A real time, continuously operating, fluxlocked superfluid interferometer

Interferometers are widely used in basic and applied sciences. These instruments using sound, light or de Broglie matter waves, have an output amplitude (e.g., the Josephson critical current in a dc SQUID), which is a sinusoidally varying function of some variable of interest (magnetic flux in the case of the SQUID). To achieve widespread practical utility, it is very useful to have a method to linearize the instrument's response. We report here a real-time flux locking technique using thermal counter flow to linearize the output of a superfluid He-4 quantum interference device (SHeQUID), an analogue of the superconducting dc SQUID. A continuously changing rotation flux through the interferometer sense loop of the SHeQUID produces a changing phase-difference in the sense loop. This change is canceled via continuous negative feedback using the phase shift caused by a thermally driven superflow. The feedback signal (injected heater power) is then a linear measure of rotation flux and is used to track the rotation signal in real time.