# There are many ways to spin a photon

### What is it about?

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

http://dx.doi.org/10.1126/sciadv.1501748

The following have contributed to this page: Dr Paul R Eastham