Quantum Information Processing with Molecular Spin Systems

Abstract

Quantum Information Processing with Molecular Spin Systems: A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Science and Engineering. The research presented in this PhD thesis is focused on the use of electron paramagnetic resonance (EPR) methods to study potential molecular spin qubits for quantum information processing (QIP) application. Of particular interest is the investigation of hyperfine interactions using pulsed methods, such as hyperfine sublevel correlation (HYSCORE) and electron-nuclear double resonance (ENDOR). Chapter I provides a background of the main EPR techniques utilised throughout the thesis and Chapter II introduces the field of molecular spin qubits (MSQs), highlighting the main advances in recent years and positioning the research presented herein within the greater context. Chapter III presents an investigation of a family of heterometallic rings of general formula [Cr7M(Etglu)F3(CO2tBu)15(L)], where Etglu = N-ethyl-D-glucamine, M = Ni(II) (1) or Cd(II) (4) and L = H2O (1), 4PhPy (2 and 4) or 3PhPy (3). These systems behave as molecular spin qubits showing both electron spin coherence and coherent Rabi oscillations at low temperature, which could be characterised in detail by pulse EPR methods. The hyperfine couplings are the main cause of decoherence in MSQs, herein HYSCORE and ENDOR were used to understand the origin and magnitude of these interactions and how they are affected by chemical modifications. The 1H and 14N hyperfine couplings were modelled considering a dipolar part, calculated from a point-dipole model, and an isotropic part, determined tentatively from simulations. The values obtained account for approximately 0.90 % of spin density delocalised onto the Etglu-14N in 1 – 4 and 0.80% in PhPy-14N in 1 – 3. The spin density on PhPy-14N in 4 is close to zero, owing to the presence of a diamagnetic cation, Cd(II). The carboxylate-1H can be reproduced considering only dipolar contribution, but the Etglu-1H are accounted for with an isotropic part. The robustness of these systems is shown with the hyperfine couplings, which see minimal change when the ligand at the Ni(II) site is varied, justifying their use as building blocks. Chapter IV expands the methodology developed in Chapter III to the dimers of the heterometallic rings. The Etglu-14N hyperfine coupling is not affected considerably by dimerization, which implies that the inter-ring electronic structure is kept the same after the modification and the unitary qubit is not disturbed much by the chemical linkage, at the same time that there is an approximately two-fold increase in the hyperfine coupling in the 14N of the linkers, showing that there is a shift in the electronic density towards the bridging ligand. Comparing the slight differences in the values obtained across the dimers, we can see that the Etglu-14N hyperfine decreases with increasing ring–ring distance and the reverse is observed in the ligand-14N. The same trend is true for the spin densities, which shows that the spin delocalisation is greater the longer the linker. Chapter V shows the use of EPR methods to study the coherence and electronic structure of two complexes of general formula [M(hfac)2L]–, where M = Zn(II) (1) and Ni(II) (2), hfac– is a fluorinated co-ligand and L is the conjugated base of the nitrophenol-substituted nitronyl-nitroxide (NIT) radical. Coherence times of 1.78 μs and 3.7 μs were observed for 1 and 2 respectively at 5.2 K. Using the CPMG pulse sequence, the time constants observed were increased to 18 μs and 7 μs. Rabi oscillations were detected for both compounds 1 and 2 at low temperatures. Hyperfine couplings to 1H, 14N, 19F and 67Zn were detected. The fluorine-19 region in 1 was modelled considering only the dipolar part of the hyperfine, because of Zn(II) being diamagnetic. Owing to the presence of a paramagnetic metal ion on 2, Ni(II), the spin density is more evenly distributed across the whole molecule, when compared to 1, where it is mainly based on the nitroxide.

Date of Award	28 Feb 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Floriana Tuna (Main Supervisor), Eric Mcinnes (Co Supervisor) & Richard Winpenny (Co Supervisor)