Under certain circumstances1,2, 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 superconductors3,4,5,6,7,8. 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 suggested9. Here we report the discovery of a metastable superfluid 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 ofa 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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We thank D. Rokhsar, D.-H. Lee, S. Vitale, D. Weiss, S. Bahcall, R. Chiao, A. Smerzi and D. J. Van Harlingen for conversations. This work was supported in part by the National Science Foundation, the Office of Naval Research, the Packard Foundation, and the A.P. Sloan Foundation.
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Nature Physics (2014)
Nature Physics (2013)