'Twisted' Waves Could Boost Capacity of Wireless Spectrum

New submitter Ogi_UnixNut writes "In Venice, Italy, physicists have shown that it is possible to use two beams of incoherent radio waves, transmitted on the same frequency but encoded in two different orbital angular momentum states, to simultaneously transmit two independent radio channels. In principle this allows the implementation of an infinite number of channels in a given, fixed bandwidth, even without using polarization, multiport or dense coding techniques. It's potentially a boon for congested spectrum problems, although at the moment I suspect it would only work for directional links."

Comments

Re:So what are they orbiting then?

[tibit]

The notion of "what are they orbiting" is nonsensical here -- we're talking about quantum objects. It's like saying that electrons "orbit" the nucleus: in the description of their motion, the concept of a classical "path" doesn't quite apply either, and classical mechanics can't describe what an electron does when bound to the nucleus! Now, Maxwell's theory is "classical" in a way, but it describes AFAIK an aggregate (macroscopic) behavior of inherently non-classical, quantum objects, the photons. To get the behavior at the quantum level right, you need quantum electrodynamics (QED).

It is well known from Maxwell's theory that electromagnetic radiation carries both energy and momentum. The momentum may have both linear and angular contributions; angular momentum has a spin part associated with polarization and an orbital part associated with spatial distribution

- from "Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes" by Allen et al. In the same paper, you can read that you can measure those properties of light using fairly simple opto-mechanical instruments:

A suspended lambda/2 birefringent plate undergoes torque in transforming right-handed into left-handed circularly polarized light. Suspended cylindrical lenses undergo torque in transforming a Laguerre-Gaussian mode of orbital angular momentum -l*hbar per photon, into one with +I*hbar per photon.