Quantum Control of Magnetic Molecules by Floquet Engineering and Cavity QED

Magnetic molecules are a promising candidate for realizing scalable spin qubits and quantum sensing at elevated temperatures. Swapping out the transition or lanthanide metal center(s) and redesigning the surrounding ligand field enables tremendous chemical tunability towards specific applications. In my talk, I will focus on approaches to realize spin Hamiltonian engineering via external quantum control. Coupling an effective S = 1 spin directly to an above-bandwidth, time-periodic magnetic field enables continuous tuning of the three energy levels with a key feature: The spin coherence-enhancing clock transition between any pair of energy levels is preserved under linearly polarized control. The main limitation of such direct Floquet engineering of the spin is that the driving amplitude required to enable anything beyond precision tuning can be prohibitively large. A more practical path forward may be indirect control via phonons: If a particular phonon is strongly coupled to the metal center(s), then applying control to that mode will indirectly affect the spin via spin-phonon coupling. More precisely, one may resonantly drive the phonon mode coherently (Floquet engineering) or resonantly couple the phonon mode passively to a cavity (QED). Looking forward, defining multimode control protocols will enable designer spin Hamiltonians via quantum optimal control techniques.