The act of observing a quantum object fundamentally perturbs its state, resulting in a random walk toward an eigenstate of the measurement operator. Ideally, the measurement is responsible for all dephasing of the quantum state. In practice, imperfections in the measurement apparatus limit or corrupt the flow of information required for quantum feedback protocols, an effect quantified by the measurement efficiency. Here, we demonstrate the efficient measurement of a superconducting qubit using a nonreciprocal parametric amplifier to directly monitor the microwave field of a readout cavity. By mitigating the losses between the cavity and the amplifier, we achieve a measurement efficiency of $(72\pm 4)\%$. The directionality of the amplifier protects the readout cavity and qubit from excess backaction caused by amplified vacuum fluctuations. In addition to providing tools for further improving the fidelity of strong projective measurement, this work creates a test bed for the experimental study of ideal weak measurements, and it opens the way toward quantum feedback protocols based on weak measurement such as state stabilization or error correction.

中文翻译：

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使用不可逆的微波放大器进行有效的量子位测量

观察量子物体的行为从根本上扰乱了它的状态，从而导致随机走向测量算子的本征态。理想情况下，测量负责量子态的所有移相。实际上，测量设备中的缺陷会限制或破坏量子反馈协议所需的信息流，这种影响由测量效率来量化。在这里，我们演示了使用不可逆参数放大器直接监视读出腔的微波场来有效测量超导量子位的方法。通过减轻空腔和放大器之间的损耗，我们实现了$（72\pm 4）％$。放大器的方向性可保护读数腔和量子位免受放大的真空波动引起的过度背作用。除了提供进一步改善强投射测量保真度的工具外，这项工作还为理想的弱测量的实验研究创建了测试平台，并且为基于弱测量（例如状态稳定或错误校正）的量子反馈协议开辟了道路。 。