A post-selected optical measurement yields negative conditional atomic-excitation response

What changed

Angulo and colleagues measured the small phase shift of an off-resonant probe caused by resonant light passing through a cold rubidium cloud. Comparing probe traces with and without a later photon detection isolated the inferred contribution of an extra transmitted photon. For longer, narrower-band pulses, its integrated response had the opposite sign to the average-photon response. The authors interpreted this result as evidence for a negative conditional atomic-excitation time related to anomalous group delay.

Scientific scope

Angulo and colleagues measured the small phase shift of an off-resonant probe caused by resonant light passing through a cold rubidium cloud. Comparing probe traces with and without a later photon detection isolated the inferred contribution of an extra transmitted photon. For longer, narrower-band pulses, its integrated response had the opposite sign to the average-photon response. The authors interpreted this result as evidence for a negative conditional atomic-excitation time related to anomalous group delay. The experiment used coherent light, weak probing and post-selection, with finite integration windows and reported statistical and systematic uncertainties. A conditional weak response is distinct from the always nonnegative unconditional excited-state population. Some data disagree quantitatively with the theory, and a bypass control retains a residual signal. The observation does not demonstrate time travel or superluminal information transfer. The cited preprint and supplement date from 2024; the 2026 journal version may differ.

Evidence

The linked original works support the scoped statement and contribution attribution. WRS has not independently replayed experiments or reconstructed a full mathematical proof. The experiment used coherent light, weak probing and post-selection, with finite integration windows and reported statistical and systematic uncertainties. A conditional weak response is distinct from the always nonnegative unconditional excited-state population. Some data disagree quantitatively with the theory, and a bypass control retains a residual signal. The observation does not demonstrate time travel or superluminal information transfer. The cited preprint and supplement date from 2024; the 2026 journal version may differ.

Contributors

• The named authors in the research record · SCIENCE: These are the listed authors of the cited contribution sources. Paper authorship does not quantify individual effort. An incomplete list is explicitly marked; same names are not automatically merged.

• Experimental evidence that a photon can spend a negative amount of time in an atom cloud — author preprint v1 with complete supplementary information

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