What did you do last nanosecond? Experimentally asking photons and atoms about their past.
Aephraim Steinberg (University of Toronto, Canada)
If there are two problems you would think quantum mechanicists & opticians had beaten to death, they might be quantum tunneling and the propagation of single photons through a cloud of two-level atoms.
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?” or “how do photons get slowed down, and where is the energy spending its time?” – the answers are not so simple.
I will describe two experiments looking into aspects of “quantum retrodiction.” In the first, we measure how long Bose-condensed atoms spend inside a potential barrier (created by a far-detuned laser beam focused to 1 micron) before being transmitted; I will also talk about some predictions regarding what insidious effects actually observing a particle in the barrier could have, and new timescales this discussion reveals about tunneling.
In the second, we measure the amount of time atoms spend in the excited state when a resonant photon is not absorbed by those atoms, but propagates clear through. We find, surprisingly, that the answer need not even be a positive number. I will connect this to better-known aspects of optical propagation. At the end I will address the situation of a photon which is tuned precisely to resonance, yet also observed at a particular moment in time – are its atomic interactions governed by the well-defined energy of its preparation, the well-defined time of its detection, or both?
[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).
[4] 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).
[5] 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, Amal Vijayalekshmi Sivakumar, Howard M. Wiseman, & Aephraim M. Steinberg, APL Quantum 2, 036108 (2025).
[6] Experimental evidence that a photon can spend a negative amount of time in an atom cloud, Daniela Angulo, Kyle Thompson, Vida-Michelle Nixon, Andy Jiao, Howard M. Wiseman, & Aephraim M. Steinberg, quant-ph/2409.03680 (2024).