Less Probable Tunneling Takes Less Time

How much time does a tunneling particle spend in a barrier? A Larmor clock, one proposal to answer this question, measures the interaction between the particle and the barrier region using an auxiliary degree of freedom of the particle to clock the dwell time inside the barrier. We report on precise Larmor time measurements of an ultra-cold gas of ⁸⁷Rb atoms tunneling through an optical barrier. The data capture distinctive features that confirm longstanding predictions of tunneling times. In particular, we demonstrate that atoms spend less time tunneling through higher barriers and that this time decreases for slower incident particles. For the lowest measured incident energy, at least 93(3)% of atoms tunneled through the barrier spending an average of 0.59(2)ms inside. This is 0.11(3)ms faster than atoms traversing the same barrier but with energy matching that of the barrier's peak and 0.21(3)ms faster than when the atoms traverse a barrier with 23% less energy.