Superfluid Acoustic Analogs of Fiber Optic and Ring Laser Gyroscopes

Longitudinal acoustic waves in superfluid ⁴He (first sound) propagate at the speed of sound relative to the fluid, and with extremely low loss at low temperatures. We have recently demonstrated acoustic attenuation lengths exceeding 600km at 8kHz and 40mK[1]. Due to macroscopic quantization, superfluid contained in a ring will remain fixed to the non-rotating frame while the container rotates at sufficiently low rates. As a result, acoustic waves traveling around the ring which are co-rotating or counter-rotating will arrive at a detector with a phase shift, resulting in an acoustic Sagnac effect. This approach may form acoustic analogues of the fiber-optic gyroscope (FOG) and the ring laser gyroscope. The low value of the speed of first sound compared to the speed of light creates long Sagnac time delays, and the ultra-low acoustic loss may make long path length sensing coils possible, both are key properties for high sensitivity to rotation. Very large mismatch between the acoustic impedance of the helium compared to the soild walls of the containing tubing, guide and confine the acoustic waves. Analytic and numerical computations show that leakage of energy into the tubing of ~10^-4 of the total energy is typical. Together with low acoustic dissipation of pure metals suggest that very long acoustic path lengths in the helium are feasible. These effects form a possible alternate route to ultra-sensitive rotation sensors which do not require a junction structure (such as a Josephson junction for the superfluid) or the measurement of small, low frequency mass currents.

[1]L.A. De Lorenzo and K. C. Schwab. "Ultra-High Q Acoustic Resonance in Superfluid ⁴He." Journal of Low Temperature Physics 186 (2017): pp. 233-240.