Quantum Multiplexing with the Orbital Angular Momentum of light

Quantum Multiplexing with the Orbital Angular Momentum of light
Authors: Juan Carlos Garcia-Escartin, Pedro Chamorro-Posada
(Submitted on 29 Jan 2009)

http://arxiv.org/abs/0901.4740

Abstract: The orbital angular momentum, OAM, of photons offers a suitable support to carry the quantum data of multiple users. We present two novel optical setups that send the information of n quantum communication parties through the same free-space optical link. Those qubits can be sent simultaneously and share path, wavelength and polarization without interference, increasing the communication capacity of the system. The first solution, a qubit combiner, merges n channels into the same link, which transmits n independent photons. The second solution, the OAM multiplexer, uses CNOT gates to transfer the information of n optical channels to a single photon. Additional applications of the multiplexer circuits, such as quantum arithmetic, as well as connections to OAM sorting are discussed.

Twisted radio beams could untangle the airwaves

http://www.newscientist.com/article/dn16591-twisted-radio-beams-could-untangle-the-airwaves.html

The human race is not only exhausting tangible resources such as oil. The radiofrequency spectrum available for wireless communication is becoming the increasingly crowded, with virgin "veins" of frequency running short.

However, Swedish physicists say that twisting radio beams into a helical shape as they are transmitted could help ease the congestion.

Radio frequency encompasses electromagnetic waves between 3 kilohertz and 300 gigahertz, and as wireless communications technology advances much of that range is being used.

Satellite TV, wireless computer networks and cellphones are among the growing technologies vying for space up to 30 gigahertz, with some technology even beginning to extend beyond 100 gigahertz leaving a dwindling supply of virgin terrain to exploit.

Physicist Thomas Leyser at the Swedish Institute of Space Physics in Uppsala, Sweden, thinks he has a novel solution. Along with an international team of physicists, he has demonstrated that it is possible to put a spin on radio beams during their transmission to produce a twisted beam.

"Twisted laser beams have been researched since the 1990s, but it has only now become possible to create twisted beams at the much lower radio frequencies," he says.

Detecting orbital angular momentum in radio signals

A good overview of some of the technical challenges of detecting RD signals with OAM

http://www.slac.stanford.edu/spires/find/hep/www?irn=7762801

Electromagnetic waves with an azimuthal phase shift are known to have a well defined orbital angular momentum. Different methods that allow for the detection of the angular momentum are proposed. For some, we discuss the required experimental setup and explore the range of applicability.

Good overview of diagnostic opportunities of OAM

http://www.aip.org/pnu/2005/split/721-3.html

Optical Vortices" Might Extract Abundant Information From Matter

"Optical vortices" might extract abundant information from matter, providing a new and potentially wide-ranging optical tool, a Spain-US team has proposed theoretically. An ordinary light beam, when viewed head-on, looks like a bright circle. But a special light beam called an "optical vortex," when viewed head-on, looks like a bright ring surrounding a dark central core (see www.aip.org/png/2001/133.htm). Optical vortices are the simplest kind of beam carrying a property called "orbital angular momentum" (see Update 639).

Extensively studied since the early 1990s, such light beams, when viewed from the side, trace out a three-dimensional corkscrew pattern (see figure at www.aip.org/png/2005/229.htm); the pattern represents regions of constant phase (for example, regions of maximum electric field). This spiraling of light represents an extra "degree of freedom" that researchers can use as a new handle to optically encode information and subsequently to retrieve information from objects the beam strikes. In conventional laser beams, the energy flows parallel to the beam axis, like water in a jet.

However, for light with orbital angular momentum (OAM), the energy spirals around the beam axis. Ordinary beams carry only "spin angular momentum," encoded in the polarization of light. All possible spin states can be constructed with just two polarization states (vertical and horizontal, or clockwise and counterclockwise). For light with nonzero OAM, however, many states are possible, with higher states denoting tighter corkscrews (and consequently, a faster spiraling of energy; see figure at www.aip.org/png/2005/229.htm). For this reason, one can encode a huge amount of information in an OAM beam by creating light made of a superposition of many OAM states.

The researchers call the different OAM components "spiral spectra." In the "digital spiral imaging" concept now put forward by Lluis Torner at the new Institute for Photonic Sciences (ICFO) in Barcelona and his colleagues, a light beam of a convenient shape illuminates a sample to be probed. The sample scatters the beam and alters its spiral components. Breaking down the altered beam into its individual orbital-angular momentum components (and thereby analyzing the “spiral spectrum” of the scattered beam) can yield a wealth of information from the object.

The spiral spectra would, for example, be sensitive to nonuniformities in geometrical and structural properties of objects, and could be potentially useful for detecting biological and chemical agents, for probing biological specimens sensitive to OAM light, and might even aide recent proposals to increase the amount of data that can be imprinted on a compact disk using OAM. (Torner, Torres, Carrasco, Optics Express, Feb. 7, 2005; contact Lluis Torner, http://www.icfo.es ; for more background on OAM light, see Physics Today, May 2004, and New Scientist, 12 June 2004).

OAM in RF applications & measuring properties of objects

For the longest time OAM was associated with optical photonic applications. I have suspected for some time it could be used in RF frequency bands as well. A recent paper discusses this:

"Utilization of Photon OAM in Low Frequency Radio Domain", B. Thide

Even more exciting is the use of RF OAM to probe properties of objects:

Recent digital spiral imaging experiments (Torner et al., Opt.
Express, 13, 873–881, 2005; Molina-Terriza et al., J. Eur. Opt.
Soc., Rapid Publ., 2, 07014, 2007) have demonstrated that
probing with OAM gives a wealth of new information about the
object under study.