Toronto Physicists Measure -0.82 Negative Weak Value for Atomic Excitation Time
Updated
Updated · The Brighter Side of News · Jul 18
Toronto Physicists Measure -0.82 Negative Weak Value for Atomic Excitation Time
1 articles · Updated · The Brighter Side of News · Jul 18
Summary
University of Toronto researchers reported a negative weak value for how long transmitted photons left rubidium-85 atoms excited, with measurements ranging from -0.82 ± 0.31 to +0.54 ± 0.28.
The result came from weak signal pulses crossing a cold atomic cloud while an off-resonant probe beam tracked phase shifts from the cross-Kerr effect, letting the team infer excitation tied specifically to transmitted photons.
Negative values appeared under conditions where group delay also turned negative, showing a measurable phase response can reverse sign even though no information traveled faster than light or backward in time.
Physical Review Letters published the peer-reviewed work on April 13, 2026, sharpening earlier claims from a 2024 release by framing the finding as a negative weak value rather than literal negative time.
The experiment strengthens the view that quantum interference and postselection can give negative conditional averages real laboratory consequences, extending a broader debate over how weak values should be interpreted.
Could the strange 'negative time' effect measured in atoms be harnessed to build ultra-fast quantum computers?
Is 'negative time' a real physical effect or a statistical illusion created by the experiment's design?
University of Toronto Physicists Measure Negative Atomic Excitation Time, Challenging Quantum Intuition
Overview
In April 2026, University of Toronto physicists made a groundbreaking discovery by directly measuring a negative weak value for the time atoms remain excited after interacting with a single photon. Unlike earlier experiments, which showed photons arriving early and were dismissed as artifacts of group delay, this new approach measured the actual excitation stored in atoms and found negative values independently. This direct measurement removes previous doubts and confirms the reality of 'negative time' as a genuine quantum effect, marking a major advance in our understanding of quantum mechanics.