Material collective modes
Material collective modes comes in many forms and flavors. An example of a collective mode is the collective precession of spins in a ferromagnetic material – the spin wave. Typically, spin waves can be accessed in the microwave regime and strongly couple to microwave fields through a dipolar interaction. This interaction can be observed using cQED as an avoided crossing in the photon spectrum as the spin wave modes cross it.
Extending our material systems to superconductors, an analogous mode to the spin wave is predicted to exist, which instead of a coherent precession of spins is the coherent precession of Cooper pairs – a Cooper pair spin wave. Such spin waves exist only in unconventional superconductors where Cooper pairs are formed as spin triplet pairs. Coupling our microwave photons to triplet-candidate superconductors may thus allow us to interrogate their underlying pairing symmetry.
Material collective modes both offer new ways to probe materials and their underlying symmetries, as well as to interact coherently with microwave fields. This may allow us to store information through global excitations within a dissipationless material, which can be controlled using standard circuit QED techniques. Exploring this new field of 'superconducting magnonics' is another research direction in our lab.
While the Cooper pair spin waves typically would exist at an energy scale in the low microwave regime, other collective modes in superconductors reside closer to the gap edge. To access these modes require beyond-microwave cQED techniques. To this end, we are also developing cQED techniques in the millimeter wave regime. To learn more about these developments, see ‘Hybrid qubits’.