Pushing the sensitivity boundaries of X-band EPR cryoprobe using a fast microwave switch

We present a new design of an X-band EPR cryoprobe based on a fast microwave switch and a cryogenic low-noise microwave amplifier that are placed close to the sample in the same cryostat. The probehead supports high-power (100 W) pulsed EPR experiments and is compatible with standard EPR resonators and samples. In contrast to the directional coupler design of the EPR cryoprobe reported previously, the fast microwave switch fully isolates the microwave amplifier from input thermal noise without microwave power suppression allowing us to approach the sensitivity limit of cryoprobes for pulsed EPR experiments. We benchmark the performance of our cryoprobe setup against a standard commercial EPR instrument revealing a significant sensitivity improvement, which reduces the measurement time by a factor of about $250\times$ at 6 K sample temperature. We also show that the sensitivity of our new X-band cryoprobe design matches that of a standard Q-band setup for double electron–electron resonance experiments.