Lasers, neutrons reveal long-lived quantum state

Lasers, neutrons reveal long-lived quantum state

A laser-driven magnetic state breaks physical rules of matter, toward quantum advancements

Researchers at the Department of Energy’s (DOE’s) Oak Ridge National Laboratory (ORNL) and their collaborators used lasers and neutrons to observe a long-lived quantum state in a magnetic material, opening a new way to study materials systems driven far from their equilibrium. Manipulation of quantum magnets is important for advancing quantum information science and could reduce loss of transported information, improve memory devices and benefit other quantum technologies.

The team studied magnons, quantum quasiparticles whose wave-like excitations are critical to magnetic behavior in materials. Magnons can carry information, and quantum magnets can support long lifetimes and memory for data transfer. The team’s findings, published in Nature Communications, show how a laser can push a magnetic system out of its normal state, how neutrons can reveal the details of that state, and how the new state can persist at low energy instead of transiently fading away.

In the experiment, rapid laser pulses drove the material out of its normal resting state, and ORNL’s Spallation Neutron Source, a DOE Office of Science user facility, let the team study the microscopic details of the magnetic behavior inside the material — a unique combined approach not typically used for nonequilibrium studies.

"Because we were taking a new approach, mechanics, safety personnel, hardware and software engineers, and the research team all had to think outside the box to make this project work," said ORNL’s Chengyun Hua.

And it did work, with surprising results: The measurements showed that the material violated a physical rule of matter called “detailed balance.” In a system at rest, the detailed balance rule links energy-gaining and energy-losing events in a predictable way. Under the rapid laser pulses, that balance broke. The neutron data showed extra signals only on one side.

Perhaps more surprisingly, the nonequilibrium state did not quickly disappear. The lopsided excess magnons persisted, indicating that the researchers were not seeing a brief heating effect. Instead, they were seeing a long-lived nonequilibrium steady state sustained by periodic laser pulsing.

Findings create platform to study other nonequilibrium quantum states

"No one before has directly probed a detailed balance violation in a condensed matter system," said Hua. "Theoretically it makes sense it can happen, but experimentally it was pretty awesome to see."

The observation of this rule-breaking behavior makes the system a strong platform for studying other nonequilibrium quantum physics in real time. The insights it could generate when used on various materials may help guide future work on quantum materials, coherent transport and low-loss electronics. The study also gives scientists a new benchmark for what neutron scattering can reveal when materials are observed while they are actively being driven instead of sitting at rest.

Furthermore, the success of the methodology is also exciting for the team that first envisioned the approach — Hua, Barry Winn, and Alan Tennant. Their work reflects a sustained, multi-year effort at ORNL to develop the specialized infrastructure required for such complex measurements. The team's early explorations were enabled by an ORNL seed grant, which provided the initial platform for high-risk, high-reward nonequilibrium physics.

The study’s result also points toward a broader quantum computing future. For ORNL researchers, the study helps build an experimental foundation for understanding new quantum materials and for exploring how such materials may eventually be studied with quantum computers as well as used in future information technologies.

Inelastic neutron scattering, simulation, data analysis and modeling were supported by the DOE Office of Science, Basic Energy Sciences (Materials Sciences and Engineering Division). Additional theory work was supported by the Quantum Science Center, a DOE National Quantum Information Science Research Center.

UT-Battelle manages ORNL for DOE’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time.  For more information, please visit energy.gov/science.  – Emily Tomlin