Watching an optical vortex reverse its spin

Watching an optical vortex reverse its spin. Vortices occur in whirlpools, tornadoes, Bose-Einstein condensates (BECs), and many other systems. In an optical beam, a vortex is a spiral phase ramp—like the thread of a screw—circulating around a dark spot in the beam where the phase is undefined and the intensity vanishes. It is generally accepted that, once created, a vortex cannot reverse its direction of rotation without first being destroyed. Researchers have built devices to reverse optical vortices, but were unable to watch the reversal itself. Now a Barcelona-Tucson collaboration has observed in detail such a reversal in an optical vortex that freely propagated in vacuum. The key to both reversing and observing the spiral staircase of phase was giving it some intrinsic spatial structure within the beam: The researchers passed the specially prepared laser beam through a cylindrical lens and monitored its interference with a reference beam as it propagated. They clearly observed clockwise rotation of the phase beyond the lens. But just after the focal plane, the screwlike discontinuity collapsed to a line discontinuity then reemerged with a counterclockwise rotation. A spherical lens did not generate such a reversal. The scientists also confirmed that the beam’s angular momentum was conserved throughout the experiment. They see some implications of their work for quantum entanglement and teleportation, and for elucidating vortex behavior in BECs. (G. Molina-Terriza et al., Phys. Rev. Lett. 87 , 023902, 2001 http://dx.doi.org/10.1103/PhysRevLett.87.023902 .)