CQP Research Optical Quantum Computing
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In 2001 all-optical quantum computing became feasible with the discovery that scalable quantum computing is possible using only single photon sources, linear optical elements, and single photon detectors. Although it was in principle scalable, the massive resource overhead made the scheme practically daunting. However, several simplifications were followed by proof-of-principle demonstrations, and recent approaches based on cluster states or error encoding have dramatically reduced this worrying resource overhead, making an all-optical architecture a serious contender for the ultimate goal of a large-scale quantum computer. Key challenges will be the realization of high-efficiency sources of indistinguishable single photons, low-loss, scalable optical circuits, high efficiency single photon detectors, and low-loss interfacing of these components. [1] J. L. O'Brien Science 318, 1567 (2007) [2] A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, J. L. O'Brien Science 320, 646 (2008) [3] B. P. Lanyon et al. Nature Physics 5, 134 (2009) [4] A. Politi, J. C. F. Matthews, J. L. O'Brien Science 325, 1221 (2009) [5] A. S. Clark, J. Fulconis, J. G. Rarity, W. J. Wadsworth, and J. L. O'Brien Phys. Rev. A 79, 030303(R) (2009) |
Quantum Cryptography|
Quantum entanglement is the main resource to endow the field of quantum information processing with powers that exceed those of classical communication and computation. In view of applications such as quantum cryptography or quantum teleportation, extension of quantum-entanglement-based protocols to global distances is of considerable practical interest. Here we experimentally demonstrate entanglement-based quantum key distribution over 144 km. One photon is measured locally at the Canary Island of La Palma, whereas the other is sent over an optical free-space link to Tenerife, where the Optical Ground Station of the European Space Agency acts as the receiver. This exceeds previous free-space experiments by more than an order of magnitude in distance, and is an essential step towards future satellite-based quantum communication and experimental tests on quantum physics in space. [1] T. Schmitt-Manderbach et al. Phys. Rev. Lett 98, 010504 (2007) [2] R. Ursin et al. Nature Physics 3, 481 (2007) [3] O. Ahonen, M. Möttönen, and J. L. O'Brien Phys. Rev. A 78, 032314 (2008) [4] A. Laing, V. Scarani, J. G. Rarity and J. L. O'Brien arXiv:1003.1050 |
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Integrated Quantum Photonics
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Quantum technologies based on photons will likely require an integrated optics architecture for improved performance, miniaturization and scalability. We demonstrate high-fidelity silica-on-silicon integrated optical realizations of key quantum photonic circuits, including two-photon quantum interference with a visibility of 94.8%; a controlled-NOT gate with an average logical basis fidelity of 94.3%; and a path entangled state of two photons with fidelity >92%. These results show that it is possible to directly "write" sophisticated photonic quantum circuits onto a silicon chip, which will be of benefit to future quantum technologies based on photons, including information processing, communication, metrology and lithography, as well as the fundamental science of quantum optics. [1] A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, J. L. O'Brien Science 320, 646 (2008) [2] J. C. F. Matthews, A. Politi, A. Stefanov and J. L. O'Brien Nature Photonics 3, 346 (2009) [3] G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M. Ams, M. J. Withford, J. L. O'Brien Optics Express 17, 15, 12546-12554, (2009) [4] A. Politi, J. C. F. Matthews, J. L. O'Brien Science 325, 1221 (2009) [5] A. Politi, J. C. F. Matthews, M. G. Thompson, J. L. O'Brien IEEE Journal of Selected Topics in Quantum Electronics 15, 1673 (2009) |
Quantum Metrology|
Precision measurements are important across all fields of science. In particular, optical phase measurements can be used to measure distance, position, displacement, acceleration, and optical path length. Quantum entanglement enables higher precision than would otherwise be possible. We demonstrated an optical phase measurement with an entangled four-photon interference visibility greater than the threshold to beat the standard quantum limit - the limit attainable without entanglement. These results open the way for new high-precision measurement applications. [1] T. Nagata, R. Okamoto, J. L. O'Brien, K. Sasaki, S. Takeuchi Science 316, 726 (2007) [2] K. J. Resch et al. Phys. Rev. Lett. 98, 223601 (2008) [3] R. Okamoto et al. New J. Phys. 10, 073033 (2008) [4] J. C. F. Matthews, A. Politi, A. Stefanov and J. L. O'Brien Nature Photonics 3, 346 (2009) |
