Sources of microwave loss and noise in an electro-optical converter*

A distributed network of superconducting circuits connected by low-loss optical fibers would improve quantum-enhanced computation and communication compared with isolated processors. The key element of this network is a microwave-to-optical converter, ideally with bidirectional high efficiency and minimal added noise. We use a SiN membrane mode simultaneously coupled to a superconducting LC circuit and optical Fabry-Perot cavity to realize our converter. Recent progress has been made to cool the mechanical mode to its motional ground state, but excess noise introduced from the microwave circuit inhibits quantum coherent conversion. Additionally, we find that the microwave loss has a strong positive correlation with circulating power and temperature of the superconducting circuit. Here, we will present efforts to investigate magnetic flux trapping and film quality as sources of the microwave loss and noise.