  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 ultra low power threshold is due to a combination of very high
quality factor of $4.2\times 10^8$ and relatively strong nonlinear effects. The
observed repetition rate of the comb varies from 0.7 to 2Hz and spans over tens
of Hertz. The demonstrated system is fully excited via piezoelectricity and
does not require mode spectra engineering and 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 analogue to
Kerr frequency combs excited in monolithic optical microresonators. The
ultra-low power regime opens a way of integrating this phononic system with
quantum hybrid systems such as impurity defects and superconducting qubits.
