## There are many ways to spin a photon: Half-quantization of a total optical angular momentum

• K. E. Ballantine, J. F. Donegan, P. R. Eastham
• Science Advances, April 2016, American Association for the Advancement of Science

# There are many ways to spin a photon

One of the characteristics of a beam of light is its angular momentum, or how much it twists. Angular momentum is carried by individual photons (particles of light), so when light from the mirror hits your eye in the morning, every photon twists it a little. Until now, it was thought that in all forms of light angular momentum would be a multiple of Planck's constant, the physical constant that sets the scale of quantum effects. We've shown that, in certain cases, the angular momentum of each photon takes only half of this value.

### Why is it important?

Theoretical physicists since the 1980s have speculated how quantum mechanics works for particles which 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. Our work shows, for the first time, that some of these ideas can be realised with light.

### Perspectives

##### Dr Paul R Eastham
University of Dublin Trinity College

Our discovery used an effect discovered in the same institution, Trinity College Dublin, almost two hundred years before. In the 1830s mathematician William Rowan Hamilton and physicist Humphrey Lloyd discovered that, on passing through certain crystals, a ray of light became a hollow cylinder. We used this "conical refraction" to generate beams with a screw-like structure. When we analysed these beams within the theory of quantum mechanics we found that an angular momentum of the photon would be a half-integer. Using a specially constructed device (an interferometer) we measured the flow of angular momentum in a beam of light, and also the variations in this flow caused by quantum effects. This revealed a tiny shift, one-half of Planck’s constant, in the angular momentum of each photon. 