Information transfer with electromagnetic (EM) fields is possible due to the fact these fields transport physical observables from a source to a (possibly very) remote observer in the form of volumetric densities. Therefore, it is only after these densities have been integrated over a finite volume that they appear as observables and can be used for extracting the information transferred. Consequently, an observer’s measurements are always nonlocal averages and cannot provide exact values of the fields themselves at a given point in space and time. The EM field always carries the densities in question, but it is in the measurement process that the integrated densities become observable, making them usable for wireless information transfer. As a consequence, different sensors should be used for different observables.
In the case of EM field energy, it is the energy density of the EM field that is integrated into energy, as in radiometry and simple on-off signalling, used in early-day radio communications.
In present-day radio and optics, it is the linear momentum density (Poynting vector) that is integrated; in the case of radio this is typically done with a linear dipole antenna that senses the linear momentum transfer from the field to the charge carriers in the form of an antenna current, which is a weighted average of the current density along the antenna.
Lately, the techniques of using EM energy and/or linear momentum observables for information transfer have been augmented by the use of the angular momentum observable. This technique is rooted in the electromagnetic angular (rotational) momentum physical layer. A physical observable in its own right, albeit much more sparingly used than as the EM energy and linear momentum, the EM angular momentum exploits the rotational symmetry of the electromagnetic field and the rotational (spinning and orbiting) dynamics of the pertinent charge and current densities. Just as for the energy and linear momentum densities, the volume integrated angular momentum density, i.e. the total angular momentum emitted by any source, always tends asymptotically to a constant when the distance from the source volume tends to infinity.
Clearly, a sensor that integrates the angular momentum density into an observable, must ideally be two- or three-dimensional. We present results from recent experiments that demonstrate this fact.
A scenario with angular momentum transducers of new types, including optomechancial ones and those based on the interplay between charge, spin and orbital angular degrees of freedom, heralded by recent advances in nano-mechanics, spintronics/orbitronics, condensed-matter skyrmion, and quantum ring physics and technology, will be briefly delineated.
Swedish Institute of Space Physics, Uppsala, Sweden
Acreo Swedish ICT AB, Kista, Stockholm, Sweden
Coffee and sweets will be served!
Link for webcast: https://connect.sunet.se/fridayseminar/