Norwegian physicists propose experiment to split single photon by removing mirror

A trio of Norwegian physicists has published a theoretical study in Physical Review Letters proposing that abruptly removing a mirror while a single photon is in the process of reflecting would generate a shower of new photons across a broad spectrum. The research argues that the sharp transition in the electromagnetic field requires additional bandwidth, effectively creating multiple photons from the original single particle. While the underlying physics is considered sound, the experiment remains unperformed due to significant technical complexities.

Photons are typically considered indivisible particles of light, but they also exhibit wave-like properties as extended objects without a specific location. Under normal linear conditions, photons do not divide or combine; such nonlinear processes usually require high-intensity light sources or sensitive media. The concept relies on the relationship between time and frequency domains: a sharp event in time, such as abruptly stopping a wave, requires a broad spectrum of frequencies.

The researchers note that while the physics is sound, the experiment is complex, requiring ultrafast switching mechanisms such as semiconductors driven by laser pulses to achieve the necessary 10-femtosecond transition time. Achieving the necessary 10-femtosecond transition time would require ultrafast switching mechanisms, such as semiconductors driven by laser pulses, rather than physical mirrors. A major challenge identified by the researchers is filtering out the intense laser pulse used for switching to isolate the generated photons.

The study suggests that while the physics is sound, the experiment is complex and has not yet been performed for single photons, though evidence exists for similar effects with ultrashort pulses. Previous observations have shown that mirrors can shorten ultrashort pulses, implying the generation of new frequencies and thus photons, but this has not been observed for single photons. The exact nature of the superposition of the generated photons remains to be empirically verified.

It is uncertain whether the technical challenges, particularly filtering the switching laser pulse, can be overcome to observe the generated photons from a single photon source. The experiment has not yet been successfully conducted; the study is a theoretical proposal. The authors calculate that the transition from reflective to transmission needs to take place in about 10 femtoseconds, which is insufficient time to move a physical mirror.

Semiconductors can be driven from reflective to transmission quite rapidly, though typically in the 30 to 100 femtosecond range, using ultrafast laser pulses as the switch. Unfortunately, that laser pulse gets in the way, making it quite difficult to filter out the big laser pulse used to remove the mirror so that the photons generated by cutting the long single photon are observable.

We already have some evidence that this works, though. The mirrors described in the study are used to shorten ultrashort pulses, which means that the reflected pulses have more frequencies after reflection, and new photons must therefore be generated. We just haven't observed it for single photons yet.