Laser pulses and neutron beams have revealed a quantum state in a magnetic material that violates the familiar detailed balance rule and persists at low energy instead of rapidly disappearing.
Researchers from the U.S. Department of Energy's Oak Ridge National Laboratory (ORNL), together with colleagues, conducted an experiment on a material in which quantum quasiparticles — magnons — are responsible for the magnetic properties. They used fast laser pulses to drive the system out of equilibrium and then applied neutron scattering at the Spallation Neutron Source at ORNL to study the microscopic picture in detail. The results, published in Nature Communications in November 2026, showed a violation of detailed balance: in the ordinary state, energy is absorbed and emitted symmetrically, but under laser excitation the neutron signals became asymmetric, with an excess in only one direction.
The most unexpected thing was that this state turned out to be long-lived. The excess of magnons did not disappear quickly, as with brief heating, but was sustained by periodic laser pulses, forming a stable nonequilibrium state. Imagine a flow of particles that, instead of turning back, keeps moving in one direction, like a one-way street where the motion is not balanced by an opposing flow.
The team led by Chengyun Hua, including Barry Winn and Alan Tennant, spent years developing special infrastructure for such measurements. Previously, no one had probed the violation of detailed balance directly in a condensed medium, and the experiment confirmed theoretical predictions in practice.
This discovery creates a platform for studying quantum systems far from equilibrium in real time. It helps to understand how to control magnetic materials for quantum technologies, reducing information loss during transmission and improving memory devices. The method also sets a new standard for neutron scattering under active excitation of a material.
The results, published in Nature Communications, provide an experimental basis for future studies of quantum materials and their applications in information technologies.
