Physicists have just discovered a new form of light that doesn’t follow our existing rules of angular momentum, and it could shake up our understanding of the electromagnetic radiation and lead to faster, more secure optical communication.
Because of how well-studied and, well, everywhere, light is, you might assume that we’ve pretty much learnt all there is to know about it. But just last year, researchers identified a fundamental new property of light, and now a team of Irish scientists has shown that light can take on unexpected new forms.
One of the ways we measure a beam of light is through its angular momentum – a constant quantity that measures how much light is rotating. And until now, it was thought that for all forms of light, the angular momentum would be a whole number (known as an integer) multiple of Planck’s constant – a physical constant that sets the scale of quantum effects.
But researchers led by Trinity College Dublin have now demonstrated that a new form of light exists, where the angular momentum is only half of this value.
“What I think is so exciting about this result is that even this fundamental property of light, that physicists have always thought was fixed, can be changed,” said lead researcher Paul Eastham.
Let’s back up for a second here and explain what all that means.
As one of the researchers, Kyle Ballantine, explains:
“A beam of light is characterised by its colour or wavelength and a less familiar quantity known as angular momentum. Angular momentum measures how much something is rotating. For a beam of light, although travelling in a straight line, it can also be rotating around its own axis. So when light from the mirror hits your eye in the morning, every photon twists your eye a little, one way or another.”
As mind-bending as that might sound, it’s all well understood by physicists. But what they didn’t realise was that light could exist that had an angular momentum that wasn’t a whole number.
To figure this out, the team passed light through crystals to create beams of light that had a twisted, screw-like structure. They were looking for new light behaviours that might improve optical communications, but when they analysed this particular beam within the theory of quantum mechanics, it looked as though its angular moment would be a half-number – which definitely wasn’t what they’d expected to find.
They then came up with an experiment to test this prediction, and were able to construct a device that measured the flow of angular momentum within the light beam, as well as the variation in this flow caused by quantum effects.
Usually any of those quantum variations would cause the angular moment to change by whole numbers, based on our understanding of physics so far. But the experiments revealed a tiny shift – one-half of Planck’s constant – in the angular momentum of each photon.
That’s really exciting, not only because it’s a brand new form of light, but because since the 1980s, theoretical physics have predicted that quantum mechanics would enable the possibility of particles whose quantum numbers were fractions of those expected. And now, for the first time, this work proves those predictions right, using one our best-studied particles.
“The topic of light has always been one of interest to physicists, while also being documented as one of the areas of physics that is best understood,” said one of the researchers, Stefano Sanvito. “This discovery is a breakthrough for the world of physics and science alike.”
The biggest impact, other than shaking up our understanding of light, is that this new information could help to improve speed and security along fibre-optic cables, leading to faster, safer internet connections.
But before we get anywhere close to benefitting from this new form of light, another team of researchers will need to replicate and validate this work to make sure it wasn’t just a one-off. Science is often a slow process, but there’s no denying its an exciting one.
The results have been published in Science Advances.