Quantum magnonics

The field of magnonics usually operates with coherent magnons at room temperature. The density of the so-called thermal magnons that are in equilibrium with a phononic bath of a solid body is around 1018 cm-3. Thus, the operations with single magnons are not possible without using cryogenic techniques. The most straightforward estimation using Bose-Einstein distribution shows that the thermal magnon population at 10 GHz and 100 mT is around 0.01, and nowadays mT temperatures are readily accessible with commercial dilution refrigerators. The field of quantum magnonics already took its first but essential steps, as our colleagues in this section describe it. Compared to the area of quantum optics, which is already a well-established field of modern physics, magnonics offers a set of unique features inherent also to a usual room-temperature magnonics: scalability down to the atomic lattice scale, frequency range from GHz up to hundreds of THz, straightforward control of magnons by electric currents and fields, pronounced natural nonlinearity, and a manifold of nonreciprocal phenomena.