Proof-of-principle of the ultra-narrowband detection scheme at 30 GHz toward weakly interacting slim particle search

A coherent detection scheme was tested for dark photons and axion search with millimeter waves. It was shown that relative bandwidth of 169 μHz can be achieved at the center frequency of 30 GHz by commercial equipment. This is in principle a lock-in amplifier at 30 GHz. With a low noise amplifier, thermal noise can be reduced down to 1.36 × 10−24 W with one and half hours data taking and the expected signal was locked within one bin of 169 μHz resolution bandwidth by using the standard 10 MHz reference line. This will lead to a new search of dark photons in the mass range, which is complementary to any previous experiments and a future upgrade project of ALPS-II. Although a fully quantum mechanical argument will be necessary to quantify the ultimate limit of this coherent detection scheme, the use of a state-of-the-art phase-locked gyrotron and superconducting quantum sensors would lead to further improvement of millimeter-wave dark photon and axion searches.