Wits physics student helps develop method to quickly reveal structures of quantum-entangled states

31st August 2021

By: Schalk Burger

Creamer Media Senior Deputy Editor

     

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University of the Witwatersrand (Wits) PhD student Isaac Nape is part of a team of physicists who developed a new tool to reveal the hidden structures of quantum-entangled states in a study published in science journal Nature Communications on August 27.

Nape's PhD focuses on harnessing structured patterns of light for high dimensional information encoding and decoding for use in quantum communication.

Nape and his colleagues at Wits, including Wits School of Physics and Structured Light Laboratory director Professor Andrew Forbes and Wits postdoctoral fellow Dr Valeria Rodriguez-Fajardo, together with Taiwanese researcher Dr Hasiao-Chih Huang, and Dr Jonathan Leach and Dr Feng Zhu from Heriot-Watt University, in Scotland, offer a new and fast tool for measuring the dimensionality and purity of high-dimensional quantum states; relevant for quantum computing and communication.

“Quantum states that are entangled in many dimensions are key to our emerging quantum technologies, where more dimensions mean a higher quantum bandwidth, making it faster, and more resilient to noise, meaning better security, which are crucial for fast and secure communication and to speed up error-free quantum computing,” Nape explains.

“We invented a new approach to probing these so-called high-dimensional quantum states, reducing the measurement time from decades to minutes.”

In the 'Measuring dimensionality and purity of high-dimensional entangled states' study, the team outlined a new approach to quantum measurement, testing it on a 100 dimensional quantum entangled state.

With traditional approaches, the time of measurement increases unfavourably with dimension, so that to unravel a 100 dimensional state by a full quantum state tomography would take decades. Instead, the team showed that the salient information of the quantum system, or how many dimensions are entangled and to what level of purity, could be deduced in minutes.

The new approach requires only simple projections that could easily be done in most laboratories with conventional tools. Using light as an example, the team used an all-digital approach to perform the measurements, Wits says in a statement.

“Our work circumvented the problem by a chance discovery, namely that there is a set of measurements that is not a tomography and not a Bell measurement (a form of quantum information science measurement of quantum bits or qubits), but that holds important information of both,” says Nape.

“In technical parlance, we blended these two measurement approaches to do multiple projections that look like a tomography but measuring the visibilities of the outcome, as if they were Bell measurements. This revealed the hidden information that could be extracted from the strength of the quantum correlations across many dimensions.”

The combination of speed from the Bell-like approach and information from the tomography-like approach meant that key quantum parameters such as dimensionality and the purity of the quantum state could be determined quickly and quantitatively.

“We are not suggesting that our approach replace other techniques. Rather, we see it as a fast probe to reveal what you are dealing with, and then use this information to make an informed decision on what to do next,” says Forbes, who was the lead investigator on the study.

For example, the team sees its approach as changing the game in real-world quantum communication links, where a fast measurement of how noisy that quantum state has become and what this has done to the useful dimensions is crucial.

"While high-dimensional states are easily made, particularly with entangled particles of light, or photons, they are not easy to measure. Our toolbox for measuring and controlling them is almost empty," explains Nape.

A high-dimensional quantum state is analogous to the faces of a die. A conventional die has six faces, providing a six-dimensional alphabet that can be used for computing or for transferring information in communication. To make a high-dimensional die means having many more faces – 100 dimensions equals 100 faces.

“In our everyday world, it would be easy to count the faces to know what sort of resource we had available to us, but not so in the quantum world. In the quantum world, you can never see the whole die, so counting the faces is very difficult.

"The way we get around this is to do a tomography, as they do in the medical world, building up a picture from many, many slices of the object,” explains Nape.

Nape is an emerging South African talent in the study of quantum optics and won two awards this year at the South African Institute of Physics conference, and adding to his growing collection of accomplishments in the field of optics and photonics.

In May, he was also awarded the 2021 Optics and Photonics Education Scholarship from the international society for optics and photonics for his potential contributions to the field of optics, photonics or related fields.

Edited by Chanel de Bruyn
Creamer Media Senior Deputy Editor Online

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