Quantum archaeology: How much time do atoms spend “under the tunnel barrier," and how much time do photons spend trapped “inside” atoms?

Abstract: One of the most famous tidbits of received wisdom about quantum mechanics is that one "cannot ask" how a particle got to where it was finally observed, e.g., which path of an interferometer a photon took before it reached the screen. What, then, do present observations tell us about the state of the world in the past? I will describe two experiments looking into aspects of this “quantum retrodiction."

The main experiment I will focus on addresses a century-old controversy: that of the tunneling time. Since the 1930s, and more heatedly since the 1980s, the question of how long a particle spends in a classically forbidden region before being transmitted has been a subject of debate. Using Bose-condensed Rubidium atoms cooled down to a nanokelvin, we have now measured just how long they spend inside an optical beam which acts as a “tunnel barrier” for them. I will describe these ongoing experiments, as well as proposals we are refining to study exactly what happens during the time it takes to “collapse” an atom to be in the barrier.

I will also say a few words about more recent experiments, which revisit the common picture that when light slows down in glass, or a cloud of atoms, it is because the photons “get virtually absorbed” before being sent back along their way. We have carried out an experiment that lets us distinguish between the time spent by transmitted photons and by photons which are eventually absorbed, asking the question “how much time are atoms caused to spend in the excited state by photons which are not absorbed?”

[1] Measuring the time a tunnelling atom spends in the barrier, Ramón Ramos, David Spierings, Isabelle Racicot, Aephraim M. Steinberg, Nature 583, 529 (2020).

[2] Observation of the decrease of Larmor tunneling times with lower incident energy, David C. Spierings & Aephraim M. Steinberg, Phys. Rev. Lett. 127, 133001 (2021).

[3] Measuring the time atoms spend in the excited state due to a photon they do not absorb, Josiah Sinclair, Daniela Angulo, Kyle Thompson, Kent Bonsma-Fisher, Aharon Brodutch, Aephraim M. Steinberg, PRX Quantum 3, 010314 (2022).

[4] How much time does a resonant photon spend as an atomic excitation before being transmitted?, Kyle Thompson, Kehui Li, Daniela Angulo, Vida-Michelle Nixon, Josiah Sinclair, Howard M. Wiseman, Aephraim M. Steinberg, in preparation

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