Structured Light Could Increase Information Capacity

Researchers from the University of Witswatersrand (Wits) have reported on progress made in the use of structured light in quantum protocols to create a larger encoding alphabet. The researchers said that since patterns of light can be distinguished from each other, they can be used as a form of alphabet. “Light comes in a variety of patterns that can be made unique — like our faces,” professor Andrew Forbes said. “There are, in principle at least, an infinite set of patterns, so an infinite alphabet is available.”

Quantum protocols have traditionally been implemented using the polarization of light, which has only two values and can provide only a two-level system with a maximum information capacity per photon of 1 bit. Spatial modes of light — that is, patterns of light — offer a route to high-dimensional Hilbert spaces for larger encoding alphabets, promising higher information capacity per photon. Also, security would be stronger, and the robustness to noise, such as background light fluctuations, would be improved.

Researchers from the University of Witwatersrand in South Africa review the progress being made in the use of 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. Courtesy of Andrew Forbes and Issac Nape.

“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.”

The researchers reported on advances in the use of orbital angular momentum as well as vectorial states that are hybrid entangled, combining spatial modes with polarization. While scientists have pushed the boundaries in both the dimension and the photon number, full control of multiple photons entangled in high dimensions remains a challenge. “We know how to create and detect photons entangled in patterns, 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,” Forbes said. “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 for quantum communications. “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.”

The research was published in AVS Quantum Science (https://doi.org/10.1116/1.5112027).