Hybrid quantum systems combine complementary degrees of freedom, such as spins, spin waves, and single photons, to exploit their distinct advantages in coherence, nonlinearity, and interfacing. Realising them demands solid-state platforms whose materials, models, and propagating excitations remain controllable beyond idealised conditions. This colloquium traces that goal through selected results from my research. In solid-state quantum sensing, nitrogen-vacancy (NV) centres in diamond serve as embedded probes of fabrication-induced strain and electric fields in nanostructures [1] and as adaptive magnetometers that reach high sensitivity, with, on average, a single detected photon per readout [2]. In cryogenic magnonics, I then turn to propagating spin-wave spectroscopy in nanometre-thick YIG films at millikelvin temperatures [3], together with k-selective electrical-to-magnon transduction that links realistic nanoantenna near-fields to propagating spin-wave dynamics [4]. The same spin-transport principles extend beyond ordered magnets: in organic free radicals, paramagnon-polaritons propagate over millimetre ranges [5]. As an outlook, I will sketch how these lines converge: combining millikelvin Brillouin light scattering, NV-based quantum sensing, and adaptive models continuously refined from measured data, building on data-driven Hamiltonian learning [6], toward hybrid opto-magnonic quantum systems.
[1] S. Knauer et al., npj Quantum Inf. 6, 50 (2020).
[2] R. Santagati*, A. A. Gentile*, S. Knauer* et al., Phys. Rev. X 9, 021019 (2019).
[3] S. Knauer et al., J. Appl. Phys. 133, 143905 (2023).
[4] A. Höfinger, …, S. Knauer, Adv. Phys. Res. e00211 (2026).
[5] S. Knauer et al., under review (2026); arXiv:2511.10294.
[6] A. A. Gentile*, B. Flynn*, S. Knauer* et al., Nat. Phys. 17, 837 (2021).
