Twist light, carry terabits of data

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Achieved 2.56Tbps transfers over the air using light split into four beams.

Microscopically, it probably doesn't appear like this. But you get the idea—light, twisted.

The limit of communication using light isn't its speed, but the number of ways of encoding and decoding information using photons. Today's fiber optic communications rely on light of different wavelengths to carry data from multiple sources carried along the same cable: each distinct signal is carried at a separate wavelength, which are then separated at the receiving end. A second method uses the polarization of light, which relies on two distinct ways photons can spin around the direction of motion. Sending two signals that differ only by polarization doubles the rate of data transfer.

A third method involves manipulating the shape of the photons themselves: twisting the light's wavefront into a ring, so that the photons rotate rather than merely spin around the direction of motion. The size of the ring is independent both of the wavelength of the light and the polarization. In principle at least, it has no limit to the number of possible configurations. Researchers Jian Wang et al. achieved terabit (Tbit/s) data transfer rates over open-air distances of about a meter, through combining four beams of different rotations (channels). They demonstrated both the generation and receiving of distinct data from the different channels (multiplexing and demultiplexing), using a laser beam of a single wavelength to generate all of them. While this result is not currently usable in fiber optics, it's a significant step toward exploiting a thus-far unused property of light to increase the rate of telecommunication.

Electrons in atoms have three distinct quantum properties: their energy level, their orbital angular momentum, and their spin angular momentum. Angular momentum in the macroscopic world can be thought of as how difficult it is to stop a spinning object. Microscopically, it dictates the kinds of transitions that are possible between different quantum states within the atom. This gives rise to the unique spectrum different elements and molecules possess. Orbital angular momentum, as the name suggests, depends on the orbit of the electron, but spin is a property of the electron alone.

Similarly, photons have distinct quantum properties, though they cannot be trapped in the same way as electrons. They possess energy (determined by their wavelength), polarization (which is light's version of spin), and orbital angular momentum (OAM). Photon OAM is a rotation of the photon around its direction of motion; think of it as traveling in a helical pattern, akin to a spiral staircase. The amount of OAM is independent of polarization and energy, and—unlike polarization, which has only two possible states—theoretically can have an infinite number of possible values. Additionally, by combining two or more OAM states in a single photon, many possible rotation configurations can be created (including rather cool spiral patterns, which the authors showed in their paper).

Light can carry information in the form of orbital angular momentum (OAM), where the photons twist in a spiral pattern. That opens up a large number of new channels for data transfer.

The researchers started with laser light of a specific wavelength, then split it into four beams, each carrying four different bits of information. Each beam was reflected off a different phase mask, a special liquid crystal whose structure produces the helical wavefront. The particular filters made four concentric rings of wavefronts. Each of these beams was then passed through a polarizing beamsplitter (PBS), resulting finally in eight separate channels (beams of light carrying a unique set of bits). These were then combined and sent across a distance of about one meter to a receiver.

At the receiving end, the total beam was passed through polarizing filters to select each of the two polarizations. The resulting light was reflected from a second set of phase masks; a match between the particular helical wave front and the phase mask changed the light from a circular pattern back to the original unshaped wave. They achieved as high as 2.56Tbit/s data transfer.

Additionally, the researchers exchanged data between beams by reflecting both off a phase mask unmatched to either. The reflection swapped the OAM values between the beams. This allows for flexible data transfer and processing, possibly for quantum computing applications.

While this was an excellent proof-of-principle experiment, it will be a while before we'll see OAM-based telecommunication. For starters (as both the authors and accompanying commentary point out), optical communication through air is prone to many problems as the light scatters off air molecules. (However, communication through vacuum, as between spacecraft, is still a possibility, at least over distances where dispersion isn't a major issue). One meter is an insignificant distance for most communication purposes, so optical signals are generally passed through fiber optic cables. However, standard fiber optic cables are not capable of carrying this kind of multi-channel signal, while fibers that can handle it experience problems of cross-contamination between channels when carrying data under high bit-rates.

Nevertheless, opening up the OAM degree of freedom available in photons will possibly lead to significant increases in data transfer, since technical challenges often lead to novel technical solutions. While the "infinite" number of available OAM channels doesn't necessarily even mean thousands in practice, even a modest increase over the existing transfer rate that can be carried by a single frequency of light is noteworthy.

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