Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61573372, 61603413) and the Principal Fund of Air Force Engineering University, China (Grant No. XZJY2018038).
Received Date:22 November 2018
Accepted Date:21 January 2019
Available Online:01 May 2019
Published Online:05 May 2019
Abstract:Quantum entanglement possesses important applications in quantum computation, quantum communication, and quantum precision measurement. It is also an important method to improve the performance of quantum radar and quantum radio navigation. However, the penetration of light wave is poor due to the high frequency, which leads to detecting limitations in bad weather. In this context, quantum entanglement in the microwave domain has been extensively studied, and it is hopeful to overcome the above-mentioned defects in quantum optics. Although the entangled microwave preparation of continuous variable is achievable at present, there exist still some problems such as poor entanglement performance, low entanglement efficiency, complex signal processing and control, which restrict the development of entangled microwave sources. In order to improve the entanglement performance in microwave domain, a squeezing-angle locking scheme based on single photon counting is proposed. First, two Josephson parametric amplifiers (JPAs) are driven respectively by two pump signals to generate two single-mode squeezed states which are uncorrelated to each other. Next, the squeezing angle difference between the two single-mode squeezed states is adjusted to 180°, and then the two signals are mixed in a superconducting 180° hybrid ring coupler for two entangled microwave outputs. The outputs are single photon detected, and the results are sent to the data processor for solution. The squeezing angle difference between the input single-mode squeezed microwaves is estimated by Bayesian criterion and compared with the target value to calculate the error. Finally, the squeezing angle correction information is fed back into the JPA pump to control the squeezing angle of the single-mode squeezed microwave of the JPA output as well as the relative squeezing angle to reach the target value. Thus, the dual-path entangled microwave with the optimal entanglement performance is output. Comparing with the existing entangled microwave preparation schemes, a single photon counter is utilized in the scheme of this paper, which leads to a detection efficiency of 90%. In addition, the Bayesian criterion is used to estimate the output result, and the theoretical precision reaches the quantum Cramer-Rao lower bound. Meanwhile, the introduced noise level and operation difficulty are reduced, which greatly improves the property of dual-path entangled microwave preparation. Keywords:entangled microwave preparation/ squeezing angle locking/ microwave photon counting/ Bayesian estimation
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3.基于微波光子计数的压缩角锁定方案随着光子计数器的发展, 基于光子计数的测量方法逐渐成为一种重要的测量方案, 近年来对微波光子计数器的研究不断深入, 基于电流偏置约瑟夫森结的电路量子电动力学系统通过在辐射场中吸收单光子实现微波光子的探测, 但效率较低[15,17]. 图2所示的基于transmon的微波单光子探测通过位于传输线末端的transmon实现了传输过程中微波光子的探测, 且当前不断改进多transmon的设计以及超导传输线耦合以克服量子噪声对探测的影响, 探测效率已达90%[16,18]. 此外, 近年来超导隧道结、微波动态电感探测器等新的微波光子探测方法也不断地被提出[19], 探测效率也不断提升, 使得微波光子计数逐渐成为可能. 图 2 基于单transmon的微波光子探测系统 Figure2. A scheme for single-photon detection using a transmon.