July 14, 2026 -- Researchers at RPTU University Kaiserslautern-Landau have achieved a key experimental breakthrough: for the first time, the spontaneous macroscopic coherence of magnons - the quantized excitations of magnetic materials - has been directly observed. These experiments confirm a central prediction of the theory of magnon Bose-Einstein condensates. Eventually, these findings could open new avenues for signal processing, sensing technologies, and information processing. The study has been published in Nature Physics.

The three classical states of matter: solid, liquid, and gas are everyday phenomena. However, additional states exist, including plasma and the Bose-Einstein condensate (BEC). In a BEC, a large number of quantum particles no longer behave independently but instead collectively occupy a single macroscopic quantum state.

BECs were originally observed in ultracold atomic gases near absolute zero temperature. Twenty years ago, however, researchers demonstrated that a comparable phase transition can also occur in magnetic solids—and notably at room temperature. The corresponding study was carried out by the Department of Physics of TU Kaiserslautern (now RPTU Kaiserslautern-Landau), in collaboration with researchers from the Universities of Münster, Oakland, and Kyiv.

Although the existence of magnon BECs has been established for many years, direct experimental evidence for the spontaneous emergence of their macroscopic phase and its independence from external excitation signals had remained elusive. In the BEC state, magnons no longer behave as an incoherent ensemble but instead organize into a single collective quantum system characterized by a well-defined phase and frequency. Such spontaneous coherence is a defining property of all Bose-Einstein condensates. Until now, however, this hallmark feature had not been experimentally demonstrated for magnon BECs.

Direct Observation of Magnon BEC Using Phase-Resolved Microwave Spectroscopy

Researchers at RPTU have now provided this missing evidence using high-precision phase-resolved microwave spectroscopy. The measurements allowed them to directly observe both the emergence of coherence and the random seeding of the condensate phase in repeated experiments. “You can think of it as a noisy audio signal suddenly turning into a pure tone with a single well-defined frequency,” explains Professor Mathias Weiler, head of the working group Applied Spin Phenomena at RPTU. At that moment, the phase transition into a Bose-Einstein condensate takes place.

To observe the coherent state, the researchers needed a material in which magnons can persist for exceptionally long times. They therefore used yttrium iron garnet (YIG), a material known for exhibiting record-long magnon lifetimes. In the experiment, short and intense microwave pulses create a dense magnon gas. Subsequently, the magnons interact, gradually dissipate energy, and relax toward their lowest-energy state. Within only a few tenths of a microsecond, a large number of magnons accumulate in this state, forming a Bose-Einstein condensate.

“Our experiments provide the first direct evidence that magnons exhibit the defining property of a Bose-Einstein condensate,” says Professor Georg von Freymann, former head of the working group Optical Technologies and Photonics at RPTU, who is now based at Leibniz University Hannover. “This confirms a long-standing theoretical prediction.”

New Device Concepts for Signal Processing

From a fundamental perspective, the results represent an important step toward a deeper understanding of collective quantum states in condensed matter systems. At the same time, magnon BECs may ultimately enable new technological functionalities.

Particularly intriguing is the fact that, at room temperature, magnon condensates display phenomena reminiscent of superconductivity. In superconductors, electric charge can flow without resistance. In magnon Bose-Einstein condensates, by contrast, so-called spin supercurrents can emerge. In this case, spin - a fundamental quantum property of electrons – rather than charge is transported without dissipation.

This could pave the way for entirely new device concepts for analog signal processing. Potential applications include highly sensitive detectors for electric and magnetic fields as well as circuit architectures based on physical principles similar to those of Josephson junctions in superconducting systems. Today, Josephson junctions form the basis of numerous precision technologies and constitute a key building block of superconducting quantum computers.

Dr. Oleksandr Serga, co-author of the study and one of the researchers involved in the original discovery of the magnon BEC together with Professor Burkard Hillebrands, head of the working group Magnetism at RPTU, explains: “Compared with superconductivity, research on magnon Bose-Einstein condensates is still in its infancy. Superconductivity was discovered roughly one hundred years before the magnon BEC. While superconductivity experienced a major technological breakthrough with the development of robust high-temperature superconductors beginning in the 1980s, a key challenge for magnon BECs is maintaining the condensate state over sufficiently long timescales. At present, we achieve lifetimes in the order of microseconds, but we already have ideas for extending them significantly.

If this challenge can be overcome, magnon Bose-Einstein condensates could provide a new platform for energy-efficient information processing and ultrasensitive quantum sensing. The present demonstration of spontaneous coherence and the random formation of a macroscopic phase independent of the microwave excitation therefore marks an important milestone on this path.