In this thesis, I study quantum resources for quantum communication and cryptography. The first part deals with bit commitment, a cryptographic primitive in which a party commits a secret bit to another party. After shortly presenting the security issues faced by quantum bit commitment, I demonstrate the first realization of a secure relativistic bit commitment sustained for 24 hours. The second part deals with the development of efficient methods to generate, distribute and characterize photonic entangled states. I address the problem of certifying entanglement in the presence of measurement imperfections, and demonstrate a practical implementation of a measurement-device-independent entanglement witness. I then address the challenge of long-distance distribution of entanglement. I demonstrate a heralded photon amplifier and entanglement swapping protocols adapted for the distribution of single photon entanglement, and further discuss the advantages and limitations of using single photon entanglement as a resource for quantum communication.
In this thesis, I study quantum resources for quantum communication and cryptography. The first part deals with bit commitment, a cryptographic primitive in which a party commits a secret bit to another party. After shortly presenting the security issues faced by quantum bit commitment, I demonstrate the first realization of a secure relativistic bit commitment sustained for 24 hours. The second part deals with the development of efficient methods to generate, distribute and characterize photonic entangled states. I address the problem of certifying entanglement in the presence of measurement imperfections, and demonstrate a practical implementation of a measurement-device-independent entanglement witness. I then address the challenge of long-distance distribution of entanglement. I demonstrate a heralded photon amplifier and entanglement swapping protocols adapted for the distribution of single photon entanglement, and further discuss the advantages and limitations of using single photon entanglement as a resource for quantum communication.
Quantum entanglement plays a central role in many quantum communication protocols. It allows distant particles to share correlations beyond the limits of classical interactions. Entanglement is essential for superdense coding, quantum teleportation, and secure cryptographic key distribution in quantum communication. It is also a pillar for developing quantum networks, where the management and distribution of entanglement are crucial for connecting distant nodes. Although perfect entanglement is a sought-after ideal, experimental imperatives, including entanglement distribution over long distances, often limit the quality of entangled states. An important question is whether weaker entanglement still offers advantages. First, we study more general tasks than dense coding to show that simpler measurements, combined with entanglement, allow advantageous and sometimes even optimal qubit communication protocols to be obtained. We also demonstrate that simple measurements can generate quantum correlations that cannot be modelled by two classical communication bits and can constitute an optimal protocol with quantum resources. We implement a novel Bell-type inequality tailored for certifying full network non-locality (FNN) to develop certification methods guaranteeing security on an entanglement-based network. Our experiment uses two pairs of polarised entangled photons in a network configuration with three nodes, briefly referred to as a bilocal network scenario. Finally, we show that weakly entangled states can improve communication over a qubit channel using only separate (local) measurements on isotropic non-steerable two-qubit states, without interference, of individual photons, across two communication tasks: secret sharing and its stochastic variant.
During my thesis I investigated several technologies and protocols aiming at practical quantum-cryptography applications. Firstly, I developed a free-running single-photon detector for telecom wavelengths featured by an extremely low noise that competes with the state-of-the-art superconducting single-photon detectors. Then I developed a testbench for metrological characterization of single-photon detectors. A special source of spontaneous emission and a radiometer have been developed during this work. Both the source and the radiometer are based on an Erbium-doped fiber and have been built using standard commercial components. In the third part of my thesis I discuss the first experimental demonstration of a bit commitment protocol provably secure. Finally I investigated an innovative approach to self-test the randomness underlying a quantum process. This approach allows generating unpredictable strings of bits with few calibrations of the device.
The work in this thesis can be divided into three parts. The first two parts deal with optical quantum communication protocols and the third part deals with quantum cryptography. The first part of the thesis is a step towards reformulating quantum protocols in terms of coherent states. Quantum communication protocols are typically formulated in terms of abstract qudit states and operations. This leaves the question of an experimental realization open. Direct translation of these protocols, say into single photons with some d-dimensional degree of freedom, are typically challenging to realize. Multi-mode coherent states, on the other hand, can be easily generated experimentally. Reformulation of protocols in terms of these states has been a successful strategy for implementation of quantum protocols. Quantum key distribution and the quantum fingerprinting protocol have both followed this route. In Chapter 3, we characterize the Gram matrices of multi-mode coherent states in an attempt to understand the class of communication protocols, which can be implemented using these states. We also derive the closure of the Gram matrices, which can be implemented in this way, so that we also characterize those matrices, which can be approximated arbitrarily well using multi-mode coherent states. \n \nIn the second part of the thesis, Chapter 4, we describe our collaboration with an experimental group to implement the quantum fingerprinting protocol and examine the tradeoffs between the resources required to implement such protocols. It is seen that it is difficult to implement the quantum fingerprinting protocol experimentally for large input sizes. This leads us to study the tradeoff between the two resources expended during optically implemented simultaneous message passing communication protocols: the duration of the protocol and the energy required to run it. We derive general bounds on the growth of these quantities which are valid for all optical protocols. We also develop tighter bounds for the growth of these resources for protocols implementing quantum fingerprinting with coherent states. \n \nFinally in Chapter 5, we venture into quantum cryptography. We introduce a new setting for two-party cryptography with temporarily trusted third parties. In this setting, in addition to Alice and Bob, there are third parties, which Alice and Bob both trust to be honest during the course of the protocol. However, once the protocol concludes, there is no guarantee over the behaviour of these third parties. It is possible that they collaborate and act adversarially against the honest parties. We implement a variant of bit commitment in this setting, which we call erasable bit commitment. In this primitive, Alice has the choice of either opening or erasing her commitment after the commit phase. The ability to ask for an erasure allows Alice to ask the trusted nodes to erase her commitment in case the trust period is about to expire. However, this ability also makes erasable bit commitment weaker than the standard version of bit commitment. In addition to satisfying the security requirements of bit commitment, our protocol also does not reveal any information about the commitment to the third parties. Lastly, our protocol for this primitive requires a constant number of trusted third parties and can tolerate a small number of dishonest trusted nodes as well as implementation errors.
