Quantum-based security in optical fibre networks

Electronic communication is used everyday for a number of different applications. Some of the information transferred during these communications can be private requiring encryption and authentication protocols to keep this information secure. Although there are protocols today which provide some security, they are not necessarily unconditionally secure. Quantum based protocols on the other hand, can provide unconditionally secure protocols for encryption and authentication. Prior to this Thesis, only one experimental realisation of quantum digital signatures had been demonstrated. This used a lossy photonic device along with a quantum memory allowing two parties to test whether they were sent the same signature by a single sender, and also store the quantum states for measurement later. This restricted the demonstration to distances of only a few metres, and was tested with a primitive approximation of a quantum memory rather than an actual one. This Thesis presents an experimental realisation of a quantum digital signature protocol which removes the reliance on quantum memory at the receivers, making a major step towards practicality. By removing the quantum memory, it was also possible to perform the swap and comparison mechanism in a more efficient manner resulting in an experimental realisation of quantum digital signatures over 2 kilometres of optical fibre. Quantum communication protocols can be unconditionally secure, however the transmission distance is limited by loss in quantum channels. To overcome this loss in conventional channels an optical amplifier is used, however the added noise from these would swamp the quantum signal if directly used in quantum communications. This Thesis looked into probabilistic quantum amplification, with an experimental realisation of the state comparison amplifier, based on linear optical components and single-photon detectors. The state comparison amplifier operated by using the wellestablished techniques of optical coherent state comparison and weak subtraction to post-select the output and provide non-deterministic amplification with increased fidelity at a high repetition rate. The success rates of this amplifier were found to be orders of magnitude greater than other state of the art quantum amplifiers, due to its lack of requirement for complex quantum resources, such as single or entangled photon sources, and photon number resolving detectors

Implementation of Quantum Key Distribution Protocols

As a wide spectrum of the human activity rapidly transitions to a digital environment, the need for secure and efficient communication intensifies. The currently used public key distribution cryptosystems, such as the Rivest-Shamir-Adleman (RSA) protocol, source their security from the computational difficulty of certain mathematical problems. While widely successful, the security these cryptosystems offer remains heuristic and the development of Quantum computers may render them obsolete. The security that Quantum Key Distribution (QKD) guarantees, stems not from the mathematical complexity of the encryption algorithms but from the laws of Quantum Physics. Implementations of QKD protocols, however, rely on imperfect instruments and devices for information encoding, transmission and detection. Device imperfections limit the rate of information exchange and introduce vulnerabilities which can be exploited by a potential eavesdropper. This work explores practical aspects of QKD as it matures beyond proof-of-principle experiments, focusing on the Measurement Device Independent - QKD, a novel Quantum Communication protocol that offers an exceptional balance between security and efficiency. At the heart of the MDI-QKD lies the Hong-Ou-Mandel (HOM) interference which characterizes the indistinguishability of the photon states that the communicating parties independently send. This study examines the HOM interference in a realistic lab environment and concludes that exceptional interference visibility can be achieved using typical commercially available optical devices and detectors, further demonstrating the applicability of the MDI-QKD protocol. An important limiting factor for every Quantum Communication protocol is the transmission medium. Fiber - based optical networks suffer significant losses that prohibit Quantum Communication beyond metropolitan scales. While Free Space communication is an attractive alternative for long distance communication, is susceptible to losses due to the atmospheric Turbulence of the channel. As a means to improve the key generation efficiency, this work examines and experimentally demonstrates the Prefixed-Threshold Real Time Selection (P-RTS) scheme, which improves the free-space communication efficiency by rejecting detections that occur while the channel transmittance drops below a predetermined threshold

