Momentum space Josephson oscillations in a spin-orbit coupled Bose-Einstein condensate

Josephson oscillations are a key quantum phenomenon with broad applications from sensing to quantum information. While the effect has first been discovered in tunneling supercurrents, the realization of Josephson oscillations in dilute gas Bose-Einstein condensate has opened up new paths for experimental investigations.

Here, we demonstrate both experimentally and theoretically the study of Josephson oscillations in momentum space by exploiting the properties of a spin-orbit coupled BEC. By combining Raman detuning with a weak optical lattice, a supersolid-like stripe phase with periodic density modulation can be induced in the BEC. The Bogoliubov spectrum of such a supersolid-like phase corresponds to a pseudo-Goldstone mode due to the explicit breaking of translational symmetry by the weak lattice, the zero-momentum Bogoliubov gap gives the mass of the pseudo-Goldstone mode. We show that a subsequent Raman detuning quench leads to the Bogoliubov excitations in the form of spin oscillation, which can be interpreted in the language of Josephson physics. The measured oscillation frequencies are in good agreement with numerical simulations.

This combined experimental and theoretical study of Josephson oscillation in the context of spin-orbit coupled BECs establishes a new platform for the study of Josephson physics and, at the same time, provides a technique to probe the excitations of a BEC in the stripe phase.