Photons behave very strangely if you try to cut them

In Greek mythology, cutting one head off the Hydra of Lerna simply resulted in two more heads growing to replace it – and it turns out it’s even worse for photons. If you try to cut a piece off a particle of light, the result is infinitely many more of them being created.

Some particles are elementary, which means that they cannot be broken into smaller pieces. For instance, a proton can be torn into three quarks, but each quark cannot be subdivided further. But what would happen if you tried to cut an elementary particle anyway?

Johannes Skaar at the University of Oslo in Norway and his colleagues have looked into the case of a photon encountering a mirror that could do just this.

Because it is quantum, light can be described both as made from photons and as being an electromagnetic wave. Accordingly, a photon isn’t perfectly localised, like a solid object, but rather has a tail that extends across space. In the proposed scenario, the mirror would be able to move fast enough to just reflect part of the photon, equivalent to snipping off its tail.

Using quantum equations for the electromagnetic field, the team uncovered that this snipping would create a quantum state of light that is a mix, or superposition, of infinitely many photons. This happens because, at the quantum level, empty space isn’t empty, but rather filled with quantum fields, such as the electromagnetic field, all of which have tiny fluctuations and can be excited into producing particles. The mirror trimming the photon triggers such a process.

“Whenever you change a mirror or a shutter quickly, you stir up the vacuum and conjure photons out of empty space,” says Samuel Braunstein at York University in the UK. But any local measurements – observations made from nearby – would find the superposition state to be indistinguishable from a single photon on one side of the mirror and an empty vacuum on the other, underlining how differently the idea of observation works in the quantum realm compared with our everyday experience. It shows how in quantum theory “a fearsomely complicated object can masquerade as something utterly simple”, he says.

Ulf Leonhardt at the Weizmann Institute of Science in Israel says that experiments have found that a sufficiently fast shutter operating in empty space does really create photons, but experimentally testing the new idea may be more technically challenging. Manipulating light on ultrafast timescales is increasingly becoming an experimental reality, but the shutter from the new study is still faster than what is available in labs, he says. The new work also points to phenomena arising from the quantum vacuum as having to be explored further, possibly leading to refining or amending quantum field theories of electromagnetism, says Leonhardt.

In addition to being interested in questions of locality in quantum theory, which can relate to even bigger ideas such as how causality works in experiments with quantum particles, Skaar and his colleagues now want to extend their analysis to more than one photon at a time or other particles like electrons.