L. Allen et al., Phys. Rev. A (1992) A figure from the paper by Allen et al., showing the torque a Laguerre-Gaussian mode of light would impart to cylindrical lenses.

L. Allen et al., Phys. Rev. A (1992) A figure from the paper by Allen et al., showing the torque a Laguerre-Gaussian mode of light would impart to cylindrical lenses. ×

It started as “just a fun idea,” says Robert Spreeuw of the University of Amsterdam. Thirty years ago, he was kicking around Leiden University, Netherlands, after receiving his Ph.D., when he had a thought: Could a beam of light with a spiraling wave front carry orbital angular momentum (OAM) like a pinwheel? In principle, light with OAM could make another object spin.

During a coffee break, he discussed the idea with Les Allen, a visiting scientist at Leiden. In their 1992 paper, Allen, Spreeuw, and their coauthors considered Laguerre-Gaussian modes, which are donut-shaped modes produced with lasers. The phase of a donut mode spirals around the propagation axis like a corkscrew, and the researchers determined the OAM in beams with varying amounts of spiraling. They also suggested that spiraling light might induce a measurable twist to a suspended cylindrical lens upon passing through it. That test proved problematic. But another group later detected the light’s OAM using small particles held by optical tweezers.

Unlike the circular polarization of a photon, the OAM is described by more than a rotational direction: it can have a value that is any integer multiple of ℏ , which is why physicists have explored spiral modes as a compact means of carrying a lot of data. These modes also enable new tests of quantum mechanics and new ways to manipulate small objects. “I could never have suspected that [this idea] would grow so big,” says Spreeuw. “There were so many applications that surprised me.”

In her 2012 Ph.D. thesis, Jacquiline Romero describes one of those applications, which is the ability to explore entanglement in more “dimensions” than would be possible in a simple two-state system. Now at the University of Queensland in Australia, Romero says she was intrigued by the “neat family” of shapes that light can have. “I think of these shapes as an alphabet for encoding information. The number of letters in that alphabet is theoretically infinite,” she explains. OAM thus provides a unique playground for exploring quantum information phenomena. For her, the 1992 paper is both a landmark reference and a reminder of Les Allen, who passed away in 2016. “He was such a magnanimous character.”

L. Allen et al., “Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes,” Phys. Rev. A 45, 8185 (1992).