Lecocq F, Ranzani L, Peterson G A, Cicak K, Jin X Y, Simmonds R W, Teufel J D, Aumentado J
National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA.
Department of Physics, University of Colorado, 2000 Colorado Avenue, Boulder, Colorado 80309, USA.
Phys Rev Lett. 2021 Jan 15;126(2):020502. doi: 10.1103/PhysRevLett.126.020502.
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±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.
对量子物体进行观测的行为从根本上会扰乱其状态，导致其朝着测量算符的本征态进行随机游走。理想情况下，测量应对量子态的所有退相负责。在实际中，测量设备的不完善会限制或破坏量子反馈协议所需的信息流，这种效应由测量效率来量化。在此，我们展示了使用非互易参量放大器直接监测读出腔的微波场来对超导量子比特进行高效测量。通过减轻腔与放大器之间的损耗，我们实现了(72±4)%的测量效率。放大器的方向性保护读出腔和量子比特免受放大的真空涨落引起的过度反作用。除了为进一步提高强投影测量的保真度提供工具外，这项工作还为理想弱测量的实验研究创建了一个试验台，并为基于弱测量的量子反馈协议（如状态稳定或纠错）开辟了道路。
