It sounds like science fiction: a photon travels through a cloud of atoms and emerges so early that it appears to have spent a negative amount of time inside. The effect has been known since a 1993 experiment, but physicists largely dismissed it as an artifact — only the front of a light pulse makes it through, skewing the average.

Now an international team led by Aephraim Steinberg at the University of Toronto, with theorists at MIT, Griffith University and IISER Bangalore, has confirmed the effect by asking the atoms themselves. Their experiment, published in Physical Review Letters, fires a photon pulse into a cloud of rubidium atoms cooled to 60–70 microkelvin while a second, weak laser beam continuously probes whether the atoms are excited.

Because precise measurements would disturb the system (the quantum Zeno effect), the team used "weak measurements" — imprecise but accurately calibrated. Averaging roughly one million runs, collected over about 70 hours across seven sets of parameters, the atoms' own response matched the negative time inferred from arrival times: average excitation times ranged from (-0.82 ± 0.31)τ0 for the most narrowband pulse to (0.54 ± 0.28)τ0 for the most broadband pulse.

"You get the same answer if you ask the atoms, 'How long was the photon staying with you?' They will also tell you an answer, which is a negative time," says Howard Wiseman of Griffith University. Crucially, unlike the arrival-time method, the weak measurement cannot be explained by the "front of the pulse" argument.

No time travel, though: the result is fully consistent with standard physics. The team's next target is the scattered photons, which theory predicts carry extra positive excitation time that balances the negative time and keeps the overall average non-negative — a prediction never yet tested. As Wiseman puts it: even in this "really simple thing — a photon interacting with atoms — people were already doing calculations on that almost 100 years ago... it can still show surprises."

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