Quantum cryptographic system and single photon source for optical fibre networks (Paper unavailable)

Yu‐Hui Luo

Robert John Collins

The emergence of a digital communications infrastructure over recent decades has fuelled the parallel development of advanced cryptographic techniques, to secure the ever increasing quantities of digital infonnation. From its foundation, quantum key distribution has generated significant theoretical and experimental research interest since it offers what ./ is currently the only method of verifiably secure cryptographic key distribution. By using the Heisenberg Uncertainty Principle, quantum key distribution offers a technique by which authorised parties can detect the potential presence of an eavesdropping unauthorised party and take appropriate action. Previous demonstrations of quantum key distribution have typically concentrated on extending the maximum transmission distance over which communication may be performed at the expense of bit-rates. This thesis investigates a quantum key distribution system operating at a wavelength of 850 nm in standard telecommunications fibre using commercially available silicon single-photon avalanche diodes to achieve clock rates in excess of 1 GHz, with a corresponding increase in received bit-rates. The principles of currently implemented passive optical networks are then applied to the system to demonstrate multi-user quantum key distribution systems. In addition, the point-to-point (single-user) system is demonstrated at a clock-rate of 3.3 GHz using low timing jitter niobium nitride nanowire superconducting detectors to demonstrate the highest clock-rate transmission in standard telecommunications optical fibre and the highest channel loss to date. Finally, single-photon sources based on quantum dot microcavity resonators are also examined for use in quantum key distribution.

Paul D. Townsend

M. B. Ward, O. Z. Karimov, D. C. Unitt +6 more

We demonstrate the first on-demand single photon source that is compatible with standard telecom optical fibre. The source may be useful for fibre optic-based single-photon applications, including quantum metrology, quantum communications and distributed quantum computing.

Xiulai Xu, Ian Toft, Jonathan D. Mar +3 more

We report a 'plug and play' single-photon source based on single quantum dots with an integrated optical fibre output. We attach an optical fibre to the sample surface in order to measure single-dot photoluminescence, which avoids the light coupling between quantum dots and free space. The excitation light (a HeNe Laser) is coupled into the optical fibre to excite the quantum dots, and the emitted photoluminescence is collected with same optical fibre. To verify the emission of single photons, correlation measurements of the luminescence were performed using a Hanbury Brown and Twiss setup with a 50/50 beamsplitter and two single-photoncounting avalanche photodiodes. Clear antibunching with the second-order correlation function at zero time delay less than 0.1 indicates single-photon emission. The devices are stable, and reproducible for an arbitrarily long time, which is very promising for real implementations of quantum key distribution and linear optical quantum computation.

Romain Alléaume