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Quantum Information with Photonic Crystal Fibres
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Anticipated technologies that harness uniquely quantum mechanical effects include quantum computing, quantum lithography, and quantum metrology. However, the only quantum technology in existence today is quantum cryptography, where any attempt to measure information encoded in the state of a photon results in a detectable disturbance. More sophisticated quantum networks will require multiple nodes with the ability to implement small-scale quantum processing. Such networks will rely on optical fibre links, making fibre-based photon generation and information processing of key technological importance. Here we demonstrate both elements in an all-fibre realization of a CNOT gate using two heralded photonic crystal fibre single photon sources. We measure an average logical fidelity of 90% and an average process fidelity of 0.83 < F < 0.91. Using a simple model we find the remaining discrepancy to be due almost entirely to spectral properties of the photon sources, demonstrating near-perfect operation of the fibre CNOT gate itself. [1] J. Fulconis, O. Alibart, J. L. O'Brien, W. J. Wadsworth, J. G. Rarity Phys. Rev. Lett. 99, 120501 (2007) [2] A. S. Clark, J. Fulconis, J. G. Rarity , W. J. Wadsworth, J. L. O'Brien Phys. Rev. A 79, 030303¨ (2009) [3] M. Halder, J. Fulconis, B. Cemlyn, A. Clark, C. Xiong, W. J. Wadsworth, J. G. Rarity, Opt. Express 17, 4670 (2009) |
Solid State-Photonic Quantum Information Science|
The degenerate fundamental mode of a microcavity pillar structure with circular cross section splits into two linearly polarized modes when the shape is changed to elliptical. The quality factor Q of these modes is very different. This letter demonstrates that the high Q mode provides better values of the figure of merit for strong coupling applications, Q / √V, where V is the modal volume, compared to values obtainable in circular structures. The difference in Q is shown to be a consequence of the polarization dependence of the losses through the microcavity mirrors. [1] D. M. Whittaker et al. Applied Physics Letters 90, 161105 (2007) [2] S. J. Devitt, A. D. Greentree, R. Ionicioiu, J. L. O'Brien, W. J. Munro, L. C. L. Hollenberg Phys. Rev. A 76, 052312 (2007) [3] C. Y. Hu, A. Young, J. L. O'Brien, W. J. Munro, and J. G. Rarity Phys. Rev. B 78, 085307 (2008) [4] A Young, C Y Hu, L Marseglia, J P Harrison, J L O'Brien and J G Rarity New Journal of Physics 11, 013007 (2009) |
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Quantum Measurement & Control
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The indistinguishability of non-orthogonal quantum states lies at the heart of quantum mechanics - it underpins the fundamental challenge of quantum state discrimination and has been harnessed as a resource in quantum technologies. Perfect identification of an unknown quantum process that acts on the state of a quantum system (including unitary operations, measurements, and decohering processes) can be achieved via quantum process tomography, but requires infinite uses of the unknown process. Here we experimentally demonstrate that discriminating between non-orthogonal processes can always be achieved with finite uses of the unknown process, in stark contrast to the situation for quantum states. We use either entanglement or an additional known process to deterministically and unambiguously discriminate between non-orthogonal measurement processes, and qubit and quitrit unitary processes. Finally we experimentally demonstrate that non-local multipartite unitary processes can be locally distinguished - i.e. without entanglement. Our processes act on photons and are discriminated with a confidence of >97% in all cases. [1] A. Laing, T. Rudolph, J. L. O'Brien Phys. Rev. Lett. 102, 1605 (2009) [2] R. Okamoto, J. L. O'Brien, H. F. Hofmann, T. Nagata, K. Sasaki, and S. Takeuchi. Science 323, 483 (2009) [3] G. G. Gillet et al., Phys. Rev. Lett 104, 080503 (2010) |