Quantum coherent manipulation of spin information in molecular nanomagnets

Résumé

Quantum two-level systems based on spin states known as ``spin-qubits’’ are promising building blocks for the development of quantum technologies. Among different physical platforms, spin-qubits defined in single-molecule-magnets (SMMs) are promising candidates because their electronic structure can be easily tuned by chemical engineering (i.e., the molecular spin Hamiltonian can be easily modified). However, molecular spin qubits generated in SMMs faces several challenges: fragile quantum coherence, insufficient coherent control over spin states and generation of entanglement between the spin-qubits for quantum information processing applications. To address these challenges and achieve the coherent manipulation of spin information, we need to understand the relationship between spin states, molecular motions (vibrations or phonons) and charge polarization (e.g., that generated by an external E-field). The current Thesis explores the relationship between spin states, vibrations and polarization from a theoretical perspective. Initially, we study the interaction between spin states and vibrations (vibronic coupling) as an important source of spin information dissipation. In particular, a detailed modelling of vibronic couplings, supported by experimental evidence, is employed to decipher the decoherence pathways in different SMMs. Our outcomes reveal that only some molecular distortions associated to certain vibrational modes are able to strongly couple to spin degrees of freedom and, thus, promoting decoherence. Additionally, we also identified that sparse spectra between spin and phonon states are crucial to preserve quantum superpositions longer times. Secondly, we present a comprehensive study of coherent control over spin states using electrical fields in a molecular qubit system that exhibits clock transitions (HoW10). This coherent control is modelled by evaluating the spin-electric coupling (SEC); that is, finding a relation between spin states, charge polarization, and molecular distortions. The strong SEC observed in HoW10 is sufficient to allow selective addressing of the spins using a local E-field at practical level. Finally, we explore the possibility of constructing two-qubit entanglement gate in a pair of two dipolar-coupled clock-qubit (HoW10--HoW10), where electrical field is used to locally control the qubit states. The work presented in this Thesis advances the understanding of molecular spin qubits for their potential application in quantum information processing.