Conventional ultra-high sensitivity detectors in the millimeter-wave range are usually cooled as their own thermal noise at room temperature would mask the weak received radiation. The need for cryogenic systems increases the cost and complexity of the instruments, hindering the development of, among others, airborne and space applications. In this work, the nonlinear parametric upconversion of millimeter-wave radiation to the optical domain inside high-quality ($Q$) lithium niobate whispering-gallery mode (WGM) resonators is proposed for ultra-low noise detection. We experimentally demonstrate coherent upconversion of millimeter-wave signals to a 1550 nm telecom carrier, with a photon conversion efficiency surpassing the state-of-the-art by 2 orders of magnitude. Moreover, a theoretical model shows that the thermal equilibrium of counterpropagating WGMs is broken by overcoupling the millimeter-wave WGM, effectively cooling the upconverted mode and allowing ultra-low noise detection. By theoretically estimating the sensitivity of a correlation radiometer based on the presented scheme, it is found that room-temperature radiometers with better sensitivity than state-of-the-art high-electron-mobility transistor (HEMT)-based radiometers can be designed. This detection paradigm can be used to develop room-temperature instrumentation for radio astronomy, earth observation, planetary missions, and imaging systems.

中文翻译：

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基于高效电光上变频器的毫米波光子辐射计的灵敏度极限

毫米波范围内的常规超高灵敏度探测器通常会被冷却，因为它们在室温下自身的热噪声会掩盖微弱的接收辐射。对低温系统的需求增加了仪器的成本和复杂性，尤其阻碍了机载和太空应用的发展。在这项工作中，毫米波辐射的非线性参数上转换将转换为高质量（$问$提出了铌酸锂耳语画廊模式（WGM）谐振器用于超低噪声检测。我们通过实验证明了毫米波信号到1550 nm电信载波的相干上变频，其光子转换效率比现有技术高2个数量级。此外，理论模型表明，通过过度耦合毫米波WGM破坏了反向传播WGM的热平衡，有效地冷却了上转换模式并允许超低噪声检测。通过基于提出的方案在理论上估计相关辐射计的灵敏度，发现可以设计出比基于最新的高电子迁移率晶体管（HEMT）的辐射计更好的灵敏度的室温辐射计。