Structured Light for Faster Communications

Structured light is a fancy way to describe patterns or pictures of light, but deservedly so as it promises future communi­cations that will be both faster and more secure. Quantum mechanics has come a long way during the past 100 years but still has a long way to go. Now, researchers from the Uni­versity of Witwaters­rand in South Africa review the progress being made in using structured light in quantum protocols to create a larger encoding alphabet, stronger security and better resis­tance to noise.

Using structured light in quantum protocols to create a larger encoding alphabet, stronger security and better resistance to noise: This image shows the creation of hybrid entangled photons by combining polarization with a twisted pattern that carries orbital angular momentum. (Source: Forbes & Nape)

“What we really want is to do quantum mechanics with patterns of light,” said Andrew Forbes. “By this, we mean that light comes in a variety of patterns that can be made unique – like our faces.” Since patterns of light can be distin­guished from each other, they can be used as a form of alphabet. “The cool thing is that there are, in principle at least, an infinite set of patterns, so an infinite alphabet is available,” he said.

Tradi­tionally, quantum protocols have been implemented with the polari­zation of light, which has only two values – a two-level system with a maximum infor­mation capacity per photon of just 1 bit. But by using patterns of light as the alphabet, the infor­mation capacity is much higher. Also, its security is stronger, and the robust­ness to noise is improved. “Patterns of light are a route to what we term high-dimen­sional states,” Forbes said. “They’re high dimen­sional, because many patterns are involved in the quantum process. Unfor­tunately, the toolkit to manage these patterns is still under­developed and requires a lot of work.”

The quantum science community has made many recent noteworthy advances, both in the science and derived tech­nologies. For example, entanglement swapping has now been demons­trated with spatial modes of light, a core ingredient in a quantum repeater, while the means to securely communicate between nodes is now possible through high-dimen­sional quantum key distri­bution protocols. Together they bring us a little bit closer to a fast and secure quantum network.

In a similar vein, the construc­tion of exotic multi­party high-dimen­sional states for quantum computer has been realized, as has enhanced reso­lution in ghost imaging – produced by combining light from two light detectors. Yet it remains chal­lenging to break beyond the ubiquitous two photons in two dimensions for full control of multiple photons entangled in high dimensions. “We know how to create and detect photons entangled in patterns,” said Forbes. “But we don’t really have good control on getting them from one point to another, because they distort in the atmo­sphere and in optical fiber. And we don’t really know how to efficiently extract infor­mation from them. It requires too many measure­ments at the moment.”

Forbes and his co-author Isaac Nape helped pioneer the use of hybrid states. Old textbook quantum mechanics was done with polari­zation. “It turns out that many protocols can be effi­ciently imple­mented with simpler tools by combining patterns with polari­zation for the best of both worlds,” Forbes said. “Rather than two dimen­sions of patterns, hybrid states allow access to multi­dimensional states, for example, an infinite set of two-dimen­sional systems. This looks like a promising way forward to truly realize a quantum network based on patterns of light.” (Source: AIP)

Reference: A. Forbes & I. Nape: Quantum mechanics with patterns of light: Progress in high dimensional and multidimensional entanglement with structured light, AVS Quantum Sci. 1, 011701 (2019); DOI: 10.1116/1.5112027

Link: School of Physics, University of the Witwatersrand, Wits, South Africa

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