Many experiments searching for fundamental physics beyond the standard model (like DMRadio) have reached the point where their sensitivity is limited by quantum noise. As a result, these experiments require the techniques of quantum sensing to reach and go beyond the Standard Quantum Limit (SQL) of measurement.
The Irwin Group in particular is interested in quantum sensing for electromagnetic signals in the VHF (< 300 MHz) band, which is the frequency range of the axions targeted by DMRadio. Our focus is on developing new quantum sensors, like the Radiofrequency Quantum Upconverter (RQU), and using these new sensors to perform quantum measurement protocols like backaction evasion to enable more sensitive measurements of fundamental physics.
Many existing quantum metrology techniques like photon counting and squeezing have been used to enable beyond SQL precision sensitivity but these mainly work at GHz scales. At lower MHz frequencies, thermal noise becomes much more of a problem and presents new challenges to reaching the SQL. The RQU is a new quantum sensor made of a network of Josephson junctions embedded into a superconducting microwave resonator.RQUs use the nonlinearity of the Josephson junctions to convert MHz input signals into GHz output signals, which allows us to leverage existing quantum protocols to achieve beyond SQL precision in the VHF regime.
Our current focus is perfecting the fabrication process for the current generation of 3-junction RQUs and using them to demonstrate quantum protocols like backaction evasion. In the future, we plan to make more intricate multi junction devices for greater sensitivity, with the aim of incorporating them in the DMRadio readout.