Continuous measurements of two qubits
arXiv:cond-mat/0408471 · doi:10.1103/PhysRevB.71.085320
Abstract
We develop a theory of coherent quantum oscillations in two, in general interacting, qubits measured continuously by a mesoscopic detector with arbitrary non-linearity and discuss an example of SQUID magnetometer that can operate as such a detector. Calculated spectra of the detector output show that the detector non-linearity should lead to mixing of the oscillations of the two qubits. For non-interacting qubits oscillating with frequencies and , the mixing manifests itself as spectral peaks at the combination frequencies . Additional nonlinearity introduced by the qubit-qubit interaction shifts all the frequencies. In particular, for identical qubits, the interaction splits coherent superposition of the single-qubit peaks at . Quantum mechanics of the measurement imposes limitations on the height of the spectral peaks.
14 pages, 6 figures
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