Data security in photonic information systems using quantum based approaches

The last two decades has seen a revolution in how information is stored and transmitted across the world. In this digital age, it is vital for banking systems, governments and businesses that this information can be transmitted to authorised receivers quickly and efficiently. Current classical cryptosystems rely on the computational difficulty of calculating certain mathematical functions but with the advent of quantum computers, implementing efficient quantum algorithms, these systems could be rendered insecure overnight. Quantum mechanics thankfully also provides the solution, in which information is transmitted on single-photons called qubits and any attempt by an adversary to gain information on these qubits is limited by the laws of quantum mechanics. This thesis looks at three distinct different quantum information experiments. Two of the systems describe the implementation of distributing quantum keys, in which the presence of an eavesdropper introduces unavoidable errors by the laws of quantum mechanics. The first scheme used a quantum dot in a micropillar cavity as a singlephoton source. A polarisation encoding scheme was used for implementing the BB84, quantum cryptographic protocol, which operated at a wavelength of 905 nm and a clock frequency of 40 MHz. A second system implemented phase encoding using asymmetric unbalanced Mach-Zehnder interferometers, with a weak coherent source, operating at a wavelength of 850 nm and pulsed at a clock rate of 1 GHz. The system used depolarised light propagating in the fibre quantum channel. This helps to eliminate the random evolution of the state of polarisation of photons, as a result of stress induced changes in the intrinsic birefringence of the fibre. The system operated completely autonomously, using custom software to compensate for path length fluctuations in the arms of the interferometer and used a variety of different single-photon detector technologies. The final quantum information scheme looked at quantum digital signatures, which allows a sender, Alice, to distribute quantum signatures to two parties, Bob and Charlie, such that they are able to authenticate that the message originated from Alice and that the message was not altered in transmission

Quantum interferometers: principles and applications

Interference, which refers to the phenomenon associated with the superposition of waves, has played a crucial role in the advancement of physics and finds a wide range of applications in physical and engineering measurements. Interferometers are experimental setups designed to observe and manipulate interference. With the development of technology, many quantum interferometers have been discovered and have become cornerstone tools in the field of quantum physics. Quantum interferometers not only explore the nature of the quantum world but also have extensive applications in quantum information technology, such as quantum communication, quantum computing, and quantum measurement. In this review, we analyze and summarize three typical quantum interferometers: the Hong-Ou-Mandel (HOM) interferometer, the N00N state interferometer, and the Franson interferometer. We focus on the principles and applications of these three interferometers. In the principles section, we present the theoretical models for these interferometers, including single-mode theory and multi-mode theory. In the applications section, we review the applications of these interferometers in quantum communication, computation, and measurement. We hope that this review article will promote the development of quantum interference in both fundamental science and practical engineering applications.Comment: 64 pages, 40 figures. Comments are welcom

Security of advanced quantum key distribution protocols in realistic conditions

Quantum key distribution (QKD) allows two users to generate a random secret key, which they can use to securely exchange a message. Unlike many other cryptographic schemes, QKD offers information-theoretical security based on the laws of physics. In recent years, major theoretical and experimental advancements have been made. Among these are two novel protocols, memory-assisted (MA) QKD and twin-field (TF) QKD, which can both improve the secret-key rate scaling with channel length, potentially allowing QKD to be performed at longer distances. The main motivation of this thesis is to incorporate more realistic assumptions into the security proofs and performance analyses of these new protocols. One common assumption made in QKD security proofs is that the protocol is run for an infinitely long time, which allows the users to obtain a perfect statistical characterisation of the quantum channel. In this thesis, we drop this assumption for a TF-QKD variant that is well suited for experimental implementation, proving its security in the finite-key regime. We also analyse the finite-key performance of MA-QKD, concluding that it is particularly resistant to its statistical fluctuation effects. Moreover, we develop an alternative finite-key security analysis approach based on random sampling theory, and apply it to the loss-tolerant protocol, which can ensure security in the presence of flawed sources. Compared to previous finite-key security proofs of the protocol, our analysis offers better performance. Another common assumption is that the users can emit laser pulses with a continuous random phase. In practice, this is difficult to achieve, and the phase is often randomised discretely. In this thesis, we prove the security of a TF-QKD variant that relies on discrete phase randomisation, and show that, using certain post-selection techniques, it can provide higher secret-key rates than an equivalent continuously-randomised protocol

High-Quality Photon Pair Generation in Silicon Photonic Microring and Its Applications

Compact, efficient, high-quality and scalable non-classical photon sources are one of the most critical building blocks for quantum information processing and communications. Photon pair generation via spontaneous four wave mixing is being recognized as an attractive platform for quantum optics which is compatible with the rapidly-maturing classical integrated photonics technology using silicon wafers, which is adopted in industry for its low cost, high performance and scalability. Traditionally, the photon pairs generated from silicon photonic devices, however, have not compared favorably with traditional nonlinear crystals in terms of overall rate, quality and efficiency. This dissertation discusses the factors that affect the quality and efficiency of photon pair generation in silicon integrated photonic microrings, together with their design and characterization. Furthermore, we show how the microrings can be integrated with other photonic devices on the silicon-on-insulator platform, and using proof-of-concept experiments, discuss the feasibility of using these photon-pair sources with technology that is already present in, or will soon be present in, practical fiber-optic networks, such as integrated lasers and modulated classical data streams