Continuous variable quantum key distribution relies on the transmission of weakly modulated coherent states and the detection of optical field for secure communication. In this talk, we give an overview of recent developments in continuous variable quantum key distribution. In particular, we discuss a scheme that uses continuous-wave laser beams without optical pulsing. Due to its continuous wave nature, our scheme is intrinsically broadband and does not require measurement basis switching. Finally, we present a status report on integration of this system into existing communication infrastructures at the Parliamentary Triangle of Australia in Canberra.
Continuous variable quantum key distribution relies on the transmission of weakly modulated coherent states and the detection of optical field for secure communication. In this talk, we give an overview of recent developments in continuous variable quantum key distribution. In particular, we discuss a scheme that uses continuous-wave laser beams without optical pulsing. Due to its continuous wave nature, our scheme is intrinsically broadband and does not require measurement basis switching. Finally, we present a status report on integration of this system into existing communication infrastructures at the Parliamentary Triangle of Australia in Canberra.
Quantum key distribution is the first major area of quantum information science to find practical use outside of its field. Quantum key distribution involves transmitting cryptographic keys using quantum states. Any attempt to eavesdrop on the transmission will, necessarily by the laws of quantum mechanics, disturb or destroy the states. By identifying and only using keys that have not been disturbed, a perfectly secure cryptographic system can be realised. ¶ Discrete variable quantum key distribution was proposed in 1984. Since then attempts to exploit this technology have been beset with the difficulties of working with the discrete variables in question—single photon states. In the late 90’s the idea of continuous variable quantum key distribution was developed, in which the key-carrying quantum information states are properties of a bright, continuous wave laser beam. Bright laser beams are relatively simple to manipulate and can carry large amounts of information even when subject to high loss. ‘Broadband’ continuous variable quantum key distribution refers to the large information rates obtainable by using broad laser modulation sidebands. ¶ In 2005 the ANU Quantum Optics group published an experimental demonstration of broadband continuous variable quantum key distribution. This thesis documents the project work undertaken to rebuild an improved version of this experiment, and also the work undertaken to prepare for more advanced experiments. The main goal of the improved experiment was to increase the key distribution rate, while the advanced experiments were directed towards practical implementation of the system in optical fibre. ¶ The experiment rebuild was achieved with most improvements for higher bandwidth successfully implemented. Some improvements were unplanned, being necessitated by problems with the initial improvements. Although preliminary experimental data was acquired, time constraints precluded a successful cryptographic key distribution. Pilot design and major purchasing for future experiments was also completed.
Quantum key distribution provides secure keys with information-theoretic security ensured by the principle of quantum mechanics. The continuous-variable version of quantum key distribution using coherent states offers the advantages of its compatibility with telecom industry, e.g., using commercial laser and homodyne detector, is now going through a booming period. In this review article, we describe the principle of continuous-variable quantum key distribution system; focus on protocols based on coherent states, whose systems are gradually moving from proof-of-principle lab demonstrations to in-field implementations and technological prototypes. We start by reviewing the theoretical protocols and the current security status of these protocols. Then, we discuss the system structure, the key module, and the mainstream system implementations. The advanced progresses for future applications are discussed, including the digital techniques, system on chip, and point-to-multipoint system. Finally, we discuss the practical security of the system and conclude with promising perspectives in this research field.
We report a fiber-based quasi-continuous-wave (CW) quantum key distribution (QKD) system with continuous variables (CV). This system employs coherent light pulses and time multiplexing to maximally reduce cross talk in the fiber. No-switching detection scheme is adopted to optimize the repetition rate. Information is encoded on the sideband of the pulsed coherent light to fully exploit the continuous wave nature of laser field. With this configuration, high secret key rate can be achieved. For the 50 MHz detected bandwidth in our experiment, when the multidimensional reconciliation protocol is applied, a secret key rate of 187 kb/s can be achieved over 50 km of optical fiber against collective attacks, which have been shown to be asymptotically optimal. Moreover, recently studied loopholes have been fixed in our system.
Free-space quantum key distribution opens up new possibilities for the development of quantum communication systems, facilitating the deployment of quantum networks in physically hard-to-reach areas and across wide landscapes without the use of optical fibers. This approach overcomes restrictions associated with cable laying and provides a more flexible infrastructure for quantum information transmission. Together with the use of continuous-variable protocols, it becomes possible to organize secure information transfer using optimal scalable communication systems despite deleterious environmental influences. This paper reviews key achievements and technical aspects of implementing continuous-variable quantum key distribution in atmospheric channels, considering the main atmospheric effects such as turbulence, scattering, and beam wandering. We present the current state of continuous-variable quantum key distribution systems in free space and their practical implementations, including successful experimental projects and prototypes. Prospects for further development, including integration with existing telecommunications networks and potential application areas, are discussed.
Nowadays, quantum communications provide a vast field of research in rapid expansion, with a huge potential impact on the future developments of quantum technologies. In particular, continuous variable systems, employing coherent-state encoding and quadrature measurements, represent a suitable platform, due to their compatibility with both the modulation and detection systems currently employed in standard fiber-optical communications. In this work, we address some relevant aspects of the field, and provide innovative results being also experimentally oriented. In particular, we focus on two relevant paradigms: quantum decision theory and continuous variable quantum key distribution (CVQKD). In the former case, we address the problem of coherent-state discrimination and design new hybrid receivers for binary phase-shift keying discrimination, obtaining a quantum advantage over conventional detection schemes, being also robust against typical experimental imperfections. In the latter scenario, we proceed in two different directions. On the one hand, we design new CVQKD protocols employing discrete modulation of coherent states, being a feasible solution compatible with the state of the art in optical communications technologies. On the other hand, we address the more fundamental problem of performing channel losses mitigation to enhance existing protocols, and investigate the role of optical amplifiers for the task. Finally, we make a first step towards a fully non-Gaussian CVQKD scheme by proposing, for the first time, the adoption of an optimized state-discrimination receiver, commonly adopted for quantum decision theory, within the context of CVQKD, obtaining a genuine quantum enhancement over conventional protocols.