Single-Photon Avalanche Diodes (SPADs) are critical components of emerging quantum communication networks that detect single photons. They are placed in satellites for long-distance communication and are susceptible to radiation-induced displacement damage in space. This degrades SPAD performance parameters, including reduced efficiency, increased thermal dark counts, damage to the Si crystal, and increased afterpulsing rate. This paper simulates SPAD in a space radiation environment by introducing a quantum simulation framework, modeling the SPAD detector as a three-level quantum system, ground state (
∣g⟩), excited state (
∣e⟩) and a trap state (
∣t⟩). Furthermore, to model the photon as a quantum system, second quantization and Fock-space truncation are used. The interaction between the photon-SPAD closed system is simulated using the Jaynes-Cummings model, and the open-system dynamics is governed by the Lindblad master equation and Qiskit's gate-based noise channels. The key characteristics, such as the efficiency, timing jitter, thermal dark counts, and afterpulsing \& radiation effects, are obtained using quantum simulation of SPAD. This approach differs significantly from conventional TCAD simulations, which rely on semiclassical approximations that do not capture the discrete quantum statistics of single-photon interactions and the dynamics of trap states.
Single-Photon Avalanche Diodes (SPADs) are widely employed in spacebased photon detection systems for applications such as deep-space communication, LIDAR, and radiation monitoring. However, their performance is fundamentally limited by thermal dark counts, which are spurious detection events arising from temperature-induced carrier excitations. In this work, we present a quantum-mechanical modelling framework in which the SPAD is represented as a quantized two-level system |g> , |e> corresponding to the ground and excited (avalanche-triggering) states. Thermal effects are incorporated through open quantum system dynamics using the Lindblad master equation, enabling a physically consistent description of temperature-driven carrier generation and relaxation processes. To enable scalable and hardware-relevant analysis, we implement the model within a gate-based quantum simulation framework, where quantum state transitions emulate thermally induced carrier excitation events. The time evolution of the excitation probability is computed to extract the theoretical dark count rate under varying thermal conditions relevant to space environments. This approach establishes a quantum simulation platform for predicting SPAD noise behaviour in temperature-variable environments, providing a foundation for optimizing detector design and improving photon-counting reliability in space applications.
With the growing use of online communications in our modern society, information security is becoming a big concern. Along with that, the progress in quantum computers is posing severe threats to such communications. Once powerful quantum computers are available, most of today's encryption schemes, which are based on computationally hard problems, will be broken within a short period. Researchers are therefore making a great effort to establish quantum-safe encryption schemes. One such scheme is quantum key distribution (QKD), which utilizes the laws of quantum mechanics. These cryptography protocols offer unconditional security to the communication between two distant parties by providing a secure way of sharing encryption keys between them. While over the last few decades QKD has continuously progressed, it is limited to a distance of up to several hundred kilometers using terrestrial quantum links. Satellites are therefore being considered to extend the QKD range for global coverage, although implementations of the satellite-based QKD infrastructure are still in their early stage. There are many aspects of QKD that need further assessment and advancement for establishing long-term satellite-based quantum communication (QC). My thesis works were focused on developing advanced systems for single-photon detectors and quantum sources. Single-photon avalanche diodes (SPADs) are the most viable option for satellite-based quantum communications. They must travel to outer space either for receiving quantum-states in the ground-to-satellite QC or for characterizing the quantum-sources in the satellite-to-ground QC. However, while in space, SPADs exhibit damage caused by the space radiation that gradually increases their dark counts. Performing QKD is not possible when the dark counts exceed a specific threshold. Hence, methods of reducing the detectors' dark counts by mitigating the damages would help to extend the SPADs' in-space useful lifetime. Laser annealing is one such effective method, found in lab experiments, to heal the radiation-induced detector damages. We now aim to carry out this method in low Earth orbit (LEO) to verify its in-orbit effectiveness. On that goal, we are building an annealing payload (APL) for a cube satellite (CubeSat) in collaboration with the University of Illinois at Urbana Champaign (UIUC). We, the University of Waterloo team, built one of the two segments of the APL-- a space-qualified detector module containing two Excelitas C30902SH and Excelitas SLiK detectors. Our miniaturized and compact module integrates the facilities required for the detector operation and laser annealing, as well as an active detector temperature control system. The operation of the detector module is highly flexible and software controllable. Our detector module will work together with the control board containing the laser annealing system (built by the UIUC team). Once the satellite has been launched in 2020, the in-orbit experiment will enable us to study the in-space SPAD radiation damage and their healing using the integrated annealing system. During a second project, we designed and built a new simple readout circuit for the negative feedback avalanche diodes (NFADs), which are free-running single-photon detectors at telecom wavelengths. These detectors suffer from strong afterpulsing effects, which limits their overall performances. Therefore, our readout system incorporates features to suppress NFAD afterpulses. We also used this custom readout to characterize two NFADs (from Princeton Lightwave) and assessed the performance of the new electronics. Our analysis showed that even at higher detection efficiencies, a 20
