  Brillouin systems operating in the quantum regime have recently been
identified as a valuable tool for quantum information technologies and
fundamental science. However, reaching the quantum regime is extraordinarily
challenging, owing to the stringent requirements of combining low thermal
occupation with low optical and mechanical dissipation, and large coherent
phonon-photon interactions. Here, we propose an on-chip liquid based Brillouin
system that is predicted to exhibit ultra-high coherent phonon-photon coupling
with exceptionally low acoustic dissipation. The system is comprised of a
silicon-based "slot" waveguide filled with superfluid helium. This type of
waveguide supports optical and acoustical traveling waves, strongly confining
both fields into a subwavelength-scale mode volume. It serves as the foundation
of an on-chip traveling wave Brillouin resonator with a single photon
optomechanical coupling rate exceeding $240$kHz. Such devices may enable
applications ranging from ultra-sensitive superfluid-based gyroscopes, to
non-reciprocal optical circuits. Furthermore, this platform opens up new
possibilities to explore quantum fluid dynamics in a strongly interacting
condensate.
