标题： 基于约瑟夫森光电倍增管的微波光子对探测器 显示英文标题

标题： Detector of microwave photon pairs based on a Josephson photomultiplier

摘要： 我们提出了一种可行的微波双光子阈值探测器设计方案。 本质上，该方案是对现有单光子探测器——约瑟夫森光电倍增管 (JPM)——的扩展。JPM 是一种基于电容分流射频超导量子干涉仪 (RF SQUID) 的吸收式微波探测器。 为了实现双光子阈值探测器，我们利用了一个谐振器二聚体——两个集总元件谐振器通过非对称直流 SQUID 相互作用，其中一个谐振器与 JPM 电容耦合。 通过对谐振器频率和流经直流 SQUID 耦合器的外部光通量进行特定调谐，我们在谐振器之间设计了一种非微扰双光子耦合。 这种耦合使得一个谐振器中的光子对相干转换为另一个谐振器中的单个光子，从而能够对至少两个光子的量子态进行选择性响应。 我们还考虑了一种扩展耦合器设计，允许按需 \textit{原位}切换双光子耦合。 此外，我们提出了改进的JPM设计来提升其性能。 我们的计算表明，对于实际的电路参数，我们可以在不到50纳秒的时间内实现超过99%的光子对检测保真度。 该方案可作为在电路QED架构中实现高效光子数分辨探测器的基石。 显示英文摘要

摘要： We propose a viable design of a microwave two-photon threshold detector. In essence, the considered scheme is an extension of the existing single-photon detector - a Josephson photomultiplier (JPM) - an absorbing microwave detector based on a capacitively-shunted rf SQUID. To implement a two-photon threshold detector, we utilize a dimer of resonators - two lumped-element resonators interacting via an asymmetric dc SQUID, with one of the resonators capacitively coupled to the JPM. By specific tuning of the resonator frequencies and the external flux through the dc SQUID coupler, we engineer a non-perturbative two-photon coupling between the resonators. This coupling results in the coherent conversion of a photon pair from one resonator into a single photon in another resonator, enabling selective response to quantum states with at least two photons. We also consider an extended coupler design that allows on-demand \textit{in situ} switching of two-photon coupling. In addition, we propose the modified JPM design to improve its performance. Our calculations demonstrate that, for realistic circuit parameters, we can achieve more than $99\%$ fidelity of photon pair detection in less than 50 ns. The considered scheme may serve as a building block for the implementation of efficient photon-number-resolving detectors in circuit QED architecture.