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.
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.
This doctoral dissertation deals with the subject of single-photon technology applications. Particular emphasis was placed on the use of single-photon sources in quantum communication, metrology, and further development of quantum computers. At the beginning of this thesis, the development history of cryptography is described, starting from steganography and ending with quantum key distribution protocols. Later, the key elements of the quantum network, such as end nodes and transmission channels, are presented. Various types of sources and single-photon detectors are discussed. Methods for amplifying the signal and correcting errors that occur during transmission are also mentioned. In the further part of this thesis, two examples of single-photon sources are described, along with their experimental implementation. These sources are based on the nonlinear spontaneous parametric down-conversion (SPDC) process, which has also been described in detail. Parameters that specify the quality of a single-photon source (such as conversion efficiency, phase-matching conditions, singleness and photonic density matrix) are also characterized, both mathematically and experimentally. Furthermore, this doctoral dissertation consists of definitions and elaborated descriptions of quantum optical phenomena, which should be useful for those who are not yet familiar with the subject of single-photon sources and their applications or just start their work in this field.
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.
One of the main issues in the domain of quantum information and communication is the generation, manipulation and detection of several qubits on a single chip. Several approaches are currently investigated for the implementation of qubits on different types of physical supports and a variety of quantum information technologies are under development: for quantum memories, spectacular advances have been done on trapped atoms and ions, while to transmit information, photons are the ideal support thanks to their high speed of propagation and their almost immunity against decoherence. My thesis work has been focused on the conception, fabrication and characterization of a miniaturized semiconductor source of entangled photons, working at room temperature and telecom wavelengths. First the theoretical concepts relevant to understand the work are described (chapter 1). Then the conception and fabrication procedures are given (chapter 2). Chapter 3 presents the optoelectronics characterization of the device under electrical pumping, and chapter 4 the results on the optical losses measurements and the nonlinear optical characterization (second harmonic generation, spontaneous parametric down conversion and joint spectral intensity reconstruction). Chapters 5 and 6 focus on the characterization of the quantum state generated by a passive sample (demonstration of indistinguishability and energy-time entanglement) and its utilization in a multi- user quantum key distribution protocol (polarization entanglement). Finally the work on the first electrically driven photon pairs source emitting in the telecom range and working at room temperature is presented (chapter 7).
During this thesis work, two approaches to single-photon detection at telecom wavelengths have been investigated. The first one focuses on improving the performance of commercially available semiconductor single photon avalanche diodes (SPADs) to meet quantum communication requirements, and the second one describes the development and characterization of large active-area superconducting nanowire single-photon detectors (SNSPDs). On the application side, quantum random number generation (QRNG) was implemented using two schemes based on two different types of single photon detectors activated by a LED.
Quantum technologies are driving more and more interest both in academia and industry thanks to their promising performances in terms of security. In reality, practical systems suffer from imperfections compared to theoretical models which could be exploited by an eavesdropper if no protection is implemented. Bridging the gap between theory and practice is therefore one of the main challenges of this field today. During this thesis, I worked on different aspects of the practical security of quantum technologies. This ranges from the modeling of the entropy source of a quantum random number generator (QNRG) to the study of the vulnerabilities of quantum key distribution (QKD) implementations against hacking. In the first part, I focus on the modeling of a commercial QRNG chip from ID Quantique. More specifically, I present the model we developed for the quantum entropy source of the device. We estimate that this fully integrated device can provide a quantum min-entropy of 0.98 per bit. Importantly, this near-unity quantum entropy is achievable without post-processing reducing the power consumption of the chip, making it suitable for mobile devices. Moreover, we show that this high-quality entropy is robust against imperfections. On the side of QKD security, I begin by studying the behavior of negative-feedback avalanche diode (NFAD) detectors under a blinding attack. After showing their vulnerability to this attack and testing the resilience of a countermeasure based on the monitoring of the mean photocurrent, I present an improved countermeasure. This allows Bob to detect more advanced blinding strategies where Eve modulates her blinding power to reduce her impact on the mean photocurrent. In the last part of this thesis, I present a novel method to prevent the blinding attack based on statistical measurements with a multi-pixel detector. Through this approach, we can estimate an upper bound on Eve's information on the key exchanged. An analysis of the finite-key effects estimates that this countermeasure can be effective for distances up to 250km. The applicability of this countermeasure with current technology is shown with a 2-pixel superconducting detector.
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.