Engineered quantum systems enabling novel capabilities for communication, computation, and sensing have blossomed in the last decade. Architectures benefiting from combining distinct and complementary physical quantum systems have emerged as promising platforms for developing quantum technologies. A new class of hybrid quantum systems based on collective spin excitations in ferromagnetic materials has led to the diverse set of experimental platforms which are outlined in this review article. The coherent interaction between microwave cavity modes and collective spin-wave modes is presented as the backbone of the development of more complex hybrid quantum systems. Indeed, quanta of excitation of the spin-wave modes, called magnons, can also interact coherently with optical photons, phonons, and superconducting qubits in the fields of cavity optomagnonics, cavity magnomechanics, and quantum magnonics, respectively. Notably, quantum magnonics provides a promising platform for performing quantum optics experiments in magnetically-ordered solid-state systems. Applications of hybrid quantum systems based on magnonics for quantum information processing and quantum sensing are also outlined briefly.

Recent progress in nanotechnology has allowed to fabricate new hybrid systems where a single two-level system is coupled to a mechanical nanoresonator [1][2][3][4][5][6]. In such systems the quantum nature of a macroscopic degree of freedom can be revealed and manipulated. This opens up appealing perspectives for quantum information technologies [7], and for the exploration of quantum-classical boundary. Here we present the experimental realization of a monolithic solid-state hybrid system governed by material strain [8] : a quantum dot is embedded within a nanowire featuring discrete mechanical resonances corresponding to flexural vibration modes. Mechanical vibrations result in a time-varying strain field that modulates the quantum dot transition energy. This approach simultaneously offers a large light extraction efficiency [9,10] and a large exciton-phonon coupling strength g 0 . By means of optical and mechanical spectroscopy, we find that g 0 /2π is nearly as large as the mechanical frequency, a criterion which defines the ultrastrong coupling regime [12]. A single quantum two-level system coupled to a micron-size mechanical oscillator constitutes a hybrid system, which connects two different worlds: the classical and the quantum one. This new kind of interaction opens up the possibility of creating macroscopic non-classical states of motion, such as phonon Schrödinger cats or phonon number states. In the case of strain-mediated coupling, it is predicted that the two level system can even be used to cool the mechanical resonator down to its ground state [8] or conversely to achieve phonon lasing [13].Such appealing prospects have recently motivated the development of several kinds of hybrid systems, like for instance: (i) a single spin embedded in a mechanical resonator coupled together by an external magnetic field gradient [4,14,15], (ii) a few elementary charges (single electron or Cooper pair) coupled by electrostatic forces with a vibrating gate [2, 6, 16], or (iii) quantized current loops in superconducting qubits attached to a mechanical oscillator interacting via a magnetic field [17]. However, despite theoretical proposals highlighting the potential of using material strain to mediate a large coupling between Figure 1. Hybrid system and experimental setup. a Scanning electron microscope picture of a representative cone shaped nanowire. The quantum dots (QDs) layer is materialized by the dashed white line. b and c, Nanowire deformation in the first order flexural vibration mode. The stress field is plotted in blue to red color scale: due to its excentric inplane position, the quantum dot (yellow triangle) experiences in b a compressive strain that shifts its transition energy ω0 by + δω and in c a tensile strain that shifts its transition energy by − δω. d Experimental setup: single QD optical measurements are carried out using a spectrometer, and the measurement of the nanowire free-end displacement δx is realized by means of a balanced split photo-diode (SPD). The voltage difference v between th...

A ferromagnetic sphere can support optical vortices in the form of whispering gallery modes and magnetic quasivortices in the form of magnetostatic modes with nontrivial spin textures. These vortices can be characterized by their orbital angular momenta. We experimentally investigate Brillouin scattering of photons in the whispering gallery modes by magnons in the magnetostatic modes, zeroing in on the exchange of the orbital angular momenta between the optical vortices and magnetic quasivortices. We find that the conservation of the orbital angular momentum results in different nonreciprocal behavior in the Brillouin light scattering. New avenues for chiral optics and optospintronics can be opened up by taking the orbital angular momenta as a new degree of freedom for cavity optomagnonics.