A fascinating phenomenon of modern physics is the Bose-Einstein condensation (BEC). Such a condensate provides us with a novel form of quantum matter and offers us an exciting perspective for the exploitation of macroscopic quantum properties. It describes an extreme physical state of particles, the bosons, which are inherently indistinguishable. All bosons in a condensate are in the same quantum mechanical state. This puzzling property was originally predicted in 1924 by Albert Einstein based on a theory for an ideal gas developed by Satyendra Nath Bose. The most well-known phenomena associated with the BEC today are superconductivity, which allows electrical currents to flow without resistance, and superfluidity, which allows liquids to flow with a viscosity of zero. Bosons can be electrons and photons, but also quasi-particles, which are the quantized excitations of many-particle systems. We study specifically BEC of magnons, which are the quasiparticles of spin waves in a magnetic system.

The generation of a Bose-Einstein condensate is a tricky task, since this process must by definition be spontaneous. Creating the conditions for the formation of a condensate means not creating any order or coherence that will excite the particles to behave in the same way - the particles must do so independently of each other. 

We succeeded in a new and very practicable way how to create a BEC, and that is via a rapid cooling process. 

Usually Bose-Einstein condensates are created by lowering the temperature to near absolute zero or by injecting a large number of particles into a small volume at room temperature. However, the new rapid cooling process is much simpler. It works very well for magnonic nanostructures. All one needs is the magnon system and a convenient heat source, such as an electric-current carrying wire integrated into the nanostructure. 

This work was a joint effort between the team of Andrii Chumak and my team. Initially, Andrii was an Assistant Professor in my group, and this work was completed after he joined the University of Vienna as a Full Professor. Andrii is an expert in magnon physics in magnetic nanostructures, including the important technological aspects of how such nanostructures can be made. His work was largely supported by a starting grant from the European Research Council (ERC Starting Grant MagnonCircuits). My own team studies transport phenomena in magnonic Bose-Einstein condensates (we are fortunate to have received funding from an ERC Advanced Grant), so it was very beneficial to combine our expertise. 

Originally, Andrii and his doctoral student Michael Schneider wanted to study another aspect of magnonic nanocircuits, which are subject to a temperature gradient, when strange things happened. “At first we thought that there was really something wrong with our experiment or data analysis," explained Michael. “Then we realized that a magnonic BEC could have been generated.” 

So, what exactly have we done? We reduced the temperature of a magnetic nano-structure on a very rapid time scale, much faster than any internal relaxation time. Such a nano-structure can be considered as a reservoir for magnons as well as for phonons – another type of bosons existing in every solid and associated with vibrations of its crystal lattice. In a first step, the we heated the nano-structure up to 200 °C to create a large number of phonons. Since phonons and magnons constantly interact with each other, the system rather soon came into equilibrium exhibiting also a large number of magnons. Next, the nano-structure was rapidly cooled down to room temperature. The phonons could escape into the colder substrate, while the number of magnons remained confined to the nanostructure. As a result, the magnonic system became over-populated and formed a spontaneous Bose-Einstein condensation of the magnons into the same quantum state. Particularly the nanoscopic size of the investigated structure, which was around hundred times smaller than the thickness of a human hair, allowed for the realization of the fast enough cooling. 

This novel method for generating a Magnon-Bose-Einstein condensate now paves the way for the use of macroscopic quantum magnon states in conventional spin electronics and on-chip solid state quantum computation. The ultimate goal of this field of research is to combine modern semiconductor processors with quantum computer units that allow specialized tasks of high complexity to be solved.