Radio vortices

Quantum mechanics, no one will be surprised to hear, is weird. In particular, photons can carry angular momentum - circularly polarized light can set objects spinning. But it turns out that light can carry orbital angular momentum as well. It's really not very clear to me quite what this means in terms of photons. In terms of classical waves, it's weird but I think I get it: if you look at the spatial distribution of the light in a beam, you may find that the phase is constant across the whole beam. But you might also find that the phase varies. Now, it has to be continuous, but you can imagine that as you make a circle around the beam center, you find the phase increases by an integer multiple of two pi. This gives you a continuous phase in a way that is topologically different from the constant-phase situation. As I understand it, this is what is called wrapping number.

Now this would just be another weirdness from the world of (classical!) waves except that there seem to be applications for it. In particular there's a paper on the arxiv about using this for communications purposes.

Encoding many channels in the same frequency through radio vorticity: first experimental test
We have shown experimentally that it is possible to propagate and use the properties of twisted non-monochromatic incoherent radio waves to simultaneously transmit to infinity more radio channels on the same frequency band by encoding them in different orbital angular momentum states. This novel radio technique allows the implementation of, at least in principle, an infinite number of channels on one and the same frequency, even without using polarization or dense coding techniques. An optimal combination of all these physical properties and techniques represents a solution for the problem of radio band congestion. Our experimental findings show that the vorticity of each twisted electromagnetic wave is preserved after the propagation, paving the way for entirely new paradigms in radio communication protocols.
These authors point out that even in the far field, on a radio beam the different angular momentum modes are orthogonal, so that they can in principle be used to encode independent signals. That makes a certain sense, but I am left wondering how one is to produce and detect such signals. For transmission, they "mechanically modified" a parabolic reflector. As shown above, the mechanical modification is jaw-droppingly simple: they cut it and bent it into a spiral. I'm still trying to wrap my brain around why this works.

Detection, it seems to me, poses a greater challenge. The modes with nonzero angular momentum have a zero in intensity at the beam center (necessarily, to allow the topology to work out). As I understand it, then, in order to measure these modes, you must have receivers distributed over a substantial fraction of the beam width. This poses a substantial problem even for point-to-point microwave links - after all if you covered the entire beamwidth with receivers you'd lose almost no power, so the ratio of transmitted power to received power tells us just how much smaller our receivers are than the beams. I don't even understand what this would mean for omnidirectional patterns like those used in almost all mobile applications.

Still, the authors think this is great, even arranging for a massive Marconi-style public demonstration at the Palazzo Ducale in Venice. They attracted more than two thousand spectators, but the cynic in me has to wonder how many of the spectators had even the foggiest notion of what was being demonstrated.

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