Brillouin Optomechanics in the Quantum Ground State

Microwave to optical transducers convert quantum states from platforms such as superconducting circuits into the thermal noise-free optical regime, promising a route towards a quantum network using telecom fibers as links. A widespread approach is to use a mechanical resonator as intermediate system that couples to both microwaves and optical photons. One requirement for such a transducer is that the number of noise photons added in the process is much smaller than one, which can only be achieved if the intermediate mechanical resonator is in its quantum ground state. We present the demonstration of a Brillouin cavity optomechanics system operating in a dilution refrigerator, compatible with future integration with superconducting circuits. Using optical sideband asymmetry measurements, we show that the GHz frequency mechanical modes are in the quantum ground state of motion. In addition, we present a series of measurements confirming that the phonon occupation is not affected by direct heating from incident laser power.