In a newly published study, three physicists at the University of Oslo explored a fascinating quantum question: What happens if you “truncate” a photon? While an elementary particle cannot literally be cut in half, its light pulse can be partially blocked or reshaped, creating unusual quantum states. Their findings were published in the peer-reviewed journal Physical Review Letters.
The researchers explain that while a photon cannot be physically divided into two pieces, part of an optical pulse can be removed with an optical shutter, similar to briefly interrupting a stream of light. This leads to a surprisingly unexplored question: What quantum state remains after a photon is truncated?
Why Truncating a Photon Is So Complex
Optical pulses are brief laser bursts that let a few photons pass through. However, partially blocking them is complex because a single photon has “infinite tails” extending beyond its main wave packet.
The researchers discovered that truncating a photon does not simply leave behind a smaller photon. Instead, it produces a complex quantum state containing potentially an unlimited number of photons. Remarkably, even closing the optical shutter gradually leads to the creation of new photons in principle.
Like the mythical Hydra, truncating a photon does not simply reduce it—it can generate a swarm of new photons. Because a photon’s wave function extends infinitely, the Oslo team modeled truncation in an idealized quantum scenario.
A Simplified Model to Test Photon Truncation
Using a simplified theoretical model, the researchers reduced the system to one dimension and a single light polarization, then directed a single photon toward a mirror-like optical shutter that interrupted its path.
The result was striking. Rather than splitting the photon, truncation produced either a vacuum or a complete single photon. Neither portion retained evidence that it had once belonged to a larger wave packet.
The team emphasized that this outcome applies only to this highly idealized quantum setup, created specifically to investigate how photons behave under truncation. Even so, they believe the framework could help researchers better understand perturbed photon states and questions related to quantum causality.
Although many optical systems can be explained by classical physics, this study shows that cutting a single photon with a barrier can produce surprisingly complex quantum states, containing components corresponding to anything from zero to potentially infinitely many photons.
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