Quantum symmetry breaking demonstrated for the first time

Quantum symmetry breaking has been demonstrated in the lab for the first time — with startling implications for the ability to better control quantum systems.

The figure describes the dynamics of two spins as a harmonious couple-dance. Different from a solo-dance of a single spin, the couple-dance would present more unique and charming features, such as parity-time symmetry breaking demonstrated in the work. (Guoyan Wang & Lei Chen)

For the first time, researchers have observed a break in a single quantum system. The observation — and the technique used to observe it— has implications for the understanding of how quantum particles interact to produce matter and allow the world to function as we know it — and the physics that may lie beyond it.

Parity-Time (PT) Symmetry describes the properties of a quantum system — the evolution of time for a quantum particle, if the particle is even or odd and whether it moves forward or backward in time, the state of oddness or evenness remains the same in the balanced system. When this parity changes, the balance — or symmetry — of the system breaks.

In order to better understand quantum interactions and develop next-generation devices, researchers need to develop the ability to control the symmetry of systems. If they can break it — they could also manipulate the spin state of the quantum particles as they interact. Thus resulting in controlled and predicted outcomes.

Yang Wu, an author on the paper and a PhD student in the Hefei National Laboratory for Physical Sciences at the Microscale and Department of Modern Physics at the University of Science and Technology of China: “Our work is about that quantum control.”

A new method for controlling quantum systems

Wu and his PhD supervisor Rong and colleagues used a nitrogen-vacancy centre in a diamond as their platform. Here a nitrogen atom with an extra electron is surrounded by carbon atoms, which creates the perfect capsule to further investigate the PT symmetry of the electron.

The electron is a single-spin system, meaning the researchers can manipulate the entire system just by changing the evolution of the electron spin state.

A schematic representation of a nitrogen-vacancy centre in a diamond (Haque, Sumiaya, 2017)

Using what Wu and Rong label a dilation method — the researchers applied a magnetic field to the axis of the nitrogen-vacancy centre, thus pulling the electron into a state of excitability. They then applied oscillating microwave pulses, changing the parity and time direction of the system and causing it to break and decay with time.

Wu explains: “Due to the universality of our dilation method and the high controllability of our platform, this work paves the way to study experimentally some new physical phenomena related to PT symmetry.

Jiangfeng Du, who is also an academician of the Chinese Academy of Sciences and corresponding author on the paper, concludes: “The information extracted from such dynamics extends and deepens the understanding of quantum physics.

“The work opens the door to the study of exotic physics with non-classical quantum systems.”

Original research: https://science.sciencemag.org/content/364/6443/878