Quantum cryptography uses the quantum properties of individual photons to provide two or more users with means to communicate securely and efficiently. Specifically, quantum key distribution (QKD) focuses on the secure communication between two distant parties and guarantees the security of a transmitted message between them. Although QKD is the most mature application of quantum cryptography, applications of quantum mechanics in communication systems are not limited to QKD and other cryptographic protocols can be implemented as well. This thesis focuses on the implementation of quantum cryptographic systems over deployed fiber. A first QKD system was built and used to demonstrate the BB84 protocol over deployed fiber between the University of Calgary and SAIT. The system implemented a novel tool called quantum frames that allow for the quantum signals to be routed, for clock synchronization, and for channel stabilization in a network scenario. The same system was used to examine the scalability of the secret key generation rate in different steps of the process in order to detect potential bottlenecks for key generation rate of high speed systems. A second QKD system was developed to demonstrate the measurement-device independent QKD protocol. The system worked over deployed fiber between the University of Calgary, SAIT, and Foothills Hospital in the City of Calgary. The advantage of the measurement-device independent QKD protocol over previous QKD protocols is that it eliminates the possibility of side-channel attacks that target single photon detectors used in QKD systems. This demonstration also involved the first demonstration of a Bell state measurement over deployed fiber. Bell state measurements were further studied by using novel single photon detectors, which allowed demonstrating a highly efficient Bell measurement. Finally, a cryptographic protocol known as private queries was demonstrated between the University of Calgary and SAIT. Quantum private queries were demonstrated for the first time over deployed fiber by developing and implementing an error correction scheme to make the protocol noise and loss tolerant.
Quantum communication attempts to harness the unique rules of quantum mechanics to perform communication tasks that are difficult or impossible using classical rules. To realize these benefits, information must be carried on quantum systems. Photons make excellent carriers because they interact very little with the environment, move quickly, and can naturally store quantum information in their polarization. However, it is notoriously difficult to detect a single photon without destroying it. Standard detectors simply absorb the photon, losing the quantum information. This is a critical outstanding problem in quantum communication, as advanced protocols need to know exactly when a photon has arrived at a receiver after transmission through the atmosphere or an optical fibre before performing further quantum-information-processing tasks. Certifying a photon’s presence is of particular interest in tests of Bell’s inequalities, which have only recently been performed without loopholes arising from photon loss, and in device-independent quantum cryptography, which relies on such Bell tests for security. \n \nIn this thesis, I first present work on directly reducing losses in a successful loophole-free Bell test. This is an extremely difficult task that cannot be extended for long-distance communication. Therefore I then focus on ways to circumvent loss by detecting photons without destroying them while preserving their quantum information. First I analyze theoretically a way to herald photons using only linear-optical elements (beam splitters and phase shifters) and extra ancilla photons. Similar but older methods have been demonstrated experimentally by other groups, and my improvements will help future advanced quantum communication protocols. \n \nThe main experiment in this thesis certifies the presence of a photon in a rather simple way: split the photon into two using a nonlinear optical crystal, then detect one of the pair to herald the other. I show in this first proof-of-principle experiment that photonic qubit precertification indeed preserves qubit states, with up to (92.3 ± 0.6)% fidelity and rates of 1100 events per hour. With reductions in detector dark counts, precertification could outperform direct transmission, even with extremely lossy fibre links. \n \nFinally, I present two sources of photons based on nonlinearities in optical fibres. One of the limitations of the photon splitting scheme for heralding is the low success probability due to the very low likelihood of splitting a photon in two. In these fibre photon sources I investigate increasing the splitting likelihood in four-wave mixing through advanced materials and fibre designs. I use polarization-maintaining fibres to generate entangled photon pairs as a prerequisite to precertification, with (92.2 ± 0.2)% fidelity to a maximally-entangled state. Then I show that one type of highly nonlinear chalcogenide glass, never before used for photon pair generation, could outperform standard nonlinear crystals by two orders of magnitude, with calculated conversion efficiency up to 10^-3. To approach the single photon regime, I generate photon pairs in chalcogenide microwires with pump powers as low as 480 nW and verify their timing correlations. \n \nThese advances demonstrate the capability of nondestructive precertification of photons to allow advanced quantum communications protocols even over long distance, and could lead to the first implementations of fully device-independent quantum cryptography.
In this thesis we explore the benefits of relativistic constraints for cryptography. We first revisit non-communicating models and its applications in the context of interactive proofs and cryptography. We propose bit commitment protocols whose security hinges on communication constraints and investigate its limitations. We explain how some non-communicating models can be justified by special relativity and study the limitations of such models. In particular, we present a framework for analysing security of multiround relativistic protocols. The second part of the thesis is dedicated to analysing specific protocols. We start by considering a recently proposed two-round quantum bit commitment protocol. We propose a fault-tolerant variant of the protocol, present a complete security analysis and report on an experimental implementation performed in collaboration with an experimental group at the University of Geneva. We also propose a new, multiround classical bit commitment protocol and prove its security against classical adversaries. This demonstrates that in the classical world an arbitrarily long commitment can be achieved even if the agents are restricted to occupy a finite region of space. Moreover, the protocol is easy to implement and we report on an experiment performed in collaboration with the Geneva group.