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We describe in detail the theory underpinning the measurement of density matrices of a pair of quantum two-level systems ͑''qubits''͒. Our particular emphasis is on qubits realized by the two polarization degrees of freedom of a pair of entangled photons generated in a down-conversion experiment; however, the discussion applies in general, regardless of the… (More)

- Xiaobo Zhu, Shiro Saito, Alexander Kemp, Kosuke Kakuyanagi, Shin-ichi Karimoto, Hayato Nakano +7 others
- Nature
- 2011

During the past decade, research into superconducting quantum bits (qubits) based on Josephson junctions has made rapid progress. Many foundational experiments have been performed, and superconducting qubits are now considered one of the most promising systems for quantum information processing. However, the experimentally reported coherence times are… (More)

Linear optics with photon counting is a prominent candidate for practical quantum computing. The protocol by Knill, Laflamme, and Milburn ͓2001, Nature ͑London͒ 409, 46͔ explicitly demonstrates that efficient scalable quantum computing with single photons, linear optical elements, and projective measurements is possible. Subsequently, several improvements… (More)

- T C Ralph, A Gilchrist, G J Milburn, W J Munro, S Glancy
- 2003

quantum information, Schrödinger cat states, coherent states, conditional measurements We show that quantum computation circuits using coherent states as the logical qubits can be constructed from simple linear networks, conditional photon measurements and " small " coherent superposition resource states. We show that quantum computation circuits using… (More)

We define the task of quantum tagging, that is, authenticating the classical location of a classical tagging device by sending and receiving quantum signals from suitably located distant sites, in an environment controlled by an adversary whose quantum information processing and transmitting power is unbounded. We define simple security models for this task… (More)

We present a new control algorithm and system design for a network of quantum repeaters, and outline the end-to-end protocol architecture. Such a network will create long-distance quantum states, supporting quantum key distribution as well as distributed quantum computation. Quantum repeaters improve the reduction of quantum-communication throughput with… (More)

- T D Ladd, P Van Loock, K Nemoto, W J Munro, Y Yamamoto
- 2008

We describe a system for long-distance distribution of quantum en-tanglement, in which coherent light with large average photon number interacts dispersively with single, far-detuned atoms or semiconductor impurities in optical cavities. Entanglement is heralded by homodyne detection using a second bright light pulse for phase reference. The use of bright… (More)

- A G White, D F V James, W J Munro, P G Kwiat
- 2001

We report the creation of a wide range of quantum states with controllable degrees of entanglement and entropy using an optical two-qubit source based on spontaneous parametric down-conversion. The states are characterized using measures of entanglement and entropy determined from tomographically determined density matrices. The tangle-entropy plane is… (More)

- A Gilchrist, Kae Nemoto, W J Munro, T C Ralph, S Glancy, Samuel L Braunstein +1 other
- 2004

We outline a toolbox comprised of passive optical elements, single photon detection and superpositions of coherent states (Schrödinger cat states). Such a toolbox is a powerful collection of primitives for quantum information processing tasks. We illustrate its use by outlining a proposal for universal quantum computation. We utilize this toolbox for… (More)