Description:

The post-inflationary QCD axion is a sharp BSM theory target that spans a frequency range from 5 to 50 GHz known as the classical window. At these higher frequencies, linear amplifiers as cavity haloscope receivers are severely degraded in sensitivity by the standard quantum limit. For calorimetric measurements, this limit can be evaded through the use of direct photon counting. At mK temperatures, the occupation number of photons in these frequencies can be three to four orders of magnitude below 1, providing remarkable enhancement to QCD axion scan rates.

We present our work to develop a tunable single microwave photon detector (SMPD) that could be used in an axion cavity haloscope experiment in the 5 to 7 GHz range. The detector architecture is modeled after an existing transmon-based design, initially developed by the Quantronics group at Paris-Saclay University. We describe the design's fundamental working principles, which include four-wave mixing, dispersive readout, and cyclic readout operation. We also detail the effort to make a tunable SMPD from 6 to 7 GHz, chosen to line up with existing high-volume cavity haloscopes, in connection with the ADMX-VERA (Volume-Enhanced Resonant Axion) working group. Lastly, we report on the T1 performance of a prototype device, hypothesize what it may be limited by, and present the working plan for building a high-efficiency and low-dark-count tunable SMPD.

Osmond Wen, Postdoctoral Scholar, Stanford University

Host: Susan Dittmer