We demonstrate an inexpensive, bench-top method, which we here name cavity pre-cooling (CPC), for removing, just prior to performing a measurement, a large fraction of the deleterious thermal photons that would otherwise occupy the electromagnetic modes of a microwave cavity (or some alternative form of rf resonator) at room temperature. Our method repurposes the input of a commercial HEMT-based low-noise amplifier (LNA) to serve as a photon-absorbing cold load that is temporarily over-coupled to the cavity. No isolator is inserted between the LNA and the coupling port of the cavity. In a proof-of-concept experiment, the noise temperature of a monitored microwave mode drops from a little below room temperature to approx 108 K. Upon incorporating our pre-coolable cavity into a time-resolved (tr-) EPR spectrometer, a commensurate improvement in the signal-to-noise ratio is observed, corresponding to a factor-of-5 speed up over a conventional tr-EPR measurement at room temperature for the same precision and/or sensitivity. Simulations indicate the feasibility, for realistic Q factors and couplings, of cooling the temperatures of the modes of a cavity down to a few tens of K, and for this coldness to last several tens of microseconds whilst the cavity and its contents are optimally interrogated by a microwave tone or pulse sequence. The method thus provides a generally applicable and extremely convenient approach to improving the sensitivity and/or read-out speed in pulsed or time-resolved EPR spectroscopy, quantum detection and other radiometric measurements performed at or near room temperature. It also provides a first-stage cold reservoir (of microwave photons) for deeper cooling methods to work from.
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Chen et al. (2024) studied this question.
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