Pulsed Cavity Electro-optics for Ground-state Microwave-to-optical Conversion*

In pursuit of quantum microwave-to-optical (MO) converters, excessive noise induced by the parametric optical drive at milli-Kelvin temperatures remains a major obstacle. Here we present an experimental study of the microwave noise in an electro-optic transducer under intense optical drives. The integrated electro-optical transducer leverages the Pockels effect of aluminum nitride microrings, which is flip-chip bonded to a superconducting resonator. Harnessing the pulsed drive scheme, we observe efficient bi-directional MO conversion, with near-ground state microwave thermal excitation (n_e = 0.09±0.06), despite the fact that the optical drive peak power exceeds the cooling power of the dilution refrigerator at its base temperature. Time evolution study suggests different mechanisms of light-induced microwave noise, among which the main contribution is the superconductor absorption of stray light scattered off the chip-fiber interface. Our results provide guidelines to further suppress microwave noise in cavity electro-optics systems, which is an essential step towards quantum transduction between microwave and optical frequencies.