Abstract:

One of the famous conclusions typically drawn from the famous Bohr-Einstein debates is that in quantum mechanics, we cannot discuss what particles were doing before they were measured: when a photon in a two-slit interferometer hits the screen, we have no right to ask where it was beforehand.  This contrasts with “classical” science ranging from archaeology to cosmology, where present observations provide us our information about the past.  Yet the real world (including dinosaurs and galaxies!) is quantum, so it is important to recognize that present measurements do contribute to knowledge of the past, but also to understand the limits on this “retrodiction."  These questions have become especially relevant given the major role that “post-selection” plays in many quantum information and quantum optics protocols.

I will concentrate largely on one, century-old, controversy related to these questions, and our just-submitted experiment attempting to resolve it, but I will also introduce approaches to combining information from state preparation and post-selection to most fully describe a quantum state at intermediate times, and give a brief overview of some ongoing experiments to study some of the puzzles they raise, particularly related to the “weak measurement” formalism of Aharonov, Albert, and Vaidman.

The question of the time a particle takes to tunnel through a classically forbidden region has been a topic of debate since the 1930s, complicated by the fact that the group delay which predicts the arrival time of a wave packet peak may seem to be faster than light.  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 (i.e., to get new information from the post-selection).  

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 a “clock” which ticks only while the atoms are in the barrier  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 real and imaginary components of this “complex time.”

For more information see:   https://www.quantumperspectives.org/