Daniel C. McCarthy
For years, physicists have known that photons can exhibit orbital angular momentum. Spin manifests when a photon's transverse components are out of phase, whereas orbital angular momentum affects the entire phase front of the beam. The mechanical effects are visible in optically trapped specimens: Spin rotates trapped items on their own access axes, similar to the rotation of the Earth, where orbital angular momentum rotates them around the beam's axis, similar to the path of a satellite.
In theory, this quality could have big benefits for logic-based applications in communications and computing. Although a single photon can have only two spin states, it can incorporate multiple orbital-angular-momentum values and thereby encode more information. Until recently, established detection techniques could test individual photons for only one orbital-angular-momentum value at a time.
However, researchers at Glasgow and Strathclyde universities, both in Glasgow, UK, collaborated to break that barrier. Led by Miles Padgett and Johannes Courtial, the group demonstrated an interferometric technique that distinguishes individual photons in several arbitrary orbital-angular-momentum states.
In the experimental setup, input beams are routed according to their orbital-angular-momentum state with a theoretical efficiency of 100 percent.
It splits a HeNe laser beam in two using a Mach-Zehnder interferometer, and inserts a Dove prism into each arm. One prism is set on its side to rotate the beam 180°. Orbital angular momentum is measured in terms of hbar.
Where they converge at a beamsplitter, beams with even hbar values (i.e., 2 hbar, 4 hbar) interfere constructively with the rotated beam and exit one port. Beams with odd hbar values (i.e., 1 hbar, 3 hbar) interfere destructively and exit from another side of the beamsplitter cube.
By cascading interferometers, the researchers were able to subdivide photons further, sorting even-valued photons with multiples of 4 or 8, for example.
The number of output ports can be substantial, allowing single-photon detectors in each port to measure the orbital angular momentum of individual photons.
"It turns out the even ones are very easy to sort further," Courtial said. "We add one hbar to the odds to convert them back to even."
The next step, he continued, is to make the arrangement more compact. Because tolerances for the arm lengths are a tenth of the wavelength, a smaller arrangement will improve the stability of the prisms. Courtial speculated that each interferometer could be constructed from one block of glass or, alternatively, holograms could replace some functions.
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