Quantum information processing with tunnel junction circuits

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Single electron box. The tunnel barrier is an oxide layer 1 nm thick. Its role is to let charge from the reservoir go to the island  in the form of single electrons. When both the electron reservoir and the island are superconducting, the charge of the island can vary by 2 electrons at a time and the circuit becomes a Cooper pair box. The states with 0 or 1 Cooper pair in the island are the basis states of a possible candidate for a solid-state qubit.

Micrograph of a QuBit based on the Cooper pair box: the Quantronium. This QuBit is fabricated by e-beam lithography and double angle evaporation of two aluminum layers. The first layer is oxidized in order to form Josephson junctions where the two layers overlap. The rotating 01 object added on top of the device symbolizes a quantum superposition of its two lowest energy eigenstates.

Quantum physics has been thought for long as setting constraints on the knowledge we can extract experimentally from the physical world. Only recently has it begun to be viewed as opening rather than restricting information processing: some information manipulations which would be impossible according to classical physics become possible for a "quantum" computer . Several implementations of quantum computers have been proposed. Preliminary experiments in which isolated atoms and ions are used as quantum bits are so far the more advanced. Nevertheless, it seems doubtful that they can lead to ''scalable'' implementations. Solid-state systems, in particular superconducting nanocircuits, are more promising in this respect. They are derived from  single-electron transfer tunnel junction circuits ( see review article ). One such system is the single Cooper pair box, which was developed in our laboratory [ Bouchiat et al. (1998) ]. In 1999, quantum coherent Rabi oscillation of the box have been observed and a lower bound on the decoherence time in the continuous observation mode has been measured  [Nakamura Y., Pashkin Yu. and Tsai J.S., Nature 398, 786 (1999)]. Is  the Cooper pair box the best superconducting quantum bit? Recently, we have improved the Cooper pair box to make a new quantum circuit nicknamed Quantronium, in which we have measured the decoherence time, in the observation-free mode. A ratio of about 10^4 between this decoherence time and the quantum bit clock period has been reached. The Quantronium is consequently the best solid state quantum bit ever made. Its coherence time is sufficient to implement simple quantum gates, which could be the elementary bricks of a quantum processor. We are currently setting up an experiment aimed at coupling two Quantroniums.

right.GIF (78 octets) More on tunnel junction circuits

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