Physicists model what happens when a photon is cut off mid-reflection
A new theory paper says abruptly removing a mirror during reflection could turn one photon into a spread of new colors.
By Hana Yoshida · Markets Reporter
3 min read
A single photon reflecting from a mirror may not behave like a particle choosing one route if the mirror vanishes partway through the interaction. A theory paper by three Norwegian physicists, published in Physical Review Letters, finds that the abrupt cutoff could generate additional photons across a wider range of frequencies.
The question targets a strange edge case in quantum optics: a photon is treated as one indivisible quantum of light, yet its electromagnetic field can extend over time and space. According to the authors, interrupting that extended field during reflection changes the problem from ordinary reflection into a rapid time-dependent event.
Why a mirror switch changes the result
In a common quantum optics example, a single photon hitting a partially reflective mirror enters a superposition of being reflected and transmitted. When detectors are placed on both paths, only one detector registers the photon, according to the standard description cited by the researchers.
That reasoning would suggest a similar result for a fully reflective mirror removed during reflection: the photon would be found either reflected or transmitted, with the timing of the removal affecting the probabilities. The new analysis says that picture misses what the sudden change does to the photon’s field.
The authors frame the issue through the link between time and frequency. A smooth, long-lasting wave can be described with a narrow frequency range, while a sharp change in time requires many frequencies. In the mirror scenario, the reflected wave is cut off and the transmitted wave begins abruptly when the mirror is switched away.
According to the paper, those sharp transitions require extra bandwidth beyond the original photon’s frequency. The result is that the interrupted photon is associated with newly generated light at multiple frequencies, rather than a clean single-frequency outcome.
A single photon could become a spectrum
The study says the cutoff leaves the light in a quantum superposition involving reflected and transmitted components. Because the interruption can create more than one photon, detectors on both sides could register light from the same event, unlike the standard partially reflective mirror case.
That does not mean a photon is split into two half-photons. The analysis instead treats the fast change in the mirror’s optical properties as an event that can create photons, producing a spread of colors from the original single-frequency input.
The researchers also point to related evidence from work with ultrashort optical pulses. Mirrors whose reflectivity is rapidly changed are already used to shorten such pulses, and the shortened reflected pulses contain a broader set of frequencies. The authors say that broader spectrum implies photon generation, though the single-photon version has not yet been observed.
Testing the idea will be difficult
The paper says an experiment would need a source that emits single photons on demand with an extremely narrow spectral bandwidth. That narrow bandwidth would stretch the photon over enough time for extra photons from the cutoff to be detected.
The mirror also could not be a moving physical object. The authors calculate that the transition from reflective to transmissive would need to occur in about 10 femtoseconds, far faster than a mechanical mirror can move.
Some semiconductors can change from reflective to transmissive in roughly 30 to 100 femtoseconds when triggered by ultrafast laser pulses, according to the researchers. The same laser pulse creates a practical problem: it could swamp the much weaker light produced by cutting off the single photon, making the generated photons hard to isolate.
This story draws on original reporting from Ars Technica.