Entangled pairs of microwave photons are commonly produced in the narrow frequency band of a resonator, which represents a modified vacuum density of states. We generate and investigate the entanglement of a stream of photon pairs, generated in a semi-infinite broadband transmission line, terminated by a superconducting quantum interference device (SQUID). A weak pump signal modulates the SQUID inductance, resulting in a single time-varying boundary condition, and we detect all four quadratures of the microwave radiation emitted at two different frequencies separated by 0.7~GHz. Power calibration is done in-situ, and we find positive logarithmic negativity and two-mode squeezing below the vacuum in the observed radiation, indicating entanglement.

中文翻译：

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单一时变边界条件下微波辐射中宽带纠缠的观察

通常在谐振器的窄频带中产生纠缠的微波光子对，这表示状态的真空密度发生了变化。我们生成并研究在半无限宽带传输线中生成的光子对流的纠缠，该纠缠被超导量子干扰设备（SQUID）终止。弱的泵浦信号调制SQUID电感，从而产生一个随时间变化的边界条件，并且我们检测到以0.7〜GHz分开的两个不同频率发射的微波辐射的所有四个正交。功率校准是在原位进行的，我们发现对数正负性和所观察到的辐射中真空以下的双模压缩，表明存在纠缠。