Quantum experiment reveals negative weak time for photons in atomic cloud

University of Toronto physicists measured a negative weak value for the time transmitted photons leave atoms excited in a rubidium cloud, using weak measurement and postselection to link a negative group delay with a measurable phase shift in a probe beam. The result clarifies how quantum interference and dwell-time concepts govern light–matter interactions, without implying backward time travel, and has spurred further work extending the weak-value framework.