Researchers at the University of Toronto have published findings in Physical Review Letters describing an experiment involving photons passing through rubidium atoms that challenges conventional understanding of time at the quantum level.
The experiment centers on photons, or quantum particles of light, traveling through a cloud of rubidium atoms. These atoms resonate with photons, meaning the photon’s energy can temporarily transfer to the atoms as an excitation before the photon is released. For this resonance to work effectively, the photon must have well-defined energy matching the rubidium atom’s excitation requirements. However, quantum uncertainty principles create a tradeoff: if the photon’s energy is well-defined, its timing becomes uncertain, requiring the light pulse to have a long duration.
When a photon successfully passes straight through the cloud without being scattered, calculations reveal an unexpected result. The photon appears to arrive at the far side of the cloud so early that it seems to have spent a negative amount of time inside it. This phenomenon has been documented for decades, with observations dating to 1993, but scientists largely dismissed it as an artifact explainable by only the front portion of the long-duration pulse making it through while the rest scattered.
The current research took a different approach by directly measuring the interaction time through weak measurements of the rubidium atoms themselves. Rather than making precise measurements that would disrupt the quantum system, researchers used an imprecise but carefully calibrated weak laser beam to probe whether atoms were excited. By averaging millions of experimental runs, they obtained an accurate measurement of the photon’s dwell time within the atomic cloud.
The results showed that the negative dwell time derived from weak measurements exactly matched the negative time suggested by the photons’ arrival times. Crucially, this negative dwell time cannot be explained by the earlier theory that only the pulse’s front portion successfully traverses the cloud. The findings indicate that negative dwell time represents a genuine physical phenomenon with measurable effects on the atomic system, rather than a mathematical illusion, while remaining consistent with established quantum physics principles.
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