According to Science Alert, one of the measurable characteristics of a beam of light is known as angular momentum. Until recently, it was believed that the angular momentum of photons, which are the particles that are assumed to make up light, would always be a multiple of Planck's constant - a number that sets the scale of quantum effects.

Recent PhD graduate Kyle Ballantine and Professor Paul Eastham, both from Trinity College Dublin School of Physics, along with Professor John Donegan from CRANN, have demonstrated a new form of light where the angular momentum of each photon is only half of Planck's value.

Ballantine explains, "A beam of light is characterized by its color 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."

What is remarkable about this discovery is that the angular momentum in this new light is not a whole number. The team passed light through special crystals to create beams that had a screw-like structure, and then measured the variations in the flow of angular moment resulting from quantum effects. Each photon had gained a shift equal to one-half of Planck's constant in angular momentum.

This discovery will have significant impacts for the study of light waves in areas such as secure optical communications, and more importantly, also proves theories from the 1980's that quantum mechanics allows for particles with quantum numbers which are a fraction of what was predicted.

The results have been published in Science Advances as K. E. Ballantine et al. *There are many ways to spin a photon: Half-quantization of a total optical angular momentum,* Science Advances (2016). DOI: 10.1126/sciadv.1501748 .