Floquet Engineering Molecular Spin Qubits

A wide range of molecules with a single metal center host an S = 1 ground state. One important application receiving great attention is the formation of qubits via the m_s = ± 1 pair of states in the triplet. The fundamental quantities characterizing the qubit are the zero-field splitting parameters D and E which represent the separation between the m_s = 0 and the m_s = ± 1 pair center, and the half-splitting between the m_s = ± 1 states, respectively. Achieving tunability of these energy splittings is the first of several requirements to optimize a single qubit. Traditional approaches undertaken by modifying the ligands surrounding the metal center, as well as the supporting surface, only enable discrete and time-fixed modifications to the energy level structure. On the other hand, by means of an external time-periodic magnetic field, the level spacings are controlled continuously and on ultrafast timescales. Theoretically, the Floquet Hamiltonian is diagonalized in the Fourier space numerically as the driving amplitude is varied, and physical intuition is built by comparing to a derived effective spin Hamiltonian in the high-frequency limit. Overall, significant tunability is achieved which cannot simply be explained by an effective Zeeman correction, i.e. the Floquet drive renormalizes the zero-field Hamiltonian parameters in a non-trivial manner. Demonstrating dynamical control of molecular spin qubits is one crucial step towards realizing commercial large-scale quantum computers that operate at room temperature.