Electron spin resonance of molecular magnets for quantum information processing

Thesis

Quantum information processors have been shown theoretically to outperform their classical counterparts at certain tasks. They comprise two level systems, which can exist in an arbitrary superposition of states: qubits. A strong candidate qubit is the molecular nanomagnet (MNM). In this thesis, electron spin resonance is used to explore the potential of Cr7M based MNMs as elements of a quantum information processor.

We explore the possibility and effect of replacing H atoms with D or a halogen atom in a S = 1/2 Cr7Ni ring. Decoherence mechanisms in the resulting compounds are found to be dominated by structural effects. We conclude that halogenation does not seem to be a productive strategy for extension of coherence times in Cr7Ni based compounds.

We examine an asymmetric dimer, in which a Cr7Ni ring is linked to a highly coherent nitrogen atom within a carbon cage. Measurement of phase memory time across a range of temperatures for the N spin, provides insight into the ring's spin dynamics. At high temperature, fluctuations on the ring are so rapid that the N spin appears magnetically disconnected; as they slow, we see a sharp rise in the decoherence rate of the N spin. At the lowest temperatures, a recovery of the decoherence rate reflects the onset of the ring’s coherent ground state.

A group of symmetric Cr7Ni-Cr7Ni dimers is investigated. Through use of double electron-electron resonance, the strength of the ring-ring dipolar interaction, governing the two qubit gate time, is estimated for each. It is found that many exhibit the hierarchy of timescales required for implementation of a two qubit gate: a short single qubit manipulation time, intermediate two-qubit gate time and long phase coherence time. A possible scheme for the future implementation of a CNOT gate is presented.

The final study explores rings of spin, S > 1/2. We present the first ever coherent measurements on a dilute oriented ensemble of the S = 3/2 rings, Cr7Zn, performing nutations at various powers. In addition, we investigate the anisotropic Mn2+ (s = 5/2, I = 5/2) defect in ZnO. We develop the 'echo kill' method for identification of a three level subsystem whose transition frequencies both fall within the cavity bandwidth. Such a subsystem is then used to perform indirectly detected nutations between the upper two levels over a range of applied powers. Finally, a method for initialisation of the subsystem into a pseudopure state is presented and shown to enhance nutation amplitude.

Authors: Kaminski, D

• DOI: 10.5287/ora-6g5bzevvd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford