How Long Does A Tunnelling Particle Take To Escape Before It Is Detected?

A strong measurement of a particle's position localizes it within the region of the measuring probe. If the particle is tunnelling through a barrier, such a measurement may disturb the tunnelling and gives the particle enough energy to exist in the previously "forbidden" region. Instead, one can imagine weakly probing such a particle's position; this may be realized with a weak Larmor measurement combined with post-selection. If this probe is present for a long time, throughout the particle-barrier interaction, the system appears time-independent and must conserve energy, so a tunnelling will not appear to be "found" inside the barrier. However, if the probe is only on for a short time, the transmission and reflection probabilities may be modified, indicating real propagation above the barrier. We discover a new time scale which corresponds to the duration over which particles are significantly disturbed in this way. This time scale exhibits a characteristic position dependence within the barrier region, which persists over a range of scattering parameters. This time scale suggests a new perspective on the decades-old tunnelling time question, as it may be interpreted as the amount of time a particle takes to "escape" the probe region undetected.