Coherent transfer between electron and nuclear spin qubits and their decoherence properties

Thesis

Conventional computing faces a huge technical challenge as traditional transistors will soon reach their size limitations. This will halt progress in reaching faster processing speeds and to overcome this problem, require an entirely new approach. Quantum computing (QC) is a natural solution offering a route to miniaturisation by, for example, storing information in electron or nuclear spin states, whilst harnessing the power of quantum physics to perform certain calculations exponentially faster than its classical counterpart. However, QCs face many difficulties, such as, protecting the quantum-bit (qubit) from the environment and its irreversible loss through the process of decoherence.

Hybrid systems provide a route to harnessing the benefits of multiple degrees of freedom through the coherent transfer of quantum information between them. In this thesis I show coherent qubit transfer between electron and nuclear spin states in a ¹⁵N@C₆₀ molecular system (comprising a nitrogen atom encapsulated in a carbon cage) and a solid state system, using phosphorous donors in silicon (Si:P). The propagation uses a series of resonant mi- crowave and radiofrequency pulses and is shown with a two-way fidelity of around 90% for an arbitrary qubit state. The transfer allows quantum information to be held in the nuclear spin for up to 3 orders of magnitude longer than in the electron spin, producing a ¹⁵N@C₆₀ and Si:P ‘quantum memory’ of up to 130 ms and 1.75 s, respectively. I show electron and nuclear spin relaxation (T₁), in both systems, is dominated by a two-phonon process resonant with an excited state, with a constant electron/nuclear T₁ ratio. The thesis further investigates the decoherence and relaxation properties of metal atoms encapsulated in a carbon cage, termed metallofullerenes, discovering that exceptionally long electron spin decoherence times are possible, such that these can be considered a viable QC candidate.

Authors: Brown, R

• Publication date: 2012
• DOI: 10.5287/ora-ko6rpgbxn
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: electron paramagnetic resonance; fullerene; qubit; quantum memory; laser; M@C82; electron spin resonance; microwave; ESR; metallofullerene; quantum computing; Y@C82; relaxation; entanglement; pulsed; decoherence; quantum information processing; electron spin; P:Si; EPR; Sc@C82; La@C82; coherent transfer; radiofrequency; N@C60; nuclear spin; phosphorous doped silicon
• Subjects: Quantum information processing; Nanostructures; Spectroscopy and molecular structure; NMR spectroscopy; Physical & theoretical chemistry; Semiconductors; Condensed Matter Physics; Silicon; Nanomaterials