Re: OT: Distinguishable angular momentum in radio photons?

Posted by Jussi Eloranta on May 04, 2012; 7:02pm
URL: http://elecraft.85.s1.nabble.com/OT-Distinguishable-angular-momentum-in-radio-photons-tp7525974p7528328.html

On 05/03/2012 10:10 PM, Erik Basilier wrote:

> The May 2012 issue of the IEEE Spectrum magazine reports that researchers in
> Italy and Sweden were able to conduct two separate instances of radio
> communication on the exact same frequency, without increasing bandwidth, and
> without time-division multiplexing, be making the transmissions differ in
> angular momentum. One transmission used linear polarization, and the other
> was given angular momentum by means of a dish with a radial cut, where the
> metal  was bent backwards/forwards on the two sides of the cut. Apparently
> this is not just a case of linear vs circular polarization, as circular
> polarization can be readily picked up by a linearly polarized antenna, and
> apparently the two channels did not interfere with each other. The
> researchers claim that this demonstration points to the possibility that the
> radio photons can be given multiple, quanized levels of angular momentum,
> making possible several more communication channels without increased
> bandwidth. Other researchers say that this is just a form of MIMO. Wikipedia
> describes MIMO as the technique of using multiple antennas as in diversity
> reception or in gain increases obtained by phasing the antennas.
>

The only source of angular momentum in a photon is its intrinsic spin (
= 1). This can have two projections, +1 and -1 (only two, unlike for
most spin one particles, which have three possible projections).
However, these two projections are directly related to the left and
right circularly polarized light, so I cannot quite see what other
source of angular momentum could arise in this case? Having said this, I
don't quite believe in the Swedish group explanation of this experiment.

Jussi Eloranta
AA6KJ

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