Observation of Momentum Space Josephson Effects

TL;DR

This paper reports the experimental observation of momentum space Josephson effects in a Bose-Einstein condensate with spin-orbit coupling, revealing coherent oscillations and linking them to fundamental particle physics phenomena.

Contribution

First experimental demonstration of momentum space Josephson effects in a BEC with spin-orbit coupling, connecting superfluid dynamics to particle physics concepts.

Findings

Observation of both plasma and regular Josephson oscillations.

Measurement of the Josephson plasma frequency related to the Bogoliubov gap.

Agreement between experimental results, theory, and simulations.

Abstract

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 exotic 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 sudden quench of the Raman detuning induces coherent spin-momentum oscillations of the BEC, which is analogous to the a.c. Josephson effect. We observe both plasma and regular Josephson oscillations in different parameter regimes. The experimental results agree well with the theoretical model and numerical simulation, and showcase the important role of nonlinear interactions. We also show that the measurement of the Josephson plasma frequency gives the Bogoliubov zero quasimomentum gap, which determines the mass of the corresponding pseudo-Goldstone mode, a long-sought phenomenon in particle physics. The observation of momentum space Josephson physics offers an exciting platform for quantum simulation and sensing utilizing momentum states as a synthetic degree.