1. Physics Department, University of California, Berkeley, California 94720, USA

Correspondence to: R. E. Packard Correspondence and requests for materials should be addressed to R.P. (e-mail: Email: packard@socrates.berkeley.edu).

Celebrated interference experiments have demonstrated the wave nature of light¹ and electrons², quantum interference being the manifestation of wave–particle duality. More recently, double-path interference experiments have also demonstrated the quantum-wave nature of beams of neutrons³, atoms⁴ and Bose–Einstein condensates⁵. In condensed matter systems, double-path quantum interference is observed in the d.c. superconducting quantum interference device⁶ (d.c. SQUID). Here we report a double-path quantum interference experiment involving a liquid: superfluid ³He. Using a geometry analogous to the superconducting d.c. SQUID, we control a quantum phase shift by using the rotation of the Earth, and find the classic interference pattern with periodicity determined by the ³He quantum of circulation.