Optomechanics with Superfluid Helium-4

We demonstrate the utility of superfluid helium-4 as an extremely low loss optomechanical element. We form an optomechanical system with a cylindrical niobium superconducting TE 011 resonator whose 40 cm 3 inner cylindrical cavity is filled with 4 He. [1] Coupling is realized via the variations in permittivity resulting from the density profile of the acoustic modes. Acoustic losses in helium-4 below 500 mK are governed by the intrinsic nonlinearity of sound, leading to an attenuation which drops as T 4 , indicating the possibility of quality factors (Q) over 10 10 at 10 mK. In our lowest loss mode, we demonstrate this T 4 law down to 50 mK, realizing an acoustic Q of 1.35·10 8 at 8.1 kHz. When coupled with a low phase noise microwave source, we expect this system to be utilized as a probe of macroscopic quantized motion, for precision measurements to search for fundamental physical length scales, and as a continuous gravitational wave detector. Our estimates suggest that a resonant superfluid acoustic system could exceed the sensitivity of current broad-band detectors for narrow-band sources such as pulsars [2].