Researchers Confirm Photons Spend Negative Time in Rubidium Clouds, Matching 2 Independent Measurements

Q: How did scientists probe a photon's negative dwell time without triggering the quantum Zeno effect and accidentally freezing the entire atomic system?

They avoided the quantum Zeno effect by not making a strong, precise measurement of the atoms in real time. Instead, the Toronto team used a weak measurement: they sent a separate, very faint probe laser through the rubidium cloud and looked for tiny phase shifts in that beam. Those phase shifts acted as an indirect readout of whether the atoms were briefly in an excited state, which is where the photon’s energy would be stored during its passage. Because the probe was so gentle, it added only minimal backaction and did not repeatedly collapse the atom-photon system into a fixed state—the kind of disturbance that would have frozen the interaction through the quantum Zeno effect. The tradeoff was that any single measurement was extremely noisy and nearly useless on its own. To recover a reliable signal, the researchers repeated the experiment roughly a million times and averaged the results. That let them estimate the conditional atomic excitation time associated with photons that were later postselected as successfully transmitted. This matters because it let the scientists “ask the atoms” directly how long the photon’s energy dwelled in them, without destroying the process they were trying to measure. The atomic weak-measurement signal matched the earlier arrival-time result, confirming that the negative dwell time was not just a pulse-reshaping artifact.

3 articles · Updated · ScienceDaily · Aug 4

Millions of weak-measurement runs showed photons transmitted through a rubidium atom cloud had a negative dwell time, confirming the effect as physically measurable rather than a pulse-shape illusion.
A weak laser probe tracked tiny phase shifts in the atoms while single-photon pulses crossed the cloud, letting researchers infer how long the photons' energy resided as atomic excitation without disrupting the interaction.
That atomic measurement exactly matched the negative time inferred from the photons' unusually early average arrival at the far side of the cloud, even though the two methods are entirely different.
The result, published in Physical Review Letters, does not violate standard physics or imply time travel, but it undercuts the long-standing view that the effect was only an artifact of the front edge of a long light pulse.

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ScienceDaily16d ago

Researchers Confirm Photons' "Negative Time" in Atomic Clouds is Measurable Reality

threads.com16d ago

Physics just got a whole lot weirder. Researchers have officially confirmed that 'negative dwell time' is a real, measurable phenomenon. By tracking photons as they pass through atoms, scientists observed them exiting

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Physicists confirm negative dwell time in quantum experiments with ultracold rubidium atoms

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If photons spend measurable negative time inside an atom cloud without breaking causality, what does this bizarre reality mean for time itself?

How did scientists probe a photon's negative dwell time without triggering the quantum Zeno effect and accidentally freezing the entire atomic system?

Researchers Confirm Photons Spend Negative Time in Rubidium Clouds, Matching 2 Independent Measurements

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