Search for Dark Matter Axion with Rydberg Atoms

Axion is a strong candidate of the dark matter in the universe. From various astrophysical arguments, the mass of the dark matter axion is expected to be in the region from 5 micro-eV to 0.1meV. At Kyoto, a novel single microwave photon detector (CARRACK) had been developed for the search of the dark matter axion. The axion is converted to a microwave photon in the strong magnetic field (7T) by Primakoff process in a cavity which is cooled to 10mK to avoid black-body radiation. The photon is then detected by a Rydberg atom, which is excited by absorbing the photon and then selectively ionized. After the extensive pioneering studies of the CARRACK detector, it was recently moved to a new laboratory and New-CARRACK collaboration was formed. In the previous work by using Rb Rydberg atoms, a stray electric field of an order of mV/cm limited the overall sensitivity of the detector because of its large Stark effect. The New CARRACK utilizes Potassium as Rydberg atom which is estimated to be much less sensitive to a stray electric field. We describe the New CARRAC detector and its sensitivity for the dark matter axion based on our spectroscopic measurements of Potassium Rydberg atoms.