Physics Club: Aephraim Steinberg, University of Toronto, “How long does a tunneling particle spend in the forbidden region? (And other problems in quantum archaeology.)”

Event time: 
Monday, April 12, 2021 - 4:00pm to 5:00pm
Online See map
Aephraim Steinberg, University of Toronto

Event description: 

Every undergraduate quantum mechanics course teaches students how to calculate the probability of tunneling, but almost none addresses how long the process “takes.” Nevertheless, the question was raised as early as the 1930s, and grew into a topic of heated debate in the 1980s (rekindled more recently by so-called “attoclock” experiments).
One of the most famous tidbits of received wisdom about quantum mechanics is that “you can’t ask” about the history of a quantum particle, e.g., which path a photon took in an interferometer before reaching the screen. This has led many to argue that one cannot talk about the histories of transmitted and reflected particles separately at all. Much work over the past decades has aimed to chip away at this blanket renunciation, and investigate “quantum retrodiction.” I will present the tunneling-time problem as one example of this much broader issue.
Using Bose-condensed Rubidium atoms cooled down below a billionth of a degree above absolute zero, we demonstrate a clear operational way of probing transmitted and reflected particles independently. We have measured how long the transmitted atoms 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 now refining to study exactly how long it would take to “collapse” an atom to be in the barrier.
Time permitting, I will also say a few words about a more recent experiment, which looks back at the common picture that when light propagates through a dispersive medium, photons get “virtually absorbed” and then re-emitted. It turns out that although it is possible to measure “the average time a photon spends as an atomic excitation,” there seems to be no prior work which directly addresses this, especially in the resonant situation. We have carried out a first experiment that lets us distinguish between transmitted photons and 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?”
Aephraim Steinberg is a Professor of Physics at the University of Toronto. He is also a founding member and past director of the Centre for Quantum Information and Quantum Control (CQIQC), and currently Director of the CIFAR programme in Quantum Information Science. Steinberg received his undergraduate degree from Yale University (where he worked with Ed Hinds), and spent a year as a research assistant at the Ecole Normale Supérieure (with Serge Haroche’s group) before carrying out his Ph.D. research on superluminal tunneling with Ray Chiao at the University of California at Berkeley. He held post-doctoral fellowships at the Université de Paris VI (with Elisabeth Giacobino) and the U.S. National Institute of Standards and Technology (with Bill Phillips) before moving to Toronto, where he’s been since 1996. He has won numerous awards, ranging from the APS doctoral thesis prize in AMO Physics to a Steacie Fellowship. He is a Fellow of the Institute of Physics (UK), the American Physical Society, the Optical Society of America, and the Royal Society of Canada. Steinberg’s interests focus on foundational issues in quantum mechanics, as well as their application to tasks ranging from information processing to precision measurement. His experimental program is two-pronged, using atoms cooled down to a billionth of a degree above absolute zero as well as entangled photon pairs, to study issues such as quantum measurement and the nature of the past in quantum mechanics, quantum information & computation, tunneling times, and the control and characterization of novel quantum states. In 2012, his Science paper on using weak measurement to observe the “average trajectories” of single photons in a two-slit interferometer was selected by Physics World as the “breakthrough of 2011.”
This meeting will be held over Zoom. Please contact Teri Eskew ( for connection information.
Host: Michel Devoret,