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UTILISATION OF ELECTROMAGNETIC ANGULAR MOMENTUM - THE OPENING OF A NEW RADIO FRONTIER

The electromagnetic (EM) field exhibits a large set of symmetries, as does a mechanical system such as a fluid, gas or plasma. To each Lie symmetry of the combined electromechanical system, there is an associated conserved physical quantity. Perhaps the most commonly known conserved physical quantity is energy. The assicuated Lie symmetry is homogeneity in time. Another conserved quantity is linear momentum, which is a manifestation of homogeneity in space. A third conserved quantity, manifesting isotropy in space, is the total angular momentum, J, of the electromechanical system.

The EM angular momentum can often be separated into two parts, the spin angular momentum part (wave polarisation), S, and the orbital angular momentum part (beam vorticity/field inhomogeneity), L, an EM degree of freedom that has been neglected for a long time but which is now coming into the fore.

Because of the conservation law J = L + S = Const., EM fields generated in -- or in any other way interacting with -- a mechanical system such as a fluid, gas or plasma will carry information on the vorticity of the mechanical system and vice versa. We describe how L can be measured at radio frequencies and utilised as a remote diagnostic of vorticity and associated turbulence and how it opens for new methods of wireless communication, including information-rich quantum communications.