Quantum dynamics in arrays of superconducting qubits

Physical systems interact with each other and in the process they exchange information between them, which leads to correlations. In the case of interacting quantum mechanical systems the induced correlations often lead to counter-intuitive