Time in quantum mechanics: tunneling, arrival, clocks

4 problems

• What is the arrival-time distribution of a free quantum particle?

  Contested

  Quantum mechanics predicts where a particle will be found but has no single rule for when it reaches a detector. Competing rules give different predictions, notably for particles with spin, which an experiment could compare.

• Is there a nonzero tunneling delay in strong-field ionization?

  Contested

  An attoclock uses a rotating laser field, like the hand of a clock, to time when an electron escapes an atom by tunneling. Experiments and analyses disagree on whether the escape takes a finite time or none.

• Does the Larmor tunneling time saturate for thick barriers?

  Open

  The Hartman effect predicts that the tunneling delay stops growing as the barrier gets thicker, which would imply speeds above that of light. Cold-atom experiments that use an atom's spin as a built-in clock can now test this.

• How long does a particle spend inside a tunneling barrier?

  Contested

  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.