Enlarging Quantum Coherence of Vanadyl Spin Qubits in Zirconium Porphyrinic MOFs

The intrinsic quantum-mechanical properties of electron spin along with the structural and chemical tunability of metal-organic frameworks (MOFs) that can control the position of the active spin carrier make molecular spin qubits embedded in porous coordination frameworks an ideal platform for investigating quantum coherence. These molecular spin qubits still exhibit relatively short relaxation times, but several recently developed strategies hold promise for extending coherence. We recently demonstrated that diluting the copper(II)-porphyrin qubits, combined with the inclusion of fullerene C₆₀ guests, can significantly prolong the relaxation times to liquid nitrogen temperature. Here, we present how the exchange of copper(II)- with the vanadyl-porphyrin spin qubits allows for a more diluted qubit framework and results in a stable system with one of the longest coherence times observed in the nonactivated MOF qubit frameworks. The fullerene inclusion proved beneficial for both the electron spin-lattice and the phase-memory times. Figures of merit T_m ∼1 μs were observed in the temperature range 5-180 K, while the coherence was detected up to 240 K and still was not T₁-limited. The detected linear dependence of the Rabi frequency on the microwave field confirmed that the tested vanadyl frameworks are viable qubit candidates.