Emerging Molecular Quantum Bits – MolEQuBe

Quantum hardware has been steadily progressing in the last years. Amongst the highlights, a few of the most advanced approaches: superconducting qubits, cold atoms, ions and photonic qubits, have recently overcome the symbolic 100 qubit mark. Yet, the search for new physical and material platforms which could revolutionize the field by providing new functionalities, or solving some of the remaining technological blockages, remains very active. In MolEQuBe we propose to explore a novel quantum material platform combining the promising optical and spin properties of rare earth ions with the unique ability of synthetic molecular chemistry for creating tailored atomic arrangements and functional supramolecular architectures compatible with SC resonators and photonic devices. This fine control will be exploited at different levels, to limit decoherence mechanisms from molecular vibrations and magnetic noise, to engineer qubit-qubit interactions and qubit-host interactions, and to accurately tune the qubit’s optical and spin transition energies. To achieve these ambitious goals, MolEQuBe gathers a consortium of experts with unique complementary competences and experimental capabilities including rare-earth-molecule chemistry, coherent and high-resolution optical spectroscopy, coherent optical storage, superconducting technologies, and microwave single-spin measurements. During the project, we will target three main objectives: (i) We will synthesize RE molecular materials that combine the unmatched narrow optical and spin linewidths of RE ions with the highly deterministic design capabilities provided by molecular chemistry to design scalable optically addressable multi-qubit platforms. (ii) We will demonstrate single molecular spin detection by depositing optimized RE molecular materials onto high-quality-factor superconducting (SC) microwave (MW) resonators and using MW photon counting. We will then harness this single molecule detection capability to carry out two-qubit gates using a small nuclear spin register within the molecule. (iii) We will create novel integrable quantum memory platforms, fabricated in low-loss polymer waveguides featuring high bandwidth and containing optimized 171Yb molecules. We will demonstrate optical-to-spin coherence transfer, and single photon level storage on RE ensembles using the atomic frequency comb memory protocol. The strong potential and feasibility of MolEQuBe’s approach is supported by recent pioneering work by the partners. On the one hand, the IRCP and ISIS teams recently reported 30 kHz optical linewidth in an isotopically pure highly concentrated Eu3+ molecular crystal (optical T2 of 10 µs). This breakthrough represents three to four orders of magnitude line narrowing over molecular centers reported so far. At the same time, the achievement of single Er3+ spin detection using a scheelite crystal by the SPEC partner opens the way for quantum demonstrations with molecular qubits.