPD technology beyond its current limits. The first thrust of this work focuses on extending SNSPD sensitivity into the mid-infrared (MIR) and far-infrared regimes. By developing a low-Tc (sub-1 K) WSi film engineered to reduce the characteristic detection energy, the threshold for long-wavelength detection is significantly lowered. Coupled with a novel detector architecture utilizing impedance-matching tapers and superconducting nanowire avalanche photon detectors (SNAPs), sensitivity is initially demonstrated at wavelengths up to 29um using a blackbody thermal source. Expanding beyond this limit, the implementation of quantum cascade lasers enables the demonstration of single-photon counting at wavelengths up to 88um, marking a major improvement over the current state of the art. To address the practical MIR detection challenges of low signal-to-noise ratio and restricted fill factors, this work also demonstrates a 1 mm x 1mm array of superconducting microwire single-photon detectors (SMSPDs) sensitive out to 3.7um, establishing a pathway for future antenna-coupled detectors. The second thrust investigates the development of micrometer-scale wires to simultaneously increase active area and operating temperature. Moving to microwire geometries raises fundamental questions regarding intrinsic detection mechanisms. Using a scanning lens setup, this work demonstrates the first direct observation of transverse coordinate effects in microwires, where the precise location of photon incidence across the wire width significantly alters the detector's slew rate and timing performance. To counteract this, new rectification schemes are introduced to produce a more uniform detection landscape. Specifically, the integration of a superconducting rail alongside the main detector is shown to successfully mitigate the transverse coordinate effect, improve timing performance, and increase the viable operating temperature of the detector. The third thrust explores the use of unconventional superconductors, namely MgB2 to achieve an appreciably higher-temperature single-photon detection pathway. This section details the distinct challenges associated with the growth and fabrication of detectors using air-sensitive materials and evaluates their long-term viability for the SNSPD platform. Ultimately, this thesis highlights the critical material and architectural parameters required to advance the field of single-photon detection. By elucidating the metrics and mechanisms that govern ideal detector performance, this work aims to broaden the understanding and future application of SNSPDs across diverse scientific disciplines. All the data presented here is my own unless otherwise mentioned.