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        "description": "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.",
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      "statement": "# A post-selected optical measurement yields negative conditional atomic-excitation response\n\nAngulo 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.\n\n## Scientific scope\n\nThe 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.\n\n## Reading status\n\nThis is a source-linked WRS catalogue note. A full explanatory guide and independent scientific reconstruction have not been completed for this entry. Read the identified original work for its complete narrative and arguments.\n\n## People and systems\n\nDaniela Angulo; Kyle Thompson; Vida-Michelle Nixon; Andy Jiao; Howard M. Wiseman; Aephraim M. Steinberg\n\nThese 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.\n\n\n\n## Original sources\n\n- [Experimental evidence that a photon can spend a negative amount of time in an atom cloud — author preprint v1 with complete supplementary information](https://arxiv.org/pdf/2409.03680v1)\n",
      "summary": "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.",
      "title": "A post-selected optical measurement yields negative conditional atomic-excitation response"
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    "title": "A post-selected optical measurement yields negative conditional atomic-excitation response",
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    "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.",
    "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.",
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