Light is a fundamental avenue of study in physics, and its properties are well established with steadfast rules and invariable constraints. So, until recently, we thought we knew just about everything there was to know about it. But now physicists from Trinity College Dublin have added a twist to the existing canon by demonstrating a new form of light with a total angular momentum that has a half-integer spin. In other words, light that does not obey the rules.
The rotation of a light beam around its axis is described by its angular momentum. That is, the amount of inertia and velocity of light as it moves forward, twisting as it does so. This angular momentum was always believed to be a set multiple of Planck's constant (a uniform ratio value describing the relationship between energy in one quantum (photon) of electromagnetic radiation to the frequency of that radiation), which meant that the sum total was always a whole number (or integer).
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"A beam of light is characterised by its color or wavelength and a less familiar quantity known as angular momentum, " said Professor John Donegan of the School of Physics at Trinity College. "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."
What the researchers discovered with their new form of light, however, was that its photons had an angular momentum just half the value of Planck's constant. Whilst this may not sound like a big deal, the finding was in fact particularly exciting because it verified some important quantum theoretical research first conducted back in the 1980s that predicted the possibility of light with half-integer spin.
To conduct their experiments, the scientists employed a phenomenon first discovered by two Trinity College researchers almost 200 years ago, where light shining through certain crystals transformed into a hollow cylinder. With a slight twist, the modern-day team used this technique to create light beams with a screwed shape by shining a laser beam first through a polarizer (an optical filter), then through the crystal, and on to be split inside an interferometer (an apparatus where wave interference is used to make precise measurements of displacement in wavelengths).
The split beam then had one half rotated in one arm of the interferometer before it was recombined with the beam that hadn't been rotated traveling through the other arm, and then the resulting output flow of angular momentum was measured. It was here that the researchers recorded variations in this flow induced by quantum effects. Expecting to see multiples of Planck's constant, the scientists instead discovered that they were witnessing photons with only half the angular momentum.
"We're interested in finding out how we can change the way light behaves, and how that could be useful," said Assistant Professor Paul 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."
The upshot of this research is that it may have applications in an optical version of so-called topological quantum computing (a theoretical computing method that employs two-dimensional quasiparticles), as well as a possible new way of using angular momentum to increase bandwidth in fiber-optic communications.
The results of this research were recently published in the journal Science Advances.
Source: Trinity College Dublin