Experimental advances in broadband continuous variable quantum key distribution

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.