mus hold-off time after each avalanche event is enough to extensively reduce the number of afterpulses and to keep the dark count rate below 100 Hz at 192 K temperature. Both the detectors showed timing jitter of less than 75 ps FWHM at their maximum efficiencies. The best figure of merit is found to be 1.6
times107, which is comparable to that of the high-performing superconducting nanowire single-photon detectors. This result demonstrates the suitability of our readout and the NFADs in various quantum optics applications, such as in long-distance quantum key distribution, where the detection rate is usually low. We then performed a blinding attack, which enables an Eavesdropper in QKD to gain information on the key, on these NFADs using bright illumination. These detectors are usually threshold detectors that generate a click when the optical power is above a certain threshold, otherwise they do not click. Blinding attack utilizes detectors' inability to resolve the photon numbers. During the experiment, we sent controlled optical pulses with a high time resolution to deterministically force detection at the detectors. The result demonstrated the NFADs' susceptibility to these attacks, which tells us to include countermeasures into the system to protect the communications. Finally, we built a quantum source to produce 785 nm polarized photons to implement decoy-state BB84 QKD. Our source utilized the sum-frequency generation scheme to generate 785 nm laser pulses. Its modulator system includes an intensity modulator and two-phase modulators in the Mach-Zehnder configuration to prepare polarized quantum states with different intensities. Our source provides a repetition rate of 500 MHz, which was successfully used in an airborne QKD demonstration with a moving receiver up to
10 km distance. To summarize, my research projects are a contribution to the development of advanced devices, particularly single-photon detectors for quantum communications.
Experimental results obtained with silicon single photon avalanche diodes (SPADs) in quantum key distribution (QKD) at short wavelengths reveal remarkable potential for application in local area networks (LAN) and for free-space transmission at high rate. Actual application prospects, however, depend on the performance level and on the suitability of practical systems using the available silicon SPAD devices. They can be essentially divided in two groups: planar p-n junction structures with a thin depletion layer (typically 1 μm); and reach-through structures with a thick depletion layer (from 20 μm to 150μm). The physical mechanisms that control the device behaviour were investigated and the effect on the key parameters of the detector (quantum detection efficiency, dark counting rate, afterpulsing probability and photon-timing jitter) were thoroughly assessed. A quantitative analysis was made of the influence of such parameters on the quantum bit error rate (QBER). Actual parameters were measured and the attainable performance and system suitability of the two device types evaluated. Comparable performance is obtained, but from a system viewpoint thin SPADs appear inherently better suited to high-rate QKD applications, because of their faster response time, ruggedness, low voltage, low power dissipation and fabrication technology, which is simple, efficient, economical and compatible with monolithic integration of detector and associated circuits.
The implementation and commercialization of quantum cryptography technologies have to face some challenges related to the development of single-photon detectors operating at 1550 nm. The main requirements are: i) high detection efficiency; ii) low noise; iii) high count rate; iv) low timing jitter. Different technologies are currently available for single-photon detection at 1550 nm, but semiconductor devices (like Single-Photon Avalanche Diode, SPAD) offer a photon detection efficiency that is inherently higher than any photocathode employed in vacuum tube detectors. Additionally InGaAs/InP SPADs can detect single photons at 1550 nm with low noise when moderately cooled by means of thermo-electric coolers. Consequently, InGaAs/InP SPAD can be the enabling technology of practical quantum key distribution (QKD) systems, provided that the maximum count rate is increased above 1 Mcps. The main limit is the afterpulsing effect that usually sets too long (< 10 μs) a hold-off time after each avalanche ignition. We present the developments achieved in InGaAs/InP SPAD device design, fabrication technology and front-end electronics, aimed at decreasing the afterpulsing effect, while not impairing photon detection efficiency and timing jitter. The new InGaAs/InP SPADs provide count rates higher than 1 Mcps and temporal response with 60 ps Full-Width at Half Maximum and very short (30 ps time constant) tail. The front-end electronics includes a wide-band pulse generator able to gate the SPAD up to 133 MHz repetition rate. Eventually, a fast avalanche-quenching scheme minimizes quenching time to less than 1 ns, thus effectively reducing afterpulsing by decreasing the total charge flowing through the junction.