Magnetic resonance of nuclear and electronic spins in molecules and semiconductors for quantum information processing
Abstract:
This thesis details three studies performed with the aim of deepening our understanding of how nuclear and electronic spins can be manipulated such that they might be used in quantum information processing.I start by discussing the possibility of using pulses of static electric fields to coherently control qudits implemented on molecular magnets. The success of this control depends on the level of spin-electric coupling (SEC) which reflects how the respective Hamiltonian changes with the application of an electric field. I present our research on a family of Mn(II)-containing molecules in which the systematic control of SEC is realised by varying the coordination environment of their spin centre. Their trigonal bipyramidal molecular structure with C3 symmetry leads to a significant molecular electric dipole moment. Due to this, as well as high polarisability of the ligands, an applied electric field induces enhanced structural distortions. This gives rise to significant experimentally observed SEC, which is further rationalised by wavefunction theoretical calculations.
I then discuss the SEC in a molecular magnet [Yb (trensal)], which similarly possesses C3 symmetry, but instead of manganese, this molecule contains a rare-earth ion of ytterbium (III). At cryogenic temperatures, [Yb (trensal)] can be described by an effective spin-1/2 Hamiltonian. However, our study shows that the significant values of SEC exhibited by [Yb (trensal)] can be only explained if the Hamiltonian is additionally equipped with the extended Stevens operators. The unique property of [Yb (trensal)] is that it demonstrates linear SEC even when the E-field is oriented perpendicularly to the C3-axis of the molecule, and that this perpendicular SEC is of the same order of magnitude as the parallel effect.
In the third study, I show how, by using electron-nuclear double resonance, we implement a logical qubit encoded on four states of an I = 3/2 nuclear spin hyperfine-coupled to an S = 1/2 electron spin qubit. The encoding protects against the dominant decoherence mechanism in such systems – fluctuations of the quantizing magnetic field. We explore the dynamics of the encoded state both under a controlled application of the fluctuation and under natural decoherence processes. Our results confirm the potential of these proposals for practical, implementable, fault-tolerant quantum memories.
Tailoring the electronic configurations of YPc2 on Cu(111): decoupling strategies for molecular spin qubit platforms
Nanoscale, 2025,17, 22163-22173
Abstract:
Molecule-based spin architectures have been proposed as promising platforms for quantum computing. Among the potential spin qubit candidates, yttrium phthalocyanine double-decker (YPc2) features a diamagnetic metal ion core that stabilizes the molecular structure, while its magnetic properties arise primarily from an unpaired electron (S = 1/2) delocalized over the two phthalocyanine (Pc) ligands. Understanding its properties in the proximity of metal electrodes is crucial to assess its potential use in molecular spin qubit architectures. Here, we investigated the morphology and electronic structure of this molecule adsorbed on a Cu(111) surface using scanning tunneling microscopy (STM). On Cu(111), YPc2 adsorbs flat, with isolated molecules showing a preferred orientation along the 〈111〉 crystal axes. Moreover, we observed two different types of self-assembly patterns when growing molecular patches. For YPc2 in direct contact with Cu(111), STM revealed widely separated highest occupied and lowest unoccupied molecular orbitals (HOMO/LUMO), suggesting the quenching of the unpaired spin. Conversely, when YPc2 is separated from the metal substrate by a few-layer thick diamagnetic zinc phthalocyanine (ZnPc) layer, we found the HOMO to split into singly occupied and singly unoccupied molecular orbitals (SOMO/SUMO). We observed that more than 2 layers of ZnPc are needed to avoid intermixing between the two molecules and spin quenching in YPc2. Density functional theory (DFT) calculations reveal that spin quenching is due to the hybridization between YPc2 and Cu(111) states, confirming the importance of using suitable decoupling layers to preserve the unpaired molecular spin. Our results suggest the potential of YPc2/ZnPc heterostructures as a stable and effective molecular spin qubit platform and validate the possibility of integrating this molecular spin qubit candidate in future quantum logic devices.