  Tunnelling, one of the key features of quantum mechanics, ignited an ongoing
debate about the value, meaning and interpretation of 'tunnelling time'. Until
recently the debate was purely theoretical, with the process considered to be
instantaneous for all practical purposes. This changed with the development of
ultrafast lasers and in particular, the 'attoclock' technique that is used to
probe the attosecond dynamics of electrons. Although the initial attoclock
measurements hinted at instantaneous tunnelling, later experiments contradicted
those findings, claiming to have measured finite tunnelling times. In each case
these measurements were performed with multi-electron atoms. Atomic hydrogen
(H), the simplest atomic system with a single electron, can be 'exactly'
(subject only to numerical limitations) modelled using numerical solutions of
the 3D-TDSE with measured experimental parameters and acts as a convenient
benchmark for both accurate experimental measurements and calculations. Here we
report the first attoclock experiment performed on H and find that our
experimentally determined offset angles are in excellent agreement with
accurate 3D-TDSE simulations performed using our experimental pulse parameters.
The same simulations with a short-range Yukawa potential result in zero offset
angles for all intensities. We conclude that the offset angle measured in the
attoclock experiments originates entirely from electron scattering by the
long-range Coulomb potential with no contribution from tunnelling time delay.
That conclusion is supported by empirical observation that the electron offset
angles follow closely the simple formula for the deflection angle of electrons
undergoing classical Rutherford scattering by the Coulomb potential. Thus we
confirm that, in H, tunnelling is instantaneous (with an upperbound of 1.8 as)
within our experimental and numerical uncertainty.
