The development of new quantum communication technologies for optical networks will enable inviolable protocols to exchange secure information. The first and more ro-bust among them is the quantum key distribution (QKD). Although in theory the security is proven unconditionally safe, the implementation in optical networks can still produce flaws due the non-ideal behavior of the optical and optoelectronic devices. In this direc-tion, this thesis presents two contributions to improve security of QKD systems. The first is the redesign of two QKD protocols to use two quantum states of light, the coherent and thermal state, with a polarization randomness. The protocols are the differential quadrature phase shift QKD and QKD using homodyne detection. The second contribution is the experimental characterization of the light emitted by avalanche photodiodes during a detection and its implication in the security of the BB84 QKD protocol.
The development of new quantum communication technologies for optical networks will enable inviolable protocols to exchange secure information. The first and more ro-bust among them is the quantum key distribution (QKD). Although in theory the security is proven unconditionally safe, the implementation in optical networks can still produce flaws due the non-ideal behavior of the optical and optoelectronic devices. In this direc-tion, this thesis presents two contributions to improve security of QKD systems. The first is the redesign of two QKD protocols to use two quantum states of light, the coherent and thermal state, with a polarization randomness. The protocols are the differential quadrature phase shift QKD and QKD using homodyne detection. The second contribution is the experimental characterization of the light emitted by avalanche photodiodes during a detection and its implication in the security of the BB84 QKD protocol.
Quantum key distribution (QKD) allows two users to communicate with information theoretic security by encoding information on single photons. This security is based on the laws of physics and as such can never be broken in theory. However, in practice, components do not always behave according to their theoretical models and these deviations can be exploited by an eavesdropper. In recent years, exposing loopholes in QKD systems, known as quantum hacking, has attracted significant attention. The components most susceptible to being hacked are the single-photon detectors, often avalanche photodiodes (APDs), as they are directly exposed to the optical channel. Whilst measurement-device-independent QKD removes detector vulnerability from the system, secure key rates with this technique can be much lower than point-to-point links. As such, mitigating attacks on QKD systems is a pressing challenge in QKD. In this thesis, the focus is on a special class of detectors, self-differencing APDs (SD-APDs), which have facilitated state-of-the art demonstrations of QKD. The susceptibility of SD-APDs to blinding attacks, the most explored and successful attack to date, was investigated and it was shown that by following best practice for their operation, such an attack would be unsuccessful. We have also proposed and developed a countermeasure such that the onus for appropriate operation could be removed from the user. We have also explored an arguably more dangerous attack, in the form of the after-gate attack. We have shown that delayed detection events, ordinarily considered detrimental in QKD, can provide inherent protection against this attack. Finally, backflashes in GHz-gated APDs were investigated for the first time and it was shown that threat they pose to QKD security is negligible. These results highlight the inherent protection to a number of attacks that self-differencing APDs possess. We stress that the findings presented in this thesis are also applicable to other types of fast-gated InGaAs APDs that don't possess self-differencing circuitry.
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 cryptography has been developed from being a theoretical proposal to having real world applications, with companies developing and selling quantum key distribution (QKD) instruments commercially. Although laws of quantum physics guarantee perfect security in the key distribution protocol, several vulnerabilities in the practical implementations have been described and demonstrated. In some of these attacks the eavesdropper has been able to acquire the entire secret key without being detected by the two communicating parties. This thesis describes an experimental setup for automatically characterizing and damaging single photon avalanche photodiodes using a high power laser to damage the diode with the intention to find new weaknesses in QKD systems. Hopefully, the discovery of loopholes will stimulate the researchers and commercial manufacturers to fix their implementations, leading to more secure systems. For a silicon avalanche photodiode (APD) in a passive-quenching circuit, a number of interesting results have been found. It has been shown that we are able to change many of the APDs key characteristics by applying strong illumination. We have been able to reduce the dark count rate of several APDs by up to 80% in a predictable way by strong laser illumination, an effect which is believed to be caused by localized annealing. This is an interesting result not only in the field of quantum cryptography, but perhaps also for improving the manufacturing process of different semiconductor components. Other parameters that have been found to change in interesting ways are the dark current and the breakdown voltage. It has been demonstrated that these changes may compromise the security of QKD systems.
