Spin Population Transfer in a Gd³⁺ Molecular Crystal Studied by Pulsed High-Field EPR

Gd³⁺ is a spin S = ⁷/₂ ion that has a half-filled 4f⁷ electron occupancy, with no first order orbital angular momentum. Thus, its eight spin levels are minimally mixed and can typically be considered pure, with very weak zero-field splitting. For these reasons, the spin states of a Gd³⁺ ion can encode N = 3 addressable qubits (a d = 2^N = 8 level qudit), with each Δm_S = ±1 transition being fully allowed via EPR, permitting spin manipulation using resonant microwave pulses. However, the Pulsed EPR spectrometer must meet several key requirements, including: (1) sufficient microwave power to achieve nanosecond time resolution; (2) wide excitation bandwidth and arbitrary waveform shaping in order to rapidly address spectrally separated spin transitions; and (3) high sensitivity for detection of small samples with low spin concentrations. The quasi-optical 94 GHz HiPER spectrometer [1] at the NHMFL meets all of these requirements and additionally allows for in situ crystal rotation. We demonstrate dynamic spin population transfer within the ⁸S_7/2 ground manifold of a molecular Gd³⁺ complex diluted into an isostructural non-magnetic Y³⁺ host crystal, paving the way towards implementation of simple quantum logic operations within a d = 8 molecular spin qudit.

[1] Subramanya et al., https://doi.org/10.1007/s00723-022-01499-3