Quantum spin dynamics

Abstract:

Molecular structures appear to be natural candidates for a quantum technology: individual atoms can support quantum superpositions for long periods, and such atoms can in principle be embedded in a permanent molecular scaffolding to form an array. This would be true nanotechnology, with dimensions of order of a nanometre. However, the challenges of realizing such a vision are immense. One must identify a suitable elementary unit and demonstrate its merits for qubit storage and manipulation, including input/output. These units must then be formed into large arrays corresponding to an functional quantum architecture, including a mechanism for gate operations. Here we report our efforts, both experimental and theoretical, to create such a technology based on endohedral fullerenes or 'buckyballs'. We describe our successes with respect to these criteria, along with the obstacles we are currently facing and the questions that remain to be addressed. © 2006 IOP Publishing Ltd.

Abstract:

The endohedral fullerene epoxide N@C60O was synthesised, isolated by High Performance Liquid Chromatography (HPLC), and characterised by Electron Spin Resonance (ESR). This nitrogen radical displays predominantly axial symmetry characteristics as expected for a monoadduct, evidenced by a zero-field splitting D parameter of 6.6 MHz and an E parameter of 0.5 MHz in powder at 77 K. Photo- and thermally-activated silencing of the nitrogen radical were observed, the latter showing the evolution of a new spin signal during heating at 100 degrees C. We suggest that loss of nitrogen spin is due to coupling with a radical formed by opening of the epoxide ring. This implies that the reaction of C60O with C60 in the solid state proceeds via a radical, rather than ionic, intermediate.

Abstract:

We examine the temperature dependence of the electron spin relaxation times of the molecules N@C 60 and N@C 70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS 2 solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S≥1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C 60, indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C 70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T 2) ever observed for a molecular electron spin, being 0.25 ms at 170 K. © 2006 American Institute of Physics.