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Twisted Light Characterization Technique Seeks to Advance Quantum Communications

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Fully understanding “twisted” light may enable its use in quantum communication applications, and an experimental technique has yielded the first characterization of the azimuthal Wigner distribution of a photon.

Twisted light has raised interest as a potential enabler of quantum communications. The discrete nature of orbital angular momentum, or OAM, one of the defining parameters of twisted light, makes it attractive for encoding quantum information. There is also no known fundamental limit to the maximum OAM value that can be coded into a photon, which could allow for quicker communication than with other systems.

To characterize twisted light the researchers looked at the images produced by the interference of a structured laser beam with a replica of that beam rotated by a given angle, including this "Pac-Man." Courtesy of Mohammad Mirhosseini.

Researchers at the University of Rochester have reported a protocol to fully characterize the transverse structure of a photon in conjugate bases of OAM and azimuthal angle. The team tested the protocol by characterizing pure superpositions and incoherent mixtures of OAM modes in a seven-dimensional space.

The Wigner distribution is a mathematical construct that completely describes a system in terms of two conjugate variables, that is, two variables linked by Heisenberg's Uncertainty Principle. Postdoctoral associate Mohammad Mirhosseini and collaborators at Rochester’s Institute of Optics have now shown how the Wigner distribution can be obtained for twisted light.

Other methods to obtain the wave function, a property that describes a quantum system in full — such as quantum tomography or direct measurements — have been demonstrated in the past. However, in their experiment, the Rochester team demonstrated that the time required for performing measurements scaled only linearly with the dimension size of the state under investigation, making the new technique suitable for quantum information applications involving a large number of OAM states.

"Apart from the potential uses in quantum communication, our work might offer a good way for describing atomic systems with quantized levels," said Mirhosseini. "The Wigner distribution of twisted light is a very complete way to understand the system. Not only does it tell us about the relation between these two linked variables, but it also tells us about the system's behavior. We showed that the Wigner distribution for twisted light superpositions contains negative values, which reveals wave-like behavior."

Mirhosseini thinks their work could also show a possible path forward for other experiments.

"Measuring time in quantum systems is not as simple as using a watch — it can prove challenging," said Mirhosseini. "The conjugate variable of OAM, angle, is in many ways similar to phase, which is itself similar to time. So perhaps the lessons learned here can be applied in other experiments to systems where we need to measure time."

The research was published in Physical Review Letters

Photonics Spectra
Jun 2016
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
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