The ability to control and measure the temperature of propagating microwave modes down to very low temperatures is indispensable for quantum information processing and may open opportunities for studies of heat transport at the nanoscale, also in the quantum regime. Here, we propose and experimentally demonstrate primary thermometry of propagating microwaves using a transmon-type superconducting circuit. Our device operates continuously, with a sensitivity down to $4\times {10}^{-4}\mathrm{photons}/\sqrt{\mathrm{Hz}}$ and a bandwidth of 40 MHz. We measure the thermal occupation of the modes of a highly attenuated coaxial cable in a range of 0.001 to 0.4 thermal photons, corresponding to a temperature range from 35 mK to 210 mK at a frequency around 5 GHz. To increase the radiation temperature in a controlled fashion, we either inject calibrated, wideband digital noise, or heat the device and its environment. This thermometry scheme can find applications in benchmarking and characterization of cryogenic microwave setups, temperature measurements in hybrid quantum systems, and quantum thermodynamics.

中文翻译：

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在量子状态下传播微波的初级测温

控制和测量低至极低温度的传播微波模式的温度的能力对于量子信息处理是必不可少的，并且可能为研究纳米尺度的热传输（甚至在量子状态下）提供机会。在这里，我们提出并通过实验证明了使用跨子型超导电路传播微波的主要测温法。我们的设备连续运行，灵敏度低至$4\times {10}^{-4}\mathrm{光子}/\sqrt{\mathrm{赫兹}}$和40 MHz的带宽。我们在0.001至0.4热光子的范围内测量高衰减同轴电缆的模式的热占用，对应于5 GHz附近频率从35 mK至210 mK的温度范围。为了以可控的方式提高辐射温度，我们要么注入校准的宽带数字噪声，要么加热设备及其环境。这种测温方案可用于低温微波设置的基准测试和表征，混合量子系统中的温度测量以及量子热力学。