Discovery of a metastable π-state in a superfluid 3He weak link

Under certain circumstances(1,2), a superconducting Josephson junction can maintain a quantum phase difference of rr between the two samples that are weakly connected to form the junction. Such systems are called 'pi-junctions' and have formed the basis of several experiments designed to investigate the much-debated symmetry of the order parameter of high-temperature superconductors(3-8) More recently, the possibility that similar phenomena might occur in another macroscopic quantum system - a pair of weakly coupled Bose-Einstein condensates - has also been suggested(9). Here we report the discovery of a metastable superfluid state, in which a quantum phase difference of pi is maintained across a weak link separating two reservoirs of superfluid He-3. The existence of this state, which is the superfluid analogue of a superconducting pi-junction, is likely to reflect the underlying 'p-wave' symmetry of the order parameter of superfluid He-3, but a precise microscopic explanation is at present unknown.