Quantum nondemolition measurements of a qubit coupled to a harmonic oscillator
نویسندگان
چکیده
We theoretically describe the weak measurement of a two-level system qubit and quantify the degree to which such a qubit measurement has a quantum nondemolition QND character. The qubit is coupled to a harmonic oscillator, which undergoes a projective measurement. Information on the qubit state is extracted from the oscillator measurement outcomes, and the QND character of the measurement is inferred from the result of subsequent measurements of the oscillator. We use the positive operator valued measure POVM formalism to describe the qubit measurement. Two mechanisms lead to deviations from a perfect QND measurement: i the quantum fluctuations of the oscillator, and ii quantum tunneling between the qubit states 0 and 1 during measurements. Our theory can be applied to QND measurements performed on superconducting qubits coupled to a circuit oscillator.
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