Under certain circumstances, a superconducting Josephson junction can maintain a quantum phase difference of π between the two samples that are weakly connected to form the junction. Such systems are called 'π-junctions' and have formed the basis of several experiments designed to investigate the much-debated symmetry of the order parameter of high-temperature superconductors. 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. Here we report the discovery of a metastable super-fluid state, in which a quantum phase difference of π is maintained across a weak link separating two reservoirs of superfluid ~3He. The existence of this state, which is the superfluid analogue of a superconducting π-junction, is likely to reflect the underlying 'p-wave' symmetry of the order parameter of superfluid ~3He, but a precise microscopic explanation is at present unknown.
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