Long Coherence Times in Nuclear Spin-Free Vanadyl Qubits

Quantum information
processing (QIP) offers the potential to create
new frontiers in fields ranging from quantum biology to cryptography.
Two key figures of merit for electronic spin qubits, the smallest
units of QIP, are the coherence time (T2), the lifetime of the qubit, and the spin–lattice relaxation
time (T1), the thermally defined upper
limit of T2. To achieve QIP, processable
qubits with long coherence times are required. Recent studies on (Ph4P-d20)2[V­(C8S8)3], a vanadium-based qubit, demonstrate
that millisecond T2 times are achievable
in transition metal complexes with nuclear spin-free environments.
Applying these principles to vanadyl complexes offers a route to combine
the previously established surface compatibility of the flatter vanadyl
structures with a long T2. Toward those
ends, we investigated a series of four qubits, (Ph4P)2[VO­(C8S8)2] (1), (Ph4P)2[VO­(β-C3S5)2] (2), (Ph4P)2[VO­(α-C3S5)2] (3), and (Ph4P)2[VO­(C3S4O)2] (4), by pulsed electron paramagnetic resonance (EPR)
spectroscopy and compared the performance of these species with our
recently reported set of vanadium tris­(dithiolene) complexes. Crucially
we demonstrate that solutions of 1–4 in SO2, a uniquely polar nuclear spin-free solvent, reveal T2 values of up to 152(6) μs, comparable
to the best molecular qubit candidates. Upon transitioning to vanadyl
species from the tris­(dithiolene) analogues, we observe a remarkable
order of magnitude increase in T1, attributed
to stronger solute–solvent interactions with the polar vanadium-oxo
moiety. Simultaneously, we detect a small decrease in T2 for the vanadyl analogues relative to the tris­(dithiolene)
complexes. We attribute this decrease to the absence of one nuclear
spin-free ligand, which served to shield the vanadium centers against
solvent nuclear spins. Our results highlight new design principles
for long T1 and T2 times by demonstrating the efficacy of ligand-based tuning
of solute–solvent interactions.