Physics Colloquium with Aephraim Steinberg

   Tunneling is one of the most quintessential quantum phenomena, and among the first we learn about (indeed, it has been known for coming on 100 years, not to mention being central to this year’s Nobel Prize).  One might have the impression that we know all about it there is to know.  And yet when you look more deeply – and ask “where are the atoms while they’re tunneling through the forbidden region, and how much time do they spend there?” – the answers are not so simple.  The question of how long particles spend “beneath” a tunnel barrier has been controversial for most of the past century, in part because by some definitions the answer appears to be faster than light.  This fits into a larger puzzle over how exactly to describe the “history” of quantum systems in general.  I will begin by introducing the problem of quantum retrodiction, and our current perspective on it, and briefly describe some of the history of tunneling-time experiments.  I will then tell you about an experiment in which we Bose-condense atoms, cool them to about 1 nK, and let them tunnel through a 1-micron-thick sheet of light as we monitor how long they spend in the “forbidden” region.  

     I will also present a follow-up experiment in which we studied reflection from the barrier, and unexpectedly observed the emergence of complicated spin textures driven by interactions.  This occurred only in reflection, and in spite of the essentially spin-independent nature of Rubidium’s collisional dynamics, and was hence a surprise.  I will explain how the indistinguishability of the atoms gives rise to an effective magnetic interaction.

We now understand it as an indistinguishable-particle effect, similar in that sense to the well-known “ISRE,” but different in that the latter is widely understood to occur only in thermal (non-condensed) gases, while our effect arises even in a condensate.  

    Finally, I will discuss the question of what the mere act of observing atoms while they are in the forbidden region does to them, and a proposed experiment to investigate this.  Our numerical and theoretical results appear to suggest a new timescale related to how long an atom in a forbidden region takes to “leak out."

 

 

REFERENCES:

[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] Spin Rotations in a Bose-Einstein Condensate Driven by Counterflow and Spin-independent Interactions, David C. Spierings, Joseph H. Thywissen, & Aephraim M. Steinberg, Phys. Rev. Lett. 132, 173401 (2024).