Microwave-to-optical quantum converters represent an indispensable component for quantum communication in future quantum networks. To maintain quantum coherence, it is critical for such devices to operate at milli-Kelvin temperatures in the quantum ground state. Integrating photonics with superconductors at milli-Kelvin temperatures is particularly challenging since the optical excitation leads to unavoidable heating and excess microwave noise, thus placing the device systems in a thermal state as opposed to the desired ground state. In this work, we demonstrate efficient bidirectional microwave-to-optical conversion with an electro-optic device fabricated on an integrated AlN photonic platform in a milli-Kelvin environment. Our device operates near its quantum ground state and meanwhile offers 0.12% conversion efficiency – a rate that is suitable for building two-node quantum network through heralding protocols. This fully integrated converter offers advantages including tunability, scalability, and high pump power handling capability. Harnessing a pulsed drive scheme, we suppress the microwave resonator’s thermal occupancy by 30 dB to as low as 0.09±0.06 quanta (92±5% ground state probability). By studying microwave noise thermodynamics, we unravel the underlying light-induced noise generation mechanisms, which provide important guidelines for future deployment of chipscale electro-optical devices as quantum links between superconducting quantum computers. [2,3]
 C. Zhong, Z. Wang, C. Zou, M. Zhang, X. Han, W. Fu, M. Xu, S. Shankar, M. H. Devoret, and H. X. Tang, Proposal for Heralded Generation and Detection of Entangled Microwave–Optical-Photon Pairs, Phys. Rev. Lett. 124, 10511 (2020).
 W. Fu, M. Xu, X. Liu, C. L. Zou, C. Zhong, and X. Han, Ground-State Pulsed Cavity Electro-Optics for Microwave-to-Optical Conversion, ArXiv Preprint ArXiv (2020).
 M. Xu, X. Han, C.-L. Zou, W. Fu, Y. Xu, C. Zhong, L. Jiang, and H. X. Tang, Radiative Cooling of a Superconducting Resonator, Phys. Rev. Lett. 124, 33602 (2020).
Hong Tang is the Llewellyn West Jones, Jr. Professor of Electrical Engineering, Physics and Applied Physics at Yale University. His research utilizes integrated photonic circuits to study photon-photon, photon-phonon and photon-spin interactions as well as quantum photonics involving microwave and optical photons. He has been on Yale faculty since 2006. He is a recipient of the NSF CAREER Award and Packard Fellowship in Science and Engineering.