Molecular Clock Qubits

This talk will focus on recent efforts aimed at protecting molecular spin qubits from decoherence caused by surrounding electron and nuclear spins (the spin bath), with emphasis on so-called clock transitions – avoided level crossings associated with the Zeeman splitting of spin states [1]. Spin clock transitions provide an optimal operating point at which the transition frequency, f, becomes insensitive to the local magnetic field, B₀. In this way, a clock qubit is immune to magnetic noise [2]. There are several strategies for generating spin clock transitions. All that is needed is an interaction term in the spin Hamiltonian that does not commute with the Zeeman interaction. For molecules with integer spin states, zero-field splitting interactions do the job. Alternatively, clock transitions may be generated in molecules possessing half-integer spin states via the on-site hyperfine interaction, a strategy that is employed widely in trapped-ion quantum devices. Crucially, in the molecular case, the hyperfine interaction can be synthetically controlled to maximize unpaired electron spin density at the relevant nucleus. A recent example involving a Lu^II ([Xe]4f¹⁴5d¹) organometallic compound has demonstrated that this is possible by varying the degree of s-orbital mixing into the spin-bearing d-orbital [3]. This approach has the added advantage of increasing the s-orbital character, thus reducing spin-orbit coupling that, in turn, suppresses spin-lattice relaxation. The talk will provide an overview of various synthetic strategies that have been employed for developing molecular clock qubits, together with the spectroscopic studies that demonstrate their enhanced coherence. The findings are supported by exact quantum dynamics simulations that demonstrate decoupling of a spin qubit from the nuclear bath at a clock transition [4].

[1] Gaita-Ariño et al., Nat. Chem. 11, (2019).

[2] Shiddiq et al., Nature 531, 348 (2016).

[3] Kundu et al., Nat. Chem. 14, 392 (2022).

[4] Kundu et al., arXiv:2106.05185 [quant-ph].