A System-Level Throughput Model for Quantum Key Distribution

Quantum Key Distribution (QKD) is an innovative technology which exploits the laws of quantum mechanics to generate and distribute shared secret keying material. QKD systems generate and distribute key by progressing through a number of distinct phases, typically in a serial manner. The purpose of this research is to identify these phases, their relationships to each other, as well as their relationship to time, memory space, computational requirements, and hardware resources. A mathematical model is developed which enables the study of critical system parameters, identifies and demonstrates potential bottlenecks that affect the overall key generation rate of serial implementations, and facilitates the analysis of design trade-offs in terms of parameters associated with specific implementations. Existing models of throughput performance make use of secure key rate equations which do not account for detailed system parameters and performance characteristics, particularly in the post-processing phases. In this research we build a model that is abstract enough to be applied to a wide range of QKD system configurations. The results of the model form an accurate prediction of throughput. The analysis contained herein provides QKD practitioners guidance in system analysis and design

An Analysis of Error Reconciliation Protocols for use in Quantum Key Distribution

Quantum Key Distribution (QKD) is a method for transmitting a cryptographic key between a sender and receiver in a theoretically unconditionally secure way. Unfortunately, the present state of technology prohibits the flawless quantum transmission required to make QKD a reality. For this reason, error reconciliation protocols have been developed which preserve security while allowing a sender and receiver to reconcile the errors in their respective keys. The most famous of these protocols is Brassard and Salvail\u27s Cascade, which is effective, but suffers from a high communication complexity and therefore results in low throughput. Another popular option is Buttler\u27s Winnow protocol, which reduces the communication complexity over Cascade, but has the added detriment of introducing errors, and has been shown to be less effective than Cascade. Finally, Gallager\u27s Low Density Parity Check (LDPC) codes have recently been shown to reconcile errors at rates higher than those of Cascade and Winnow with a large reduction in communication, but with greater computational complexity. This research seeks to evaluate the effectiveness of these LDPC codes in a QKD setting, while comparing real-world parameters such as runtime, throughput and communication complexity empirically with the well-known Cascade and Winnow algorithms. Additionally, the effects of inaccurate error estimation, non-uniform error distribution and varying key length on all three protocols are evaluated for identical input key strings. Analyses are performed on the results in order to characterize the performance of all three protocols and determine the strengths and weaknesses of each

Ideal quantum protocols in the non-ideal physical world

The development of quantum protocols from conception to experimental realizations is one of the main sources of the stimulating exchange between fundamental and experimental research characteristic to quantum information processing. In this thesis we contribute to the development of two recent quantum protocols, Universal Blind Quantum Computation (UBQC) and Quantum Digital Signatures (QDS). UBQC allows a client to delegate a quantum computation to a more powerful quantum server while keeping the input and computation private. We analyse the resilience of the privacy of UBQC under imperfections. Then, we introduce approximate blindness quantifying any compromise to privacy, and propose a protocol which enables arbitrary levels of security despite imperfections. Subsequently, we investigate the adaptability of UBQC to alternative implementations with practical advantages. QDS allow a party to send a message to other parties which cannot be forged, modified or repudiated. We analyse the security properties of a first proof-of-principle experiment of QDS, implemented in an optical system. We estimate the security failure probabilities of our system as a function of protocol parameters, under all but the most general types of attacks. Additionally, we develop new techniques for analysing transformations between symmetric sets of states, utilized not only in the security proofs of QDS but in other applications as well

Secure Optical Networks Based on Quantum Key Distribution and Weakly Trusted Repeaters

In this paper we explore how recent technologies can improve the security of optical networks. In particular, we study how to use quantum key distribution (QKD) in common optical network infrastructures and propose a method to overcome its distance limitations. QKD is the first technology offering information theoretic secret-key distribution that relies only on the fundamental principles of quantum physics. Point-to-point QKD devices have reached a mature industrial state; however, these devices are severely limited in distance, since signals at the quantum level (e.g. single photons) are highly affected by the losses in the communication channel and intermediate devices. To overcome this limitation, intermediate nodes (i.e. repeaters) are used. Both, quantum-regime and trusted, classical, repeaters have been proposed in the QKD literature, but only the latter can be implemented in practice. As a novelty, we propose here a new QKD network model based on the use of not fully trusted intermediate nodes, referred as weakly trusted repeaters. This approach forces the attacker to simultaneously break several paths to get access to the exchanged key, thus improving significantly the security of the network. We formalize the model using network codes and provide real scenarios that allow users to exchange secure keys over metropolitan optical networks using only passive components. Moreover, the theoretical framework allows to extend these scenarios not only to accommodate more complex trust constraints, but also to consider robustness and resiliency constraints on the network.Comment: 11 pages, 13 figure