Featured Article 10 May 2016

The discovery by Prof Paul Eastham from Trinity College Dublin’s School of Physics and Prof John Donegan from the SFI-backed CRANN research centre could have a major impact in terms of fibre-optic communications.

One of the measurable characteristics of light is known as angular momentum.

Up until now, in all forms of light, the angular momentum was thought to be multiples of Planck’s constant – the physical constant that sets the scale of quantum effects.

But the professors were able to discover a form of light where the angular momentum could be a fraction rather than a multiple.

Their findings were published in the online journal Science Advances.

Donegan said that his research focuses on the study of light at nanometre scale.

“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.

“Our discovery will have real impacts for the study of light waves in areas such as secure optical communications.”

Let there be light, and there was a fraction of light

In order to make this discovery, the team involved used an effect discovered in the same institution almost 200 years ago.

In the 1830s, mathematician William Rowan Hamilton and physicist Humphrey Lloyd (both Irish) found that, upon passing through certain crystals, a ray of light became a hollow cylinder.

The team used this phenomenon to generate beams of light with a screw-like structure.

Analysing these beams within the theory of quantum mechanics they predicted that the angular momentum of the photon would be half-integer, and devised an experiment to test their prediction.

Using a specially-constructed device, they were able to measure the flow of angular momentum in a beam of light. They were also able, for the first time, to measure the variations in this flow caused by quantum effects.

The experiments revealed a tiny shift, one-half of Planck’s constant, in the angular momentum of each photon.

“We’re interested in finding out how we can change the way light behaves, and how that could be useful,” explained Eastham.

“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.”

Breakthrough for the world of physics

Theoretical physicists since the 1980s have speculated how quantum mechanics works for particles that are free to move in only two of the three dimensions of space.

They discovered that this would enable strange new possibilities, including particles whose quantum numbers were fractions of those expected. This work shows, for the first time, that these speculations can be realised with light.

“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 Prof Stefano Sanvito, director of CRANN.

“This discovery is a breakthrough for the world of physics and science alike. I am delighted to once again see CRANN and physics in TCD producing fundamental scientific research that challenges our understanding of light.”

Light image via Shutterstock

John Kennedy

This article was originally published on www.siliconrepublic.com and can be found at:


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