How long does a particle spend inside a tunneling barrier?
In plain words
A particle can cross a barrier it classically could not climb, but the time spent inside has several definitions (phase delay, dwell time, Larmor time) that give different numbers, some implying faster-than-light motion. The question is which, if any, is the physical duration.
Precise statement
For a particle of energy $E<V_0$ incident on a barrier of width $L$, compare the Wigner phase delay, dwell time, Buttiker-Landauer traversal time and weak-value Larmor times $\tau_y,\tau_z$. Determine whether a single operationally defined traversal time exists (e.g. the reading of a weakly coupled internal clock conditioned on transmission) and how it scales with $L$.
What would settle it
A theory of clocks coupled to the tunneling particle whose predictions agree with Larmor-clock, attoclock and Ramsey-clock experiments.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
A 2024 analysis proposed a unified treatment via Ramsey clocks (arXiv:2404.14382); attoclock and Larmor results are still interpreted differently in 2025 (arXiv:2503.07859).
Problems built on this one
Special cases
- Is there a nonzero tunneling delay in strong-field ionization? · open
- Does the Larmor tunneling time saturate for thick barriers? · open
Results
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