Evolution of room-temperature magnon gas toward coherent Bose-Einstein condensate

Abstract

Nowadays a lot of effort is put into the creation of a practical quantum computer. For this ambitious aim different research fields propose and investigate new materials and technologies. One of such promising systems, a magnon Bose-Einstein condensate (BEC)in magnetic crystals, may allow the creation of magnon-BEC-based qubit at room temperature. A prerequisite for such a qubit is the appearance of spontaneous coherence in magnon BEC. Also, the presence of coherence of the magnon Bose-Einstein condensate is essential for understanding such fundamental properties of it as the possibility of a dissipative magnon supercurrent [1], excitation of Josephson oscillations [2], and propagation of Bogoliubov waves [3]. In most of the previous studies in this field, the magnon BEC is investigated by means of Brillouin light scattering (BLS) spectroscopy [4,5] delivering information about the spectral density of a magnon gas. Unfortunately, due to the limited frequency resolution of the optical Fabry-Pérot interferometers used in BLS facilities, the conventional BLS technique does not allow one to prove the coherence of a magnon BEC directly. The insufficient frequency resolution makes it impossible to separate the relaxation dynamics of condensed and thermal magnons. Moreover, the possible outflow of the condensate from a spatially localized probing light spot complicates the interpretation of the obtained experimental results (see [6] and the corresponding discussion in [7]). For all these reasons only indirect indications of the coherency, obtained on the magnon BEC in magnetic crystals, could be provided yet. Up to this day, a decisive proof of the full coherence magnon BEC remained elusive.In our work, we report a first direct experimental observation of the evolution of magnon BEC towards full coherence, whose degree is limited only by the natural processes of the magnon relaxation into the crystal lattice. To achieve this, we use a novel concept that enhances the coupling of the parametrically-driven magnon system with the probing electromagnetic field. The obtained electric signal was investigated time-resolved by a spectrum analyzer to reach the required frequency resolution necessary for the proper display of the magnon BEC state. A pulsed electromagnetic parametric pumping was used to inject magnons, increase, thus the chemical potential of the magnon gas, and trigger the BEC formation process. It is found that the radiation spectrum picked up after the terminization of a pump pulse is narrowed by two orders of magnitude, compared to the broad-band spectrum obtained during the process of parametric pumping. The resulting spectrum width is only limited by the magnon relaxation frequency. We study this spectrum evolution in many details and identify the conditions for BEC creation. We believe that this direct demonstration of the coherence of the magnon BEC brings the implementation of the room temperature BEC-based computing closer.This research was funded by the European Research Council within the Advanced Grant No. 694709 “SuperMagnonics” and by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) within the Transregional Collaborative Research Center – TRR 173 – 268565370 “Spin+X” (project B01). The authors are grateful to G. A. Melkov and H. Yu. MusiienkoShmarova for the fruitful discussions. **