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. 

Moreover, hybrid quantum magnonics shows a promising path to bridge the gap between different quantum technologies. It offers the unique combination of spin-, phonon-, and photon- (including microwave) quantum systems and ultimately highly integrable hybrid quantum systems. Having achieved longer coherence times and lengths, integrated quantum magnonics will allow the coupling to other quantum systems, promising efficient interaction between the sub-systems and their mutual interactive characterization and examination. Using machine learning algorithms, one may then exploit the coupling to find true quantum models of the integrated quantum magnonics circuits, typically obtained from empirical measurements and analytical models.

One of the major challenges facing quantum magnonics, in our opinion, is the transition from a single magnon in the form of a uniform precession discovered by the group of Y. Nakamura, to propagating single magnons. The comparatively large spin-wave loss is a challenge which will limit the number of magnons reaching a detector region. Nevertheless, the recently-developed 50 nm-wide single-mode spin-wave waveguides offer relatively large propagation lengths of about 10 µm which should be sufficient for quantum operations at the nanoscale. Thus, besides a set of technical questions, the main challenges will involve the realization of efficient single-magnon sources and detectors. The superconducting qubit approach seems to be the most viable solution. Superconducting systems on the basis of nonlinear Josephson circuits with discrete energy spectra allow for the detection of microwave photons and magnons with single-quantum sensitivity. These systems are analogs of a single atom described with a discrete Hamiltonian and allow for efficient electromagnetic coupling between constituents in hybrids.

A further fundamental challenge in the field of quantum magnonics is the generation of entangled magnon states. Fortunately, there are already many well-studied physical mechanisms that generate such states. For example, Brillouin Light Scattering (BLS) generates two magnons that are entangled between themselves and with a photon, parallel parametric pumping couples two magnons with a microwave photon. Three-magnon splitting  generates two entangled magnons as well. The required powers and the efficiencies of all these phenomena will possibly be an eminent challenge. The realization of the entangled magnons will enable a variety of experimental investigations, such as quantum teleportation, quantum simulation, quantum computation and quantum sensing.