Overcoming the Thermal-Noise Limit of Microwave Measurements by Precooling with an Active Cold Load

This paper introduces Active Pre-Cooling (APC), a novel method for reducing thermal photon occupation in microwave and RF resonators prior to measurement. Key aspects include:

• Coupling cavity modes to an active cold load (ACL)—specifically the input of a commercial low-noise amplifier (LNA)—to achieve substantial reductions in noise temperature.
• Room-temperature demonstrations showing mode temperatures dropping to 123 K, with a 5× improvement in signal-to-noise ratio during time-resolved EPR experiments.
• Feasibility modeling that supports extended thermal suppression using high-Q modes and cryogenic ACLs.

APC serves as a simple and powerful first-stage cooling method for improving sensitivity in EPR spectroscopy, quantum sensing, and radiometric detection, with direct applicability to both ambient and cryogenic regimes.

Recommended citation: Chen, Kuan-Cheng; & Oxborrow, Mark. (2024). "Overcoming the Thermal-Noise Limit of Microwave Measurements by Precooling with an Active Cold Load." Phys. Rev. Appl., American Physical Society. https://doi.org/10.1103/bjky-3yb3
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