How to cloak a crime in a beam of light: This approach relies on an unusual property of silica optical fibres: their refractive index, the measure of how fast light travels through the silica, changes with the brightness of the light. To open an event cloak in the stream of light passing down the fibre, a control laser injects an additional pulse of light. The increased brightness slows the light down, says team member Paul Kinsler.
As the brighter section moves along the fibre it falls progressively further behind the dimmer, faster-moving light ahead, creating a gap in illumination. The brighter laser pulse is then filtered out again, leaving all of the light with the same brightness and travelling at the same speed, and so maintaining the size of the gap.