This thesis work is about the realization of hardware and software systems for Quantum Random Number Generation (QRNG) and Quantum Key Distribution (QKD). Such systems were developed to guarantee a full functionality for a broader investigation of these two cutting edge applications of Quantum Information field. The thesis describes in details both the hardware and the software that were developed for FPGA-CPU board, Time-to-Digital converter (TDC) devices and computers, along with QRNG and QKD specific applications and their results. Randy was the first FPGA-based QRNG device to be developed; it uses a light source attenuated to single-photon level and one single-photon avalanche diode (SPAD). From the sampling of the SPAD electrical signal, the device produces random numbers through dedicated generation protocols and through the Peres unbiasing algorithm in order to maximize the output generation bit rate. Furthermore, the device allows to generate real time random numbers. This feature is used for the time setting of electro-optical components for extending Wheeler’s delayed-choice experiment to space. The same techniques were applied to a second device, LinoSPAD; it combines an FPGA-chip and a CMOS-SPADs array. Moreover, in this device, a TDC improves the photon detection time accuracy. Along with a dedicated post-processing based on Zhou-Bruk algorithm, the TDC allowed to reach a final bit rate equivalent to 300 Mbit/s. As far as QKD systems are concerned, within the collaboration among the University of Padova, the Italian Space Agency (ASI) with the Matera Laser Ranging Observatory (MLRO) and the Chinese Academy of Sciences (CAS) a TDC device management software was developed. The project aim is to realize a quantum cryptographic key exchange between the Chinese satellite Micius and MLRO. The software was designed to manage the entire data acquisition synchronized with UTC time. Furthermore, another software was designed to deal with electro-optomechanical and electro-optical components. The software is aim at the time-variant compensation of the beam angular changes through the optical path. Once again, within a collaboration between ASI and University of Padova, a full free space QKD system over tenths of kilometers was developed. It required the design of various components. This work describes the QKD source along with the dedicated FPGA board design. Such board generates the electrical impulses to control the qubit laser along with the electro-optic phase and intensity modulators.
This thesis work is about the realization of hardware and software systems for Quantum Random Number Generation (QRNG) and Quantum Key Distribution (QKD). Such systems were developed to guarantee a full functionality for a broader investigation of these two cutting edge applications of Quantum Information field. The thesis describes in details both the hardware and the software that were developed for FPGA-CPU board, Time-to-Digital converter (TDC) devices and computers, along with QRNG and QKD specific applications and their results. Randy was the first FPGA-based QRNG device to be developed; it uses a light source attenuated to single-photon level and one single-photon avalanche diode (SPAD). From the sampling of the SPAD electrical signal, the device produces random numbers through dedicated generation protocols and through the Peres unbiasing algorithm in order to maximize the output generation bit rate. Furthermore, the device allows to generate real time random numbers. This feature is used for the time setting of electro-optical components for extending Wheeler’s delayed-choice experiment to space. The same techniques were applied to a second device, LinoSPAD; it combines an FPGA-chip and a CMOS-SPADs array. Moreover, in this device, a TDC improves the photon detection time accuracy. Along with a dedicated post-processing based on Zhou-Bruk algorithm, the TDC allowed to reach a final bit rate equivalent to 300 Mbit/s. As far as QKD systems are concerned, within the collaboration among the University of Padova, the Italian Space Agency (ASI) with the Matera Laser Ranging Observatory (MLRO) and the Chinese Academy of Sciences (CAS) a TDC device management software was developed. The project aim is to realize a quantum cryptographic key exchange between the Chinese satellite Micius and MLRO. The software was designed to manage the entire data acquisition synchronized with UTC time. Furthermore, another software was designed to deal with electro-optomechanical and electro-optical components. The software is aim at the time-variant compensation of the beam angular changes through the optical path. Once again, within a collaboration between ASI and University of Padova, a full free space QKD system over tenths of kilometers was developed. It required the design of various components. This work describes the QKD source along with the dedicated FPGA board design. Such board generates the electrical impulses to control the qubit laser along with the electro-optic phase and intensity modulators.
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.
Quantum key distribution (QKD) is the most matured application of quantum communication. The time synchronization and system timing jitter are essential to a practical QKD experiment, which can directly influence the quantum bit error rate (QBER) and the final key rate. High precision time-to-digital converters (TDCs) can effectively reduce the system timing jitter, resulting in lower QBER and higher key rate. Here, we introduce a TDC based on a field programmable gate array (FPGA). Its high resolution (LSB) of 50 ps with precision (RMS) less than 50 ps, low dead time and large dynamic range can well meet the requirement of the QKD experiment. A free space QKD system setup using the FPGA-based TDC has been assembled. The timing precision of the synchronous light and the system timing jitter were determined. The results demonstrate that the FPGA-based TDC could be successfully applied in the QKD experiment.
