Influence of Electronic Spin and Spin–Orbit Coupling on Decoherence in Mononuclear Transition Metal Complexes

Enabling
the rational synthesis of molecular candidates for quantum
information processing requires design principles that minimize electron
spin decoherence. Here we report a systematic investigation of decoherence
via the synthesis of two series of paramagnetic co­or­din­ation
complexes. These complexes, [M­(C2O4)3]3– (M = Ru, Cr, Fe) and [M­(CN)6]3– (M = Fe, Ru, Os), were prepared and interrogated
by pulsed electron paramagnetic resonance (EPR) spectroscopy to assess
quantitatively the influence of the magnitude of spin (S = 1/2, 3/2, 5/2) and spin–orbit coupling (ζ = 464, 880,
3100 cm–1) on quantum decoherence. Coherence times
(T2) were collected via Hahn echo experiments
and revealed a small dependence on the two variables studied, demonstrating
that the magnitudes of spin and spin–orbit coupling are not
the primary drivers of electron spin decoherence. On the basis of
these conclusions, a proof-of-concept molecule, [Ru­(C2O4)3]3−, was selected for further
study. The two parameters establishing the viability of a qubit are
a long coherence time, T2, and the presence
of Rabi oscillations. The complex [Ru­(C2O4)3]3– exhibits both a coherence time of T2 = 3.4 μs and the rarely observed Rabi
oscillations. These two features establish [Ru­(C2O4)3]3– as a molecular qubit candidate
and mark the viability of coordination complexes as qubit platforms.
Our results illustrate that the design of qubit candidates can be
achieved with a wide range of paramagnetic ions and spin states while
preserving a long-lived coherence.