Angular momentum radio

Wireless communication amounts to encoding information onto physicalobservables carried by electromagnetic (EM) fields, radiating them intosurrounding space, and detecting them remotely by an appropriate sensorconnected to an informationdecoding receiver. Each observable is secondorder in the fields and fulfills a conservation law. In present-day radio only the EM linear momentum observable is fully exploited. Afundamental physical limitation of this observable, which represents thetranslational degrees of freedom of the charges (typically anoscillating current along a linear antenna) and the fields, is that itis single-mode. This means that a linear-momentum radio communicationlink comprising one transmitting and one receiving antenna, known as asingle-input-single-output (SISO) link, can provide only onetransmission channel per frequency (and polarization). In contrast,angular momentum, which represents the rotational degrees of freedom, ismulti-mode, allowing an angular-momentum SISO link to accommodate anarbitrary number of independent transmission channels on one and thesame frequency (and polarization). We describe the physical propertiesof EM angular momentum and how they can be exploited, discuss real-worldexperiments, and outline how the capacity of angular momentum links maybe further enhanced by employing multi-port techniques, i.e., theangular momentum counterpart of linear-momentummultiple-input-multiple-output (MIMO).