Probing the time a tunneling atom spends in the forbidden region, and other problems in quantum archaeology
Saletta Conferenze, edificio M
Prof. Aephraim Steinberg
Department of Physics and Centre for Quantum Information & Quantum Control, University of Toronto, Canada.
The question of the time a particle takes to tunnel through a classically forbidden region has a long history, complicated by the fact that the group delay which predicts the arrival time of a wave packet peak may be smaller than d/c. By now it is well understood that this result is not paradoxical, but it leaves open the question, already stressed by Büttiker and Landauer in the 1980s, of how long a particle interacts with the barrier, and of whether it is sensible to distinguish between interaction times for transmitted and reflected particles. It is also an archetypical problem for raising the issue of how much one learns about the history of a quantum system from present observations.
By preparing ultracold Rubidium atoms in an atom waveguide and cooling them to approximately 1 nK, we are able to study tunneling across a 1-μm barrier formed by a blue-detuned laser beam. Using Raman coupling to generate a fictitious magnetic field, we implement the “Larmor clock” discussed by Baz’, Rybachenko, and Büttiker. I will present our recent results characterizing how long transmitted atoms spend in the barrier region. We analyze them in terms of the weak-measurement formalism, which enables us to attach concrete and distinct interpretations for the various components of this “complex time.”
I will say a few words about future extensions of this work, as well as other ongoing experiments investigating other issues in quantum measurement and retrodiction.