Metalloporphyrin Zr-MOF Nanoparticles Film for Device Integration of Spin Qubits and Qudits.

Integration of molecular spin qubits onto superconducting circuits is one of the current proposals to build a hybrid quantum processor, in which their coupling to resonators would allow complex coherent quantum operations. As an alternative to large magnetically dilute crystals or concentrated nanoscale deposits of paramagnetic molecules studied so far, we consider here preformed nanoparticles of a Zr(IV)-based 3D MOF doped with Cu(II) and V(IV)O metalloporphyrin. Highly crystalline, phase-pure nanoparticles are obtained with an adjustable number of spin carriers and a homogeneous size that is adequate for nanoconstrictions in superconducting resonators. Continuous-wave and time-domain EPR spectroscopy show that the Cu(II) and V(IV) spins in these particles are potential qubits and qudits, respectively, with robust quantum coherence. These nanoparticles form good dispersions in methanol-chloroform mixtures, which allows the formation of monolayers of densely packed particles at the air-water interface. These Langmuir films are successfully transferred to very different substrates, including Nb resonators, homogeneously covering their most sensitive areas, i.e., the edges of the superconducting lines. Finally, the quantum coherence of the qubit nodes is maintained in the transferred nanoparticles, thereby demonstrating the viability of the present approach for the development of hybrid quantum processors.