Quantum technologies are rapidly transforming the landscape of secure communication and computation by exploiting the fundamental principles of quantum mechanics. Among these, Quantum Key Distribution (QKD) offers an unprecedented level of security in communications, provided by physical principles rather than computational assumptions. However, the realization of scalable and practical quantum networks remains challenging. Key technological hurdles include the need for high-sensitivity and high-speed single-photon detectors, stable and integrated optical platforms, and robust and fast true random number generators. Overcoming these challenges is essential to bridge the gap between laboratory demonstrations and the real-world deployment of quantum networks. This thesis addresses some of these critical issues through work in single-photon detection technologies, integrated QKD system implementation, and high-speed Quantum Random Number Generators (QRNGs). Single-Photon Avalanche Diodes are fundamental detectors for quantum technologies, enabling high-sensitivity measurements at the single-photon level. This thesis investigates the operating principles, limitations, and advancements of Single-Photon Avalanche Diodes, with a focus on high-speed dual-anode designs. Their photon detection efficiency, dark count rate, afterpulsing probability, and timing jitter are analyzed, while optimized control electronics and capacitance management strategies are developed. The characterization of Single-Photon Avalanche Diodes demonstrates their suitability for quantum communication, particularly in quantum key distribution and asynchronous heralded photon sources, with pathways for further improvements in speed and integration. Building on these detector technologies, a proof-of-principle integrated quantum key distribution system based on the BB84 protocol is presented. The system incorporates time-bin encoding, decoy-state methods, and photonic integrated circuits for both transmitter and receiver. Key challenges, such as chromatic dispersion and interferometer stability, are addressed through novel circuit and system-level designs. System-level testing and field deployment validate the feasibility of compact, high-performance integrated quantum key distribution implementations. Finally, the thesis explores quantum random number generation through semi-device-independent protocols. An experimental homodyne-based SDI-QRNG is realized, supported by custom transimpedance amplifier design and photonic integrated circuit integration. The setup achieves high bandwidth, strong quantum-to-classical noise separation, and stable operation, enabling practical random number generation rooted in fundamental quantum processes. Overall, this work advances the development of integrated quantum technologies by improving room-temperature single-photon detection, demonstrating scalable QKD prototypes, and implementing a high-speed QRNG. Together, these contributions pave the way toward secure, integrated quantum communication systems and practical quantum information applications ready to deploy in real world networks.
Quantum key distribution (QKD) is the most matured application of quantum communication. The time synchronization and system timing jitter are essential to a practical QKD experiment, which can directly influence the quantum bit error rate (QBER) and the final key rate. High precision time-to-digital convertor (TDC) can effectively reduce the system timing jitter, resulting in lower QBER and higher key rate. Here, we introduce a TDC based on a field-programmable gate array (FPGA). Its high resolution of 50 ps, low dead time and large dynamic range can well meet the requirement of the QKD experiment. A free space QKD system setup using the FPGA-based TDC has been built up. The timing precision of the synchronous light and the system timing jitter were tested. The results demonstrate that the FPGA-based TDC could be successfully applied in the QKD experiment.
This article presents a hardware and software architecture, which can be used in those systems that implement practical quantum key distribution (QKD) and quantum random-number generation (QRNG) schemes. This architecture fully exploits the capability of a System on a Chip (SoC), which comprehends both a field-programmable gate array (FPGA) and a dual-core CPU unit. By assigning the time-related tasks to the FPGA and the management to the CPU, we built a flexible system with optimized resource sharing on a commercial off-the-shelf (COTS) evaluation board, which includes an SoC. Furthermore, by changing the dataflow direction, the versatile system architecture can be exploited as a QKD transmitter, QKD receiver, and QRNG control-acquiring unit. Finally, we exploited the dual-core functionality and realized a concurrent stream device to implement a practical QKD transmitter, where one core continuously receives fresh data at a sustained rate from an external QRNG source, while the other operates with the FPGA to drive the qubit transmission to the QKD receiver. The system was successfully tested on a long-term run proving its stability and security. This demonstration paves the way toward a more secure QKD implementation, with fully unconditional security as the QKD states are entirely generated by a true random process and not by deterministic expansion algorithms. Eventually, this enables the realization of a standalone quantum transmitter, including both the random numbers and the qubit generation.
An important issue in today’s information society is the security of data transmission against potential intruders, which always put at risk the confidentiality. Current methods to increase security require that the two parties wishing to transmit information, exchange or share one or more security keys. Once the key has been identified, the information can be transferred in a provable secure way using a one-time pad, i. e. the key length is as long as the plaintext. Therefore, the security of the information transmission is based exclusively on the security of the key exchange. Quantum cryptography, or more precisely quantum key distribution (QKD), guarantees absolutely secure key distribution based on the principles of quantum physics, according to which it is not possible to measure or reproduce a state (e.g. polarization or phase of a photon) without being detected. The key is generated out from the measurement of the information encoded into specific quantum states of a photon, named qubits. For example, a qubit can be created using properties such as the polarization or the phase of a photon. Achieved goals of this thesis are the development of a new class of high speed integrated photonic sources for applications in quantum key distribution systems, capable of producing unprecedented qubit rates (100 Mbps - 1 Gbps) and transmitting those over larger distances than those achieved so far (> 200 km). More specifically the work has been focused on developing faint pulse sources which can be used in very demanding environmental conditions, such as those in Space. For the development of these sources, apart from the optical design, essential is the opto-mechanical engineering as well as the integration with the electronics. One of the objectives was to achieve very high level of integration and power efficiency, e.g. volumes and power consumption between 10 and 100 times smaller than those typical of a laboratory experiment. Moreover, work in related parts of a whole QKD transmission system has been carried out. In particular, a new scheme for a compact, fast and simple random number generator has been demonstrated successfully achieving a random number generation rate of 1:1 Gbps. Also, during the course of this thesis, the development and engineering of a free-space QKD optical link has been initiated. This thesis makes use of novel ideas to alternatively demonstrate proof-of- concept experiments, which could then further develop into commercial products. To this end, close collaborations with world-wide leading companies in the field have been established. The Optoelectronics Group at ICFO has been involved in current European Space Agency (ESA) projects to develop a small footprint and low power consumption quantum transceiver and a high-flux entangled photon source.