Physicists tend to think of light in terms of photons: massless, elementary particles âcarryingâ the forces of nature. And elementary particles, like the photon, are indivisible. That is, there is no such thing as half a photon. Or is there?

That was the question that three theoretical physicists tried to answer in a recent paper published in Physical Review Letters, bluntly titled “Truncated Photon.â To be clear, the team understands that elementary particles like photons canât be cut into two. But consider this quantum shortcut: photons exhibit the behaviors of both waves and particles. As it turns out, there is a way to split a photon using this propertyâalthough, in typical quantum weirdness, the product of this isnât anything youâd expect.

âDespite being a simple question, it appears that it has not been asked before,â the physicists, from the University of Oslo in Norway, wrote in the paper.

Some primers

The dual nature of light as both a particle and a wave was most famously demonstrated by British physicist Thomas Young in 1801. The double-slit experiment, as it came to be called, showed how, when you shine a beam of light (âparticlesâ) through two parallel slits on a screen, youâd get an interference pattern resembling the union of two ripples (âwavesâ) on the other side.

In the quantum realm, objects can also exist in superposition. This is the phenomenon that Schrödinger’s Cat thought experiment attempted to demonstrate. A cat with a sealed box containing a poisonous substance could be dead or alive. But itâs theoretically bothâin a superposition of statesâuntil we observe the cat by opening the box. In other words, the act of observation causes a quantum system to probabilistically fall into one state or the other.

Now we truncate

The latest study builds upon these quantum principles. As a wave packet, a photon would possess some spatial distribution. Using an optical shutterâfast-moving mirrors that block or release light pulsesâit might be possible to effectively separate sections of this wave. Specifically, the shutter would transform the photon into a superposition of modes either traveling right or left, according to the paper.

Using quantum field theory, the researchers calculated how this setup would affect the dynamics of a photonâs waveform. Surprisingly, their results werenât that we now had two photons, or even a photon and a vacuum. Rather, what they got was a âcomplicated state involving photon numbers up to infinity.â Basically, âcutting away a part of the photonâ resulted in a âbunch of new photons,â they wrote.

In an accompanying Synopsis commentary, the team explained that, in removing the mirror, a âtug on the quantum fieldâ pulls out enough photons from the nearby vacuum. This forms a âsharp edgeâ consisting of increasingly more superposed photons, until you get a quantum state of infinite photons.

But wait, it gets weirder

Hereâs the cherry on top. If you tried to measure the states on either side of the split wave packet, youâd get measurements that look âexactly like a single-photon stateâ on the left and a vacuum on the right, except for a very narrow transition region, according to the paper. So, down on the quantum level, thereâs a constant, infinite flow of photons. But if you tried to measure it, youâd see either one photon or nothing at all. (Yeah. Welcome to the quantum world.)

âThe truncated photon state is thus an example of a very complicated state that produces the exact same measurement statistics as very simple states, as long as one is interested only in local observables to the left or right of the transition region,â the researchers added in the study.

Again, the study is entirely theoretical; no actual photons were harmed during the course of this project. However, the exercise demonstrates a previously unexplored behavior of key concepts in quantum field theory. The experimental design isnât too complicated, so it wouldnât be impossible for experimentalists to replicate the calculations in real life.