T1 Anisotropy Elucidates Spin Relaxation Mechanisms in an S = 1 Cr(IV) Optically Addressable Molecular Qubit

Paramagnetic molecules offer unique advantages for quantum information science owing to their spatial compactness, synthetic tunability, room-temperature quantum coherence, and potential for optical state initialization and readout. However, current optically addressable molecular qubits are hampered by rapid spin-lattice relaxation (T1) even at sub-liquid nitrogen temperatures. Here we use temperature- and orientation-dependent pulsed electron paramagnetic resonance (EPR) to elucidate the negative sign of the ground-state zero-field splitting (ZFS) and assign T1 anisotropy to specific degrees of freedom in an optically addressable S = 1 Cr(IV) molecular qubit. The anisotropy displays a distinct sin2(2θ) functional form that is not observed in S = ½ Cu(II) or V(IV) microwave addressable molecular qubits. The Cr(IV) T1 anisotropy is ascribed to couplings between electron spins and rotational motion in low-energy acoustic or pseudo-acoustic phonons. Our findings suggest that rotational degrees of freedom should be suppressed to maximize the coherence temperature of optically addressable qubits.