  Tunnelling is one of the most paradigmatic and evocative phenomena of quantum
physics, underlying processes such as photosynthesis and nuclear fusion, as
well as devices ranging from SQUID magnetometers to superconducting qubits for
quantum computers. The question of how long a particle takes to tunnel,
however, has remained controversial since the first attempts to calculate it,
which relied on the group delay. It is now well understood that this delay (the
arrival time of the transmitted wave packet peak at the far side of the
barrier) can be smaller than the barrier thickness divided by the speed of
light, without violating causality. There have been a number of experiments
confirming this, and even a recent one claiming that tunnelling may take no
time at all. There have also been efforts to identify another timescale, which
would better describe how long a given particle spends in the barrier region.
Here we present a direct measurement of such a time, studying Bose-condensed
$^{87}$Rb atoms tunnelling through a 1.3-${\mu}$m thick optical barrier. By
localizing a pseudo-magnetic field inside the barrier, we use the spin
precession of the atoms as a clock to measure the time it takes them to cross
the classically forbidden region. We find a traversal time of 0.62(7) ms and
study its dependence on incident energy. In addition to finally shedding light
on the fundamental question of the tunnelling time, this experiment lays the
groundwork for addressing deep foundational questions about history in quantum
mechanics: for instance, what can we learn about where a particle was at
earlier times by observing where it is now?
