Design, synthesis and study of coordination complexes for quantum computing

Abstract

[spa]El trabajo realizado en esta tesis doctoral se basa en el diseño, la síntesis y el estudio de complejos de coordinación, centrándose en la comprensión de sus propiedades magnéticas y la posibilidad de su aplicación en la computación cuántica. Para el diseño de estos materiales moleculares, tres diferentes propuestas han sido llevadas a cabo. En primer lugar, se han desarrollado ligandos capaces de agregar metales paramagnéticos en dos grupos diferentes, definiendo de esta manera los dos posibles bits cuánticos de una puerta lógica. Complejos de coordinación homo- y heterometálicos con NiII, CoII y CuII han sido sintetizados y caracterizados para tal efecto. La segunda estrategia seguida ha estado centrada en el diseño de complejos de coordinación lineales para su posterior ensamblaje en parejas de compuestos. Se han desarrollado ligandos que favorezcan la complejación de este tipo de topología, obteniéndose un compuesto de CoII con las propiedades estructurales idóneas para su ensamblaje. Utilizando el ligando bifuncional 4.4’-bipiridina, se ha podido unir dos entidades [Co4] obteniendo así otro prototipo de “parejas moleculares”. La tercera estrategia se ha centrado en el diseño de moléculas asimétricas para facilitar la definición de cada bit cuántico dentro de la entidad molecular. Para ello, se ha sintetizado un ligando no simétrico, que ha sido utilizado para obtener complejos dinucleares homo- y heterometálicos de iones lantánido. Se ha obtenido compuestos con todos los elementos de la serie de los lantánidos. Su estudio magnético y estructural ha mostrado que los dos centros metálicos de estas entidades moleculares son distintos, lo que ha permitido definir el espín de cada ion lantánido como un bit cuántico. El estudio magnético a muy bajas temperaturas de un compuesto de dos átomos de terbio(III), por ejemplo, ha permitido definir dos puertas lógicas: la CNOT y la √SWAP. Utilizando el espectro de energías de los estados magnéticos de la molécula, se han observado las transiciones entre dichos estados en relación a las dos operaciones lógicas.

[eng]This thesis presents different strategies for the design of molecular complexes with the requirements to be used as two-qubit quantum gates. The approaches followed towards the preparation of potential qubit systems have been carried out focusing on the synthesis of ligands with β-diketone coordination units, which are very versatile for the design of metallocluster assemblies. One of the main advantages of using this kind of ligands is that they can be easily prepared through simple Claisen condensation, providing different combinations and possibilities for the addition of big variety of donor atoms and pockets. Each ligand has been designed for the preparation of predefined magnetic coordination complexes that can fulfill the conditions of a two-qubit molecular quantum gate. The different complexes synthesized within this thesis can be defined in two different categories: Molecular pairs of well-defined and weakly coupled metal clusters, and complexes of two dissimilar and weakly coupled anisotropic metal ions. In addition, the use of the designed ligands for the preparation of metallo-helicates has been also carried out. The description and study of their helicoidal structure is also shown, using the ligand H4L1 with trivalent and tetravalent metal ions like FeIII, GaIII or UIV. - Molecules featuring two weakly coupled clusters: The approach is based on the design of molecules featuring two well defined coordination clusters that could represent ideal systems for realizing two-qubit quantum gates, as long as each cluster exhibits the appropriate spin properties. Two different ligand-based strategies have been followed for the preparation of such molecular pairs of well-defined metal clusters. The first one is based on the design of poly-β-diketone ligands exhibiting two groups of coordination pockets, which serve to aggregate metals in close proximity. Following this approach, the ligand is responsible for having metals grouped into two clusters, as well as for keeping each metal group together within each subsystem. The structural characteristics of H4L1 provides the requirements for the construction of this kind of clusters, since it might align and separate metals into two dimetallic entities. The goal has been achieved by using the deprotonated ligand that organizes the metals in two groups within molecular linear arrays, saturating the equatorial positions. Compounds with NiII, CuII and CoII metal ions are described and studied. The second strategy is based on the preparation of poly-β-diketone ligands with an additional X donor atom in the middle for acting as a “template” for the aggregation of metals into linear clusters, further linked as molecular pairs by auxiliary ligands Organic ligands like H4L2 or H2L3 fulfill the requirements to aggregate closely spaced metals, which can be then used as building blocks to be linked into molecular pairs by other bifunctional external ligands. An example using CoII is described and studied. - Dinuclear complexes of anisotropic metal ions: The synthesis of complexes of two dissimilar and weakly coupled lanthanides has been used as approach for the construction of molecular prototypes of CNOT quantum gates. The ligand-based strategy considers the design of non-symmetric ligands as a possible way of having two inequivalent lanthanide qubits within a molecule. The ligand H3L4 exhibits a collection of donor groups disposed to favor the aggregation of two metals in different coordination environments. The use of lanthanide ions are good candidates for encoding quantum information following such approach, since they can exhibit strong anisotropy and very well isolated ground state doublets ±mJ (effective S = ½). In addition, lanthanide ions have been proved to have spin states with long decoherence, with T2 timescales that can reach values up to 7 μs.[41] A detailed study of a vast number of dinuclear homo- and heterometallic lanthanide coordination complexes is exposed, including an exhaustive study for some of them to prove their possibilities as CNOT and √SWAP quantum gates.