Exploring the mechanisms of transverse relaxation of copper-phthalocyanine spin qubits*

Quantum information science has catalyzed a search for suitable qubit platforms that would enable powerful, at-scale quantum devices. Paramagnetic spins in molecular crystals have emerged as promising spin-qubits, owing to their coherence time and potential scalability via synthetic chemistry. A key factor governing qubit performance is the phase relaxation time, which limits the number of quantum operations. Here, we combined numerical methods and dynamical decoupling experiments to investigate dephasing of electron spins in copper-phthalocyanine (CuPc) embedded in a $eta$-XPc matrix, where X represents an non-paramagnetic atom. We find that at cryogenic temperatures the CuPc electron dephasing is dominated by interactions with off-resonance electrons in the material — a factor overwhelmingly dominant over interactions with nuclear spin species. We confirm this insight by comparing experimental results (including echoes and spin-locking experiments) with simulations. Our research further unveils that the XPc matrix has a marginal impact, even when X contains hydrogen nuclear spins, in principle enhancing the nuclear spin bath. Other effects, such as the stability of the molecular crystal as a function of X, influencing the T1 longitudinal relaxation times, might be more pronounced. 

These insights are crucial for tailoring the molecular matrix to achieve specific thermal or optoelectronic properties, hinging significantly on the choice of the X atom, with potential impacts on advancing molecular spin qubit research