When a two-level system—a qubit—is used as a probe of a larger system, it naturally leads to answering a single yes-no question about the system state. Here we propose a method where a single qubit is able to extract, not a single, but many bits of information about the photon number of a microwave resonator using continuous measurement. We realize a proof-of-principle experiment by recording the fluorescence emitted by a superconducting qubit reflecting a frequency comb, thus implementing multiplexed photon counting where the information about each Fock state—from 0 to 8—is simultaneously encoded in independent measurement channels. Direct Wigner tomography of the quantum state of the resonator evidences the backaction of the measurement as well as the optimal information extraction parameters. Our experiment unleashes the full potential of quantum meters by replacing sequential quantum measurements with simultaneous and continuous measurements separated in the frequency domain.

中文翻译：

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多路复用光子数测量

当一个两级系统——一个量子位——被用作一个更大系统的探测器时，它自然会导致回答关于系统状态的一个是-否问题。在这里，我们提出了一种方法，其中单个量子位能够使用连续测量提取有关微波谐振器光子数的信息，而不是单个，而是许多位。我们通过记录由反射频率梳的超导量子位发出的荧光来实现原理验证实验，从而实现多路复用光子计数，其中关于每个 Fock 状态的信息（从 0 到 8）在独立的测量通道中同时编码。谐振器量子态的直接 Wigner 断层扫描证明了测量的反作用以及最佳信息提取参数。