耦合到谐振子的量子比特的量子非破坏测量
Quantum non-demolition measurements of a qubit coupled to a harmonic oscillator
摘要：
我们从理论上描述了两能级系统（量子比特）的弱测量，并量化了这种量子比特测量具有量子非破坏（QND）特性的程度。量子位耦合到经历投影测量的谐振子。从振荡器测量结果中提取关于量子比特状态的信息，并且通过振荡器的后续测量的结果来推断测量的QND特性。我们使用正算子值测度（POVM）形式来描述量子比特的测量。两种机制导致偏离完美的QND测量：（i）振荡器的量子波动，以及（ii）测量期间量子位态$|0>$和$|1>$之间的量子隧穿。我们的理论可以应用到超导量子位耦合到电路振荡器上进行QND测量。
英文摘要：
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 non-demolition (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 by 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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