Qubit Control Limited by Spin–Lattice Relaxation in a Nuclear Spin-Free Iron(III) Complex

High-spin
transition metal complexes are of interest as candidates for quantum
information processing owing to the tunability of the pairs of MS levels for use as quantum
bits (qubits). Thus, the design of high-spin systems that afford qubits
with stable superposition states is of primary importance. Nuclear
spins are a potent instigator of superposition instability; thus,
we probed the Ph4P+ salt of the nuclear spin-free
complex [Fe­(C5O5)3]3– (1) to see if long-lived superpositions were possible
in such a system. Continuous-wave and pulsed electron paramagnetic
resonance (EPR) spectroscopic measurements reveal a strong EPR transition
at X-band that can be utilized as a qubit. However, at 5 K the coherent
lifetime, T2, for this resonance is 721(3)
ns and decreases rapidly with increasing temperature. Simultaneously,
the spin–lattice relaxation time is extremely short, 11.33(1)
μs, at 5 K, and also rapidly decreases with increasing temperature.
The coincidence of these two temperature-dependent data sets suggests
that T2 in 1 is strongly
limited by the short T1. Importantly,
these results highlight the need for new design parameters in pursuit
of high-spin species with appreciable coherence times.