Quantum Computation & Communication Theory

Quantum communication protocols enable absolutely secure communications and the ability to share data between distributed quantum processors. The Quantum Communication Theory section of this program focuses particularly on quantum communication protocols in which the information is encoded on the optical field variables – so-called continuous variable (CV) protocols, and seeks tighter security proofs and to develop techniques for increasing the reach of quantum communication to a global scale via quantum repeater technologies. Objectives of the program will be to:
(i) Tighten security proofs for CV quantum key distribution [T.C. Ralph, Phys.Rev.A. 61, 010302(R) (1999)], so as to extend the versatility and speed of such protocols (in collaboration with CIs Symul and Lam and PI Sharma).
(ii) Develop repeater technologies for CV systems based on entanglement distillation via noiseless amplification [G.Y. Xiang, T.C. Ralph, A.P. Lund, N. Walk and G.J. Pryde, Nature Photonics 4, 316 (2010)] and solid state memory [G. Hetet, et al, Phys Rev Lett 100, 023601 (2008)], so as to increase their reach (in collaboration with CIs Pryde, Symul, Sellars, Buchler, James and Huntington and PIs Leuchs and Furusawa).
(iii) Explore new quantum communication applications.

Quantum Computation allows the efficient solution of computational problems with great practical significance that are intractable on normal classical computers. Arguably, optical implementations have the best chance of reaching problem sizes (40 – 50 qubits for specific problems) that are beyond the ability of classical computation in the next decade. The Optical Quantum Computation Theory section of this program focuses on the design and optimisation of quantum circuits for medium-scale (5 – 50 qubit) demonstrations and applications, and the collaboration on their experimental implementation. Both particle approaches, based on entanglement across additional degrees of freedom [Xiao-Qi Zhou, Timothy C. Ralph, Pruet Kalasuwan, Mian Zhang, Alberto Peruzzo, Benjamin P. Lanyon, Jeremy L. O'Brien, arXiv:1006.2670 (2009)], and field encoding techniques based on gate teleportation [A.P. Lund, T.C. Ralph, and H.L. Haselgrove, Physical Review Letters 100, 030503 (2008)] and cluster states [Nicolas C. Menicucci, Peter van Loock, Mile Gu, Christian Weedbrook, Timothy C. Ralph, Michael A. Nielsen, Physical Review Letters 97, 110501 (2006)], are being explored and detailed models of their operation are studied. The broad objectives are:
(i) Develop techniques for implementing medium-scale quantum processing in optics, optimised for limited resources.
(ii) Design medium-scale quantum circuits to demonstrate key quantum algorithms for the first time and to perform specific application driven tasks.
(iii) Initiate and collaborate on experimental demonstrations (in collaboration with CIs Pryde, White, Huntington and Lam and PI Furusawa and Collaborator O’Brien).