Superfluid helium interferometry: an introduction

This article describes, at an introductory level, how superfluids can be us ed to measure absolute rotations. To make it self-contained to some degree, I first introduce briefly the two-fluid model for superfluid helium and th e concept of superfluid order parameter. These ideas, which were put forwar d for the superfluid heliums, are now widely used, in particular for the BE C gases which are the main topic of this volume. They are presented in the somewhat different perspective of helium physics. The second part will deal with the Josephson effects, the real engine behind superfluid interferomet ry. These effects were predicted in the early sixties for superconductors a nd were promptly observed in the laboratory. It was quickly realised that t hey would also exist in superfluids but the search took longer and conclusi ve experiments were performed in the eighties only in the B-phase of superf luid He-3. How these experiments are done, and how they can be used to meas ure the rotation of the Earth by superfluid interferometry is surveyed in t he last two sections. (C) 2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.