Encoding many channels on the same frequency through radio vorticity: first experimental test

The first radio signal transmitted and received by Guglielmo Marconi on 8 December 1895
started the wireless communication revolution [1]. Now information is mostly exchanged
through wireless channels and the rapid increase of the use of mobile devices has led to
congestion in the available radio bands even after the application of dense coding and channel
sharing techniques [2]. Therefore, it is important to try to develop new methods that make it
possible to utilize the electromagnetic (EM) spectrum better.

One way is to exploit fundamental physical properties of the EM field that hitherto
have not been utilized in radio communications. To this end, we recall that the EM field
can carry both energy and momentum. Whereas the Poynting vector S and the concomitant
linear momentum p = integral(d^3 * x *  S) (rational units) are associated with force action and therefore
with translational dynamics, the angular momentum J = integral(d^3 * x *  (x×S)) is a conserved physical
observable (constant of motion) that is associated with torque action and thus with rotational
dynamics [3]. In a beam geometry as used in radio communications, the total angular
momentum can be conveniently expressed as the sum of two components: J = +L. The
component represents the spin angular momentum (SAM), related to the polarization of
the individual EM waves of the beam and thus with photon helicity. The component L
represents the lesser-known orbital angular momentum (OAM) associated with the helicoidal
phase profile of the EM beam in the direction orthogonal to the propagation axis. In a quantum
picture L can be described as a superposition of discrete photon quantum eigenstates, each
with a well-defined OAM value  0,±1,±2, . . . [4–7]. Hence, not only in mechanics
but also in electromagnetism, OAM is a fundamental physical quantity that spans an infinite
state space [8]. It offers, in addition to the conventional translational linear momentum and
polarization (SAM) rotational degrees of freedom, which spans only a two-dimensional (2D)
state space, additional rotational degrees of freedom that are distinctly different from SAM.
Without increasing the frequency bandwidth, the OAM states can be used as a new, very large
set of communication channels that are mutually orthogonal to each other in the OAM state
space.

Here we report the results of real-world, outdoor radio experiments in the 2.4 GHz WiFi
band that demonstrate the feasibility of increasing the wireless information transfer capacity
over large distances by exploiting the OAM states [8] of EM waves. Our findings extend
previous indoor laboratory test experiments in which the transmission of optical OAM states of coherent laser [9] and radio [10] beams was demonstrated. The results reported here show that
OAM and vorticity are preserved throughout the long-distance propagation over long distances
and can indeed be utilized in radio communication.

Unlike already existing radio communication protocols that use the spatial phase distribution
generated by a set of antennae to artificially increase the transmission bandwidth, the
immediate advantage provided by a protocol based on the physical OAM states as independent
communication channels is that of using the peculiar spatial phase distribution of each of these
states as a reference pattern to generate, modulate and detect them in a better way.
OAM has found practical applications in many other fields such as radar [11],
nanotechnology [12], quantum experiments [13] and also astronomy and space sciences
[14–18], improving the resolving power of diffraction-limited optical instruments [19] and
facilitating the detection of extrasolar planets [20] and Kerr black holes [21].

New Journal of Physics 14 (2012) 033001 (http://www.njp.org/)