Quantum control of molecular spins using electron spin resonance

Thesis

Molecular magnets have been demonstrated to be promising candidates for quantum-coherent nanodevices, due to their long relaxation times and design flexibility. Building and scaling such devices presents several challenges. In this thesis, two problems related to scalability of molecular magnet architectures are investigated using electron spin resonance.

Using electric fields, rather than magnetic fields, to operate quantum devices, would radically improve addressability of individual spins. Electric field sensitivities of several molecular magnets, including frustrated Cu$_3$ triangles, Cr$_7$Ni and Cr$_7$Mn rings and crystals of HoW$_{10}$ complexes, are investigated through pulse sequences incorporating static electric fields. We find spin-electric couplings of various magnitudes, significant enough to manipulate the parameters of respective Hamiltonians to enable selective excitation and we demonstrate such excitation in the HoW$_{10}$ molecule. We identify a path towards improving electric field sensitivities, through coupling structural distortions exhibiting high electric polarizability with the molecular spin Hamiltonian.

To operate assemblies of multiple molecular magnets as quantum devices, robust multiqubit gates, able to modify the amount of entanglement in the system, need to be implemented. Quantum state tomography may be employed to investigate the practical fidelity of such a gate and real entanglement it generates. Focusing on a weakly coupled dimer of two spin-$\frac12$ molecules, we propose an implementation of a two-qubit entangling gate followed by quantum state tomography using ensemble electron spin resonance. We investigate the limitations and requirements of the gate and the tomography procedure through theoretical arguments, simulations and experiments. We conclude that while challenging, reaching and demonstrating actual entanglement is within our capabilities using ensembles of molecular magnet dimers.

Authors: Mrożek, J

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: spin-electric coupling; quantum coherent nanoscience; entanglement; quantum; electron paramagnetic resonance; electron spin resonance; molecular magnets
• Subjects: Quantum computing; Magnetism; Electron paramagnetic resonance; Quantum entanglement