  A quantum particle moving in a gravitational field may penetrate the
classically forbidden region of the gravitational potential. This raises the
question of whether the time of flight of a quantum particle in a gravitational
field might deviate systematically from that of a classical particle due to
tunnelling delay, representing a violation of the weak equivalence principle. I
investigate this using a model quantum clock to measure the time of flight of a
quantum particle in a uniform gravitational field, and show that a violation of
the equivalence principle does not occur when the measurement is made far from
the turning point of the classical trajectory. I conclude with some remarks
about the strong equivalence principle in quantum mechanics.
