Single-Photon Avalanche Diodes (SPADs) are critical components of emerging quantum communication networks, used to detect single photon. They are placed in satellites for longdistance communication and are susceptible to radiation-induced displacement damage in the space environment. This degrades their performance. In this work, SPAD is simulated in a space environment by introducing a quantum simulation framework that leverages second quantization and Fock space truncation to model the SPAD as a two-level (|g⟩,|e⟩), incorporating radiation damage through Lindblad open system dynamics. This approach differs significantly from conventional TCAD simulations, which rely on semiclassical approximations that fail to capture the discrete quantum statistics of single-photon interactions and the dynamics of trap states. In this work, Photon Detection Efficiency and timing jitter, in ideal and damping environments, are simulated as expectation values.
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.
Single photons detectors are required for applications like quantum key distribution and testing Bell's inequalities. Silicon avalanche photodiodes (Si APD's) have advantages like low dark count rate. This thesis aims to experimentally characterize Si APD's under conditions experienced in low earth orbit on board a satellite and also develop supporting algorithms and simulations. \n \nIn chapter 1, I motivate the use of Si APD's as single photon detectors on a satellite. I also briefly explain the how these APD's work and the related electronics. \n \nIn chapter 2, I describe the test setup used in the laboratory at the Institute for Quantum Computing to characterize the APD's. I then explain how one goes about measuring and/or calculating the different characteristic parameters of the APD. These characteristics include breakdown voltage, output pulse characteristics, recharge time, dark counts, detection efficiency, timing jitter of APD, saturation value and afterpulsing probability. The aim of the lab characterization is to form a baseline measurement under nominal conditions before we introduce the APD to conditions of low Earth orbit. \n \nIn chapter 3, the APD's are exposed to proton radiation which is expected in low Earth orbit. Such radiation causes displacement defects in the APD's substrate and increases dark counts. Previous work [1] has shown that thermal annealing (increasing the APD temperature for a period of time) is successful in decreasing the dark count rate to operational range. It is anticipated that multiple annealing instance would be needed in operation under constant bombardment of protons. The work in this thesis explores possibility of using annealing repeatedly by alternating instances of proton irradiation and annealing. Other parameters of the APD are also under scrutiny to see if they change over the course of this experiment. A secondary objective explores if it is better to anneal only when the dark count rate drops below a threshold value or anneal after fixed intervals of time regardless of the dark count rates. A novel algorithm is also developed to analyze the afterpulsing probability. The algorithm is applied to simulated thermal count and afterpulse timetags to demonstrate its effectiveness. It is also applied to the data set drawn from the experimental tests of proton irradiation and thermal annealing to calculate the afterpulsing probability. \n \nIn chapter 4 the APD's are exposed to the vacuum conditions that they would encounter in low Earth orbit. The bracket and radiator are set to different temperatures to simulate thermal variations in orbit. The detectors are then characterized for the aforementioned parameters to ensure they are in operable range. \n \nIn chapter 5, I distinguish between the usual notion of afterpulsing probability which relates to the probability of trapping of charge carriers and the observed afterpulsing probability which differs from the former due to dead time and recharge time. I then use first principles to derive the observed afteprulsing probability as a function of charge trapping probability, charge detrapping lifetime, dead time and thermal count rate. I further go on to lay the theoretical ground work the afterpulsing algorithm developed in section 3.3.8 by deriving the analytical expression for the histogram between time differences of consecutive time tags and then justifying the linearity of the thermal region of the histogram. \n \nFinally, in chapter 6, I devise a numerical experiment where I simulated a quantum link to implement the quantum key distribution. I model the photon statistics of a weak coherent pulsed laser and the parameters of a Si APD (the one mentioned above). I then model the coincidence algorithm to determine photon coincidences and model a simple intercept and resend attack. I calculate statistical quantities of quantum bit error ratio and sifted key generation rate.
Single-photon detection concept is the most crucial factor that determines the performance of quantum key distribution (QKD) systems. In this paper, a simulator with time domain visualizers and configurable parameters using continuous time simulation approach is presented for modeling and investigating the performance of single-photon detectors operating in Gieger mode at the wavelength of 830 nm. The widely used C30921S silicon avalanche photodiode was modeled in terms of avalanche pulse, the effect of experiment conditions such as excess voltage, temperature and average photon number on the photon detection efficiency, dark count rate and afterpulse probability. This work shows a general repeatable modeling process for significant performance evaluation. The most remarkable result emerged from the simulated data generated and detected by commercial devices is that the modeling process provides guidance for single-photon detectors design and characterization. The validation and testing results of the single-photon avalanche detectors (SPAD) simulator showed acceptable results with the theoretical and experimental results reported in related references and the device's data sheets.
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.