Researchers at UCCS have helped achieve an important experimental breakthrough: the first direct observation of the spontaneous macroscopic coherence of magnons – the quantized excitations of magnetic materials. Direct observation of a defining property of magnon Bose–Einstein condensation advances the understanding of collective quantum states and may support future information-processing and sensing technologies.
The result experimentally confirms a central prediction of the theory of magnon Bose-Einstein condensation. It demonstrates that a dense gas of magnons can spontaneously organize into a collective quantum state with a well-defined macroscopic phase, independent of the external signal used to create the magnons. Direct observation of a defining property of magnon Bose–Einstein condensation advances the understanding of collective quantum states and may support future information-processing and sensing technologies.
The study, published in “Nature Physics,” was conducted by an international team involving UCCS, RPTU University of Kaiserslautern-Landau and Fraunhofer Institute for Industrial Mathematics ITWM.
Most people are familiar with the three classical states of matter: solid, liquid and gas. Quantum mechanics, however, allows additional states, including the Bose-Einstein condensate, or BEC. A characteristic feature of a BEC is that a large number of particles or quasiparticles cease to behave independently and instead occupy a single collective quantum state.
Bose-Einstein condensates were first observed in ultracold atomic gases at temperatures close to absolute zero. Magnons are fundamentally different because they are quasiparticles associated with collective oscillations of electron spins in a magnetic material. Under suitable conditions, magnons can form a condensate even at room temperature. The existence of room-temperature magnon condensates has been established for approximately two decades. However, one of their most fundamental predicted properties – the spontaneous formation of a macroscopic phase that is not inherited from the excitation source had not previously been observed directly. The new study provides this missing experimental evidence.
“This experiment allows us to observe a magnon gas transforming from an initially incoherent state into a coherent macroscopic quantum state,” said Dmytro Bozhko, Ph.D., associate professor in the UCCS Department of Physics and Energy Science and a co-author of the study. “Most importantly, the condensate selects its phase spontaneously. This is a defining characteristic of Bose-Einstein condensation and an example of spontaneous symmetry breaking in a solid-state system.”
The research team used phase-resolved microwave spectroscopy to follow the evolution of the magnon system after it was driven far from equilibrium. Short, intense microwave pulses generated a dense gas of magnons. After the microwave excitation was removed, interactions among the magnons redistributed their energy. Within a fraction of a microsecond, a large population accumulated near the lowest accessible energy state and formed a magnon Bose-Einstein condensate. The phase-sensitive measurement technique allowed the researchers to distinguish the condensate from the microwave signal that initially generated the magnons. Repeated experiments showed that the condensate phase changed randomly from one realization to another rather than remaining fixed relative to the excitation. This random phase selection provides direct evidence that coherence emerged spontaneously within the magnon system.
“The process can be thought of as a crowd beginning a stadium chant – many independent voices spontaneously settle into the same rhythm without a conductor,” said Bozhko, “in the magnon system, however, this synchronization results from a nonequilibrium quantum phase transition.”
The study complements UCCS research focused on the dynamics, coherence, transport, and exceptionally long lifetimes of magnons. These properties are central to the emerging field of quantum magnonics, which investigates how collective spin excitations can be used to transmit, process, and store quantum information.
Significant challenges remain before magnon condensates can be incorporated into practical technologies. In particular, researchers must extend the lifetime of the condensate, improve control over its phase and spatial distribution, and develop reliable methods for coupling condensates to electronic, microwave, optical, and quantum devices. UCCS researchers from the Microwave Magnonics Group are actively working on addressing these broader challenges. The direct observation of spontaneous magnon coherence represents an important step in that effort. It confirms a long-standing prediction of Bose-Einstein condensation theory while providing a new experimental foundation for studying collective quantum behavior in magnetic materials at room temperature.
This research, as well as Dr. Bozhko’s broader research program in quantum magnonics, is supported by the U.S. National Science Foundation (NSF) through NSF CAREER Award, “Fundamental Phenomena in Magnon Condensates” (Award No. DMR-2338060).
Read the “Emergence of phase coherence in a magnon Bose-Einstein condensate” article online.