Generation of ultralow power phononic combs

We demonstrate excitation of phononic frequency combs in a bulk acoustic wave system at a temperature of 20 mK using a single tone low power signal source. The observed ultralow power threshold is due to a combination of a very high quality factor of 4.2 x 10(8) and relatively strong nonlinear effects. The observed repetition rate of the comb varies from 0.7 to 2 Hz and spans over tens of hertz. The demonstrated system is fully excited via piezoelectricity and does not require mode spectra engineering or external optical or microwave signals. It is shown that the comb profile significantly depends on geometry of excitation and detection electrodes. Observed strong Duffing nonlinearity below the generation threshold suggests that the system is a phononic analog to Kerr frequency combs excited in monolithic optical microresonators. The ultralow power regime opens a way of integrating this phononic system with quantum hybrid systems such as impurity defects and superconducting qubits.