Observation of momentum space a.c. Josephson physics

Josephson oscillations are a key quantum phenomenon with broad applications from sensing to quantum information. The momentum space Josephson effect describes the supercurrent flow between weakly coupled Bose-Einstein condensates (BECs) at two discrete momentum states. Here, we experimentally observe this phenomenon using a BEC with Raman-induced spin-orbit coupling, where the tunneling between two local band minima is implemented by the momentum kick of an additional optical lattice. A subsequent sudden quench of the Raman detuning induces coherent spin-momentum oscillations of the BEC, which are analogous to the a.c. Josephson effect. Both plasma and regular Josephson oscillations are observed in different parameter regimes. The experimental results agree well with our theoretical model and numerical simulation. This measurement of the Josephson plasma frequency also provides access to the Bogoliubov zero quasimomentum gap, which determines the mass of the corresponding pseudo-Goldstone mode, a long-sought phenomenon in particle physics. This combined experimental and theoretical study of Josephson oscillations in momentum space offers an exciting platform for quantum simulation and sensing utilizing momentum states as a synthetic degree.