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Quantum Computing Graduate Research Program: Gradiometer INSQUID

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Final rept. 1 Jun 2002-31 May 2005

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This project is focused on developing flux qubits, understanding sources of relaxation and decoherence, and investigating entanglement of two qubits. This report describes our progress toward these goals over the last three years. We outline the infrastructure of our experiment and chip fabrication process, and describe a novel, dual-current source to supply two independent flux biases to the qubits and readout SQUID. We present results showing quantum coherence in a flux qubit, including spectroscopy, Rabi oscillations, Ramsey fringes, spectroscopic linewidths, and spin echoes. The different kinds of decoherence times deduced from the last three experiments are shown to be self-consistent. We summarize a study of the effects of nonequilibrium quasi particles generated in the readout SQUID when it switches to the voltage state. These excess quasi particles are shown to persist for a remarkably long time, about I ms, and thus set an upper limit on the repetition rate at which data can be acquired. We describe the theory of a novel device for entangling two flux qubits by means of a single SQUID that serves also as the readout device. It is shown that this scheme is in principle capable of the CNOT Controlled NOT operation.

Subject Categories:

  • Magnetic and Electric Field Detection and Detectors
  • Quantum Theory and Relativity

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