Using EPR to Guide the Engineering of Organometalic Molecular Qubits

Spin coherence enables magnetic molecules to function as qubits [1]. The lifetime of the qubit must be suficiently long to enable coherent spin manipulations at convenient operational temperatures [1,2]. One impediment is the occurrence of quantum decoherence, which limits the qubit operational lifetime. Inorporation of ligands that provide nuclear spin-free environments into molecules as well as operating at atomic clock transitions are two of the most improtant strategies used nowadays to overcome this problem.  In this talk we show that sufficiently long phase memory times allowing quantum spin manipulations even at ambient temperature are achievable in low-valent molecular systems with either C₃ or C₄ symmetry, even when they are rich in nuclear spins.  Examples include low-valent lanthanide and transition metal complexes bearing cyclopentadinyl derivatives as ligands, aryloxides or imides. The exceptionally long coherence times of thes esystems enabled mapping the spin densities that infiorm on decoherence path, with the aid of pulsed EPR methods: HYSCORE, ESEEM, ENDOR.  For LnCp’₃K (Ln = Sc, Y, La or Lu), coherent Rabi oscillations were measured, including at 300 K in a single crystal [2]. Ac magnetic susceptibility data confirmed slow magnetic relaxation at low temperatures, associated with very long spin lattice relaxation times, which could be nicely characterised by pulsed EPR methods. Despite many ¹H and ¹³C nuclei present in these molecules, all electronuclear transitions of the compounds enabled to be coherently manipulated, indicative that the qubit memory time is protected in these systems due to a reduced orbital angular momentum of the ground spin state.

[1] S. McAdams, A. M. Ariciu, A. Kostopulos, J. Walsh, F. Tuna, Coord. Chem. Rev. 2017, 346, 216-339.

[2] A. M. Ariciu, D. H. Woen, D. N. Huh, A. K. Kostopoulos, C. A. P. Goodwin, N. F. Chilton, E. J. L. McInnes, R. E. P. Winpenny, W. J. Evans, F. Tuna, Nature Commun. 2019, 10, 3330.