In 2023, the Nobel Prize in Physics celebrated research on quantum entanglement, a phenomenon once called “spooky action at a distance” by Einstein, where two particles remain perfectly connected, no matter how far apart they are. But entanglement isn’t the only kind of quantum connection. There’s another, quieter link called quantum discord, and it could be just as important for powering future quantum technologies.
Until now, discord had been measured almost exclusively between two particles. But real-world quantum systems, from computing to cryptography, often involve many particles interacting at once. Measuring discord in such multi-particle systems has been a long-standing challenge.
NYU Shanghai Associate Professor of Physics Tim Byrnes and Visiting Associate Professor of Practice in Computer Science Chandrashekar Radhakrishnan proposed a mathematical method with their team to measure quantum discord in any number of particles back in 2020, something existing approaches struggle to do. Now, in a new study published in PNAS, Byrnes’ team has partnered with researchers at the Indian Institute of Science Education and Research Mohali (IISER Mohali) to put the method to the test.
Working with IISER Mohali Professor of Physics Kavita Dorai, the team carried out the first experimental measurement of tripartite (three-particle) quantum discord. “The experimental technique we used is called Nuclear Magnetic Resonance (NMR), which is a cost-effective yet powerful way to manipulate and measure quantum states,” explained Radhakrishnan. “Professor Dorai is a pioneer in using NMR for quantum computing, so this collaboration was a natural fit.”
Using NMR, the researchers prepared three quantum bits (qubits) in different configurations, some highly entangled, others less so, and measured the quantum discord present in each. The results matched the 2020 theoretical predictions, confirming that the new method works in practice.
“Back then, what we had was just a nice mathematical formula,” said Radhakrishnan. “Now we’ve shown it holds up in real experiments.”
Before this work, Radhakrishnan said, measuring many-particle quantum correlations, including entanglement, was either extremely difficult or impractical. “Our method now gives us a general way to measure discord for any number of particles,” he said.
That’s a big step forward for quantum information science, says Byrnes. “Quantum cryptography, quantum computing, quantum metrology — all these fields rely on quantum correlations,” he explained. “Discord can be a valuable resource, even when entanglement is absent. Measurement is the starting point for any application, and now that we can measure it, we’re much closer to putting it to use.”
The team hopes their work will pave the way for future experiments measuring discord in larger systems, and eventually for real-world applications where discord can play a key role.