Evolution of Room-Temperature Magnon Gas toward Coherent Bose–Einstein Condensate

Abstract

The appearance of spontaneous coherence is a fundamental feature of a Bose–Einstein condensate (BEC) and an essential requirement for possible applications of the condensates for classical and quantum computing. Using a magnon BEC in a magnetic crystal, such computing can be performed even at room temperature. Using specially shaped bulk samples of yttrium iron garnet crystal, we developed a novel efficient method of BEC investigation by direct detection of microwave radiation emitted by parametrically driven magnon gas (Fig.1a-b).By using this method, we studied the process of coherence formation in a magnon BEC and demonstrated that no coherent BEC state could be found during the action of a strong magnetic pumping field. At the same time, after the pumping is terminated, the overpopulated magnon gas evolves toward BEC and reaches full coherence, with the width of the magnon radiation spectrum decreasing by more than two orders of magnitude (Fig. 1c). The residual bandwidth is mainly determined by the lifetime of magnons, as expected for a fully coherent BEC consisting of a single magnon state.This direct demonstration of the magnon BEC coherence [1] brings the implementation of room temperature BEC-based computing closer. Moreover, the magnon BEC coupling with dynamic stray fields outside the sample is enabled by a proper choice of the sample shape giving direct spectroscopic access to the BEC. Such an approach can function as a convenient tool for integrating magnetic quantum systems into electrical environments.  (a) Experimental setup for microwave detection of magnon BEC dynamics. (b) Bulk BEC mode and one of the edge magnon modes in a cuboid YIG sample. The monotonic blue line schematically shows the profile of the static magnetic field H within the YIG sample. Letters denote three field values: A – deeply inside the sample, B – at the point where the bulk BEC mode becomes evanescent, C – at the sample edge. (c) Magnon dispersion curves in the middle of the sample (at point A) and near the edge (at point B). The green, blue, and red signal intensity lines show the microwave power spectra registered during 1 µs interval before the end of pumping as well as 2 µs and 4 µs after the pump pulse is turned off.