Oslo Physicists Show Photon Cutting Can Spawn Many Photons, Not Infinite Ones

Q: If cutting a photon in half pulls new light from the vacuum, where does this sudden burst of energy actually originate?

The extra energy would come from the device that changes the boundary condition — the moving mirror or ultrafast optical shutter — not from nowhere. In the Oslo team’s calculation, “splitting” a photon means abruptly changing how the electromagnetic field is constrained. Quantum field theory says the vacuum is not truly empty; it contains fluctuations. When the boundary is changed very quickly, that external action shakes the field and can convert some of those vacuum fluctuations into real photons. This is closely related to the dynamical Casimir effect, where rapidly changing mirrors or circuit boundaries generate light from the vacuum. So the vacuum is the source of the new photons, but not the ultimate energy source. The energy is injected by whatever mechanism removes or shifts the mirror, or switches the shutter, fast enough to disturb the field. In that sense, the apparatus does work on the quantum field, and the field responds by producing a more complicated state with additional photon components. The “infinite” burst appears only in an idealized mathematical limit where the boundary changes instantaneously. Real mirrors and shutters cannot move or switch infinitely fast, so a real experiment would produce only a finite amount of extra light, likely small. The infinity is therefore a sign that a perfectly sharp cutoff would require unbounded energy, not a prediction of a physically achievable infinite flash.

3 articles · Updated · ScienceAlert · Jul 24

University of Oslo physicists calculated that removing an optical shutter while it reflects a photon can turn a would-be "cut" photon into a superposition with 0, a few, or many photons.
The number depends on shutter speed: faster removal yields more photons, while an infinite count would require infinitely fast mirror removal, which co-author Johannes Skaar said is impossible.
The team said the effect is experimentally plausible because narrow-band photons can stretch several meters or even kilometers, easing the timing demands on the moving mirror.
Locally, the resulting state still looks like a single photon on one side of the mirror and vacuum on the other, with the complex multi-photon behavior confined to a narrow transition region.
Published in Physical Review Letters, the work extends earlier reports that mirror removal during reflection produces a highly nonclassical state and suggests vacuum energy can be converted into real photon energy.