Linear momentum the next step in studies of light.

Researchers from Trinity College Dublin have discovered a mode of light propagation that has never been observed, potentially reopening an area of research that was closed.

Light doesn’t only carry energy but it also carries linear momentum, which can be harnessed to propel solar sails forward, and angular momentum, which means that a beam of light can rotate along its own axis.

The rotation is not visible to the naked eye, but it can be detected based on how rotating light interacts with matter. In three-dimensions, the value for the angular momentum is always a multiple of a fundamental constant of the universe, known as the Planck’s constant. But things are different in other dimensions.

In a paper published in Science, the team showed that in two-dimensions, the components of the angular momentum can have fractional values, being a half-integer multiple of the Planck constant. This demonstration is the first to show this property for photons, although it has long been established that new quantum states can form when electrons are placed in reduced dimensions.

"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," said Assistant Professor Paul Eastham, senior author of the paper, in a statement.

The physicists were able to construct the new quantum state, by having a beam of light passing through a specific crystal that constrained the photons to the surface of a cylinder. The team was then able then to measure the full angular momentum and detected a signal in the fluctuations of its components that corresponded to a fractional value.

The finding could have applications in optical communications, but most importantly shows us that even light is still full of mysteries.