The counter-intuitive features of quantum mechanics make it possible to solve problems and perform tasks that are beyond the abilities of non-quantum (classical) computers and communication devices. The field of quantum information processing studies how we can achieve such improvements by representing information as quantum states.One of the early achievements of quantum information processing is the development of quantum key distribution (QKD). QKD protocols allow two participants (Alice and Bob) to achieve the classically-impossible task of generating a secret shared key even if their adversary is computationally unlimited.Unfortunately, the security promises of QKD are true only in theory; practical implementations of QKD deviate from the theoretical protocols, and many of these deviations give rise to practical attacks. In this research thesis, we study the security properties of various QKD protocols in many practical settings:First, we study practical security of semiquantum key distribution (SQKD) protocols, where either Alice or Bob is non-quantum (classical). Following practical security problems in previous SQKD protocols, we suggest a new SQKD protocol (the "Mirror protocol") which can be securely implemented, and we prove it robust and secure against a wide range of attacks (the "uniform collective attacks").Then, we study "composable security" of the first QKD protocol created by Bennett and Brassard (BB84). BB84 has its unconditional security proved against adversaries performing the most general attacks in a theoretical (idealized) setting; however, some security approaches do not prove "composable security", which requires the secret key to remain secret even when Alice and Bob actually use it for cryptographic purposes. We generalize an algebraic security approach for BB84, making it prove composable security of BB84 (and many variants of BB84) against the most general attacks.Finally, we analyze an important practical attack (named "Bright Illumination"), showing how it can be modeled as a theoretical "Reversed-Space" attack.Overall, all results aim to enhance our understanding on how to bridge the gap between theory and practice in various sub-fields of QKD, and they may help solve a major open problem in the field of QKD: constructing a realistic QKD implementation that can be proved truly and unconditionally secure against any possible attack.
Quantum Information Technology is a new multi-disciplinary area which has received a lot of attention due to its promises and its high potential in solving problems still unsolved. In this big area, Quantum Communication is too much developed. In this subarea, Quantum Key Distribution is the most advanced field. It permits two parties, named Alice and Bob, sharing a cryptography key through a secure channel (guaranteed by laws of quantum mechanics). The most of Quantum Key Distribution Systems run over optical fiber links and, in these systems, the most important part is the Single-Photon Detector. Single-Photon Detector is an equipment able to absorb a photon and generate a TTL pulse. Thus, in an ideal Single-Photon Detector, each photon incoming must trigger a TTL pulse at the output. Since the energy level of a single-photon is too much low, an avalanche photodiode is used to perform the photon absorption-carrier generation process, once this photodiode if correctly biased can trigger a detectable avalanche of carriers. After the avalanche has been started, it must be quenched in order to avoid any damage to the photodiode, which is made by an avalanche quenching circuit. The avalanche photodiode is the most important element of a Single-Photon Detector and its characterization requires much attention. In this context, this dissertation deals with theoretical and practical aspects of Single-Photon Detectors for Quantum Communication. It starts from the theory of avalanche photodiodes and quenching circuits (numerical results of quenching circuits are also shown) and follows until the characterization of a home-made Single-Photon Detector and its applications in Metrology of optical devices and in Photon-Number Resolution as well.
In this thesis I describe three independent projects that advance the development of broadband quantum cryptography. While each project pertains to a different part of the QKD chain, together they provide key developments in implementing QKD at bit rates that are practical for use in the modern telecommunications infrastructure. The first project comprises the bulk of the thesis and involves developing a novel source of correlated photon pairs for use in free-space QKD. This source is based on a birefringent semiconductor optical waveguide as a Kerr medium. We demonstrate the feasibility of using birefringent phase-matched four-wave mixing to generate correlated photon pairs. We further propose that, by reversing the process and pumping with conjugate wavelengths, one can use the same effect to produce entangled photon pairs with the same device. These pairs can then be used for QKD to realize the most secure and efficient quantum cryptographic data links. The second project examines the implications of operating a BB84 QKD protocol at clock rates that are faster than the recovery time of the constituent detectors. We show that operating such systems under conventional protocols results in a security violation that allows an eavesdropper to learn significant information about the key and present a modification to the BB84 protocol that maintains key security at fast transmission rates. This modification to the protocol will become vital to QKD viability as links become faster and clock rates go into the tens of gigahertz. We also demonstrate, rather counterintuitively, that there exists an optimal transmission rate for a QKD system that exceeds the inverse of an individual detector's dead time. The final project describes a new design for a free-space QKD link that centers around faster silicon detectors. These detectors have a peak quantum efficiency in the visible range, requiring that the system operate at a wavelength that is more susceptible to solar interference. To mitigate this effect, the link is designed around a Fraunhofer line in the solar spectrum where the background solar light levels are reduced by up to 90%. By implementing this system, we expect at least a two-fold increase in the secret key rate, coming ever closer to the goal of a 10 Mb/s QKD system compatible with first-generation ethernet technology.