Tunnelling is one of the phenomena at the heart of quantum mechanics. Despite its central role in physics, the duration of the tunnelling process has been debated since the development of quantum theory without a definite answer. Ultracold atoms provide an ideal platform to address this question due to their isolation from the external environment, the ability to create almost arbitrary potentials for the atoms, and the millisecond-level timescales which are convenient to probe in the lab.
In this talk, I will present the results of our tunnelling time experiment, which consists of a Bose-Einstein condensate in a one-dimensional waveguide impinging on a micron-size barrier. As the atoms tunnel through this thin optical potential, each of their spins acts as a “clock” to record the time spent in the barrier region through the Larmor precession induced by a pseudo-magnetic field. The precession angle is found by performing tomography on the spin state of the tunnelling atoms. With the knowledge of the Larmor frequency and the rotation angles, we find a traversal time of 0.65(7) ms and study its dependence on incident energy. Our experiment achieves a long-sought tunnelling time measurement with a clear interpretation, and the results show good quantitative agreement with theory based on the weak measurement formalism.