Opt. Express 20, 2110–2115 (2012)

Optical computing involves encoding data into the physical properties of light beams or even individual photons. Orbital angular momentum is a particularly appealing degree of freedom for this task because, unlike polarization, it can take on a wide range of values — but conventional detectors are not sensitive to this property. Martin Lavery and colleagues have now shown how transforming the optical beam offers an indirect method for detecting its orbital angular momentum.

Diffraction-based detection methods can test whether a photon has a specific orbital angular momentum — which is not that useful if you don't know what value to check. Lavery et al. instead demonstrate a technique that uses refractive optical elements. Their set-up transforms an incoming angular momentum state into a complementary transverse momentum state. The output beam is then focused by a lens onto a camera — a specific spot on the camera corresponds to a particular initial orbital angular momentum. Thus, the method checks all possible values simultaneously.

The transmission efficiency of the combined refractive elements is 85% — high enough for use at low light levels. Indeed, the team have also demonstrated that the technique works for single photons by replacing the conventional camera with a sensitive CCD (charge-coupled device) camera.