Photons, in the form of optical pulses, radio waves, and the like, have been used to transmit data for decades. The next phase will probably involve using the quantum states of the photons themselves to carry information.
In terms of these quantum states, photons possess two distinct polarization orientations, along with a theoretically infinite number of helical wave forms, in which the photons rotate around the direction they're moving. The latter have garnered a lot of interest, as they could potentially carry a lot more data than other optical methods. Possible applications include quantum computing, improved fiber optic communication, point-to-point data transfer across free space, and microscopy.
Researchers have now developed a way to produce twisted light beams using silicon chips, the starting point for compact, efficient optical communication. Xinlun Cai and colleagues shaped photons using a microscopic, ring-shaped grating, which sent twisted light out in a specified pattern. Each ring was small enough to be fabricated into integrated circuits, and capable of emitting multiple vortexes of light simultaneously. The same type of chip could also serve as a receiver for twisted light, and manipulate waves that transit through it.
Photons possess a number of quantum properties that can be used to encode information. You can think of photon polarization as like the rotation of a planet on its axis. In this view, the helical shape of the light wave—known as its orbital angular momentum (OAM)—is akin to the planet's orbit around the Sun. These properties are independent of each other, and of the wavelength of light, so they can be manipulated separately. Whereas polarization occurs as a combination of two possible orientations, the OAM theoretically can have infinite values, though in practice far fewer states are available. Nevertheless, exploiting OAM greatly expands the potentially exploitable quantum states of photons we could put to use.
The researchers created helical OAM states using a ring-shaped chamber fabricated from silicon and mounted on a chip. They used infrared laser light as the input, fed in through a waveguide—a chamber that shapes the wave in a controlled fashion. The smallest of the ring chambers was 7.8 micrometers (7.8 μm, or 0.0078 millimeters) in diameter, comparable to the 1.5 μm wavelength of the light. The interior of the chamber was scored with grooves that diffract the light, causing interference. Because of the chamber's shape, however, the interference reshaped the light into a helix, and sent it "upward", perpendicular to the ring. The image below shows the spiral patterns produced from opposite polarization of the original light.
The experimenters then combined light from multiple rings, which resulted in additional patterns. Such combinations allow for many additional wave shapes that can be used in data transmission. However, reshaping light also has applications in optical "tweezers," where light is used to manipulate other quantum systems. The clean control provided by the ring chambers could potentially lead to many other applications in quantum computing control and communication.
The researchers did not test data transfer in this paper, though they suggested ways it could be done. A major problem facing OAM communication includes loss of the coherent photon state through scattering off air molecules or absorption in fiber optic cables. Either of these processes can change the waveform, defeating the purpose of the entire exercise.
Nevertheless, this is an exciting step toward OAM-based devices. The researchers showed how simple microscale ring-shaped chambers fabricated from silicon could produce any number of possible helical waveforms, opening up many possible channels for future investigation.
Science, 2012. DOI: 10.1126/science.1226528 (About DOIs).
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