  Tunnelling lies at the heart of quantum mechanics and is a fundamental
process in attosecond science, molecular biology, and quantum devices. Whether
tunnelling takes time and how a microscopic particle transits through a barrier
have been debated since the early days of quantum mechanics. The time required
for an electron to tunnel through an atomic potential barrier has been measured
with attosecond angular streaking (attoclock), and a recent work on the
hydrogen atom claimed that electron tunnelling is instantaneous. However, the
time required for Rb atoms to tunnel through an optical potential barrier has
been measured to be on the order of milliseconds with a recent Larmor clock
measurement. The essence of electron and atom tunnelling is identical, but the
reason for the contradictory conclusions remains unknown. Here, we demonstrate
that the sub-barrier potential interaction is the root of the nonzero
tunnelling delay time. We reveal that the wave-particle duality of a tunnelling
electron must be fully taken into account when decoding the tunnelling delay
time from the attoclock measurements. Based on energy-resolved attoclock
measurements, we show that the tunnelling delay time of an electron ranges from
24 to 58 attoseconds, and the counterintuitive result that an electron with a
lower energy may spend less time in the barrier is consistent with the
velocity-dependent tunnelling time of atoms in Larmor clock measurements. Our
results unify the tunnelling time of microscopic particles by highlighting the
classically forbidden sub-barrier potential interactions of matter waves.
