ADVANCED EPR METHODS AND ELECTRON SPIN DYNAMICS IN MOLECULAR AND NANO-SCALE MATERIALS
The main objectives of the project include:
• Implementation of advanced Electron Paramagnetic Resonance (EPR) methods for quantum computation
• Development of pulsed EPR methods aimed at sensitivity enhancement
• Characterization of paramagnetic species including transition metal ions and free radicals
Electron spins and quantum computing
Over the last decade the field of quantum computation has seen spectacular progress not only in the theoretical level but also in different experimental developments involving physics, chemistry and materials science. It is now believed that essential quantum mechanical properties like quantum entanglement can be considered as fundamental resources of Nature, of comparable importance to energy, information or entropy. Apart from the early developments of quantum algorithms and their remarkable ability in solving certain computational tasks, there is currently an ever increasing interest in finding appropriate physical systems for implementing key concepts of quantum computation which reflects the great emerging challenges of faithfully storing, processing, and measuring quantum information.
Electron and nuclear spins are considered as promising elements of quantum information (qubits) because they are natural two-(or higher) state systems with relatively long decoherence times that can be controlled using well-established magnetic resonance techniques. Hybrid spin systems where an electron spin is hyperfine-coupled to a nuclear spin are of distinguished importance for quantum information processing. To this end, our research focuses on (i) finding physical spin systems with long decoherence times beyond those of endohedral fullerenes like 15N@C60, and (ii) developing new pulse sequences in order to circumvent inherent drawbacks of hybrid spin systems and thus build reliable quantum gates.
G. Mitrikas, Y. Sanakis, and G. Papavassiliou, Ultrafast control of nuclear spins using only microwave pulses: towards switchable solid-state quantum gates, Phys. Rev. A (Rapid Communications) 81 (2010), 020305 1-4.
G. Mitrikas, Y. Sanakis, C.P. Raptopoulou, G. Kordas, G. Papavassiliou, Electron spin-lattice and spin-spin relaxation study of a trinuclear iron(III) complex and its relevance in quantum computing, Phys. Chem. Chem. Phys. 10 (2008), 743-748.
Pulsed EPR methodology
The electron spin echo envelope modulation (ESEEM) effect is the key element of many powerful pulsed EPR techniques that are used to determine weak hyperfine and nuclear quadrupole interactions in solids. These include methods that are based on electron spin coherence like the primary two-pulse and refocused primary echo sequences, or schemes that are based on evolution of nuclear spin coherence like three-pulse, four-pulse, hyperfine sublevel correlation (HYSCORE) and double nuclear coherence transfer (DONUT)-HYSCORE. While some of these methods provide optimum resolution for given paramagnetic systems, they may suffer from poor sensitivity due to small modulation depths, low abundances of magnetic nuclei and/or additional multinuclear suppression effects. For this reason, a variety of experiments with improved sensitivity have been developed in the past including five- or six-pulse ESEEM, methods with matched pulses, and soft ESEEM experiments.
In the present work we study a new way to increase the modulation amplitude of ESEEM experiments by applying multiple refocusing π-pulses on the electron spin coherence created initially by a π/2-pulse. Each one of these pulses redistributes the electron spin coherence among allowed and forbidden EPR transitions and this in turn leads to a significant enhancement of the ESEEM effect, depending on the strength of the hyperfine interaction and the number of applied pulses, N. We derive analytical expressions for a general two-dimensional (2D) scheme and we explore the expected modulation enhancement of various correlation peaks as a function of k (modulation depth parameter) and N. Our study shows that these methods are particularly useful for detecting weak hyperfine couplings of magnetic nuclei having small gn factors and low natural abundances like 13C and 29Si.