Quantum Computing
Gilles Nogues, Laboratoire Kastler Brossel – CNRS

The possible developpment of quantum computing or communication over large distances will require to transmit quantum information between local quantum bits (qubits) and « flying qubits » travelling from on place to another. Among the candidates for such carriers of quantum information, the single excitation of a light field, a single photon, has many advantages. Hence the realization of a fully quantum interface between photons and other kind of qubits is a critical milestone. We study the coupling of matter with light when the latter is strongly affected by boundary conditions (like the presence mirrors in its vicinity). This allows to « trap » the light and enhance its coupling to the atomic system. In the strong regime coupling, the quantum dynamics of interaction dominates any other cause of energy loss for both partners, making the realization of the quantum interface possible. I will introduce my field of research in general and present the particular setup we use in Paris. I will then report on experiments that demonstrate that we can perform very basics protocols of quantum information processing between to qubits: a single atom and a single photon. It is also possible to control the intensity of the light in the cavity and hence increase its size. The theory predicts that quantum properties vanish at an increasing rate when the size of the system goes from the microscopic world to the macroscopic one. We have studied this frontier of the quantum world. This decoherence mechanism could have a crucial impact on practical quantum computation.

References:

- « Cavity quantum electrodynamics », Serge Haroche and Jean-Michel Raimond, Scientific American, vol. 268, no. 4, p. 54-60, 1993

− « Manipulating quantum entanglement with atoms and photons in a cavity », J.-M. Raimond, M. Brune and S. Haroche, Rev. Mod. Phys. 73 565 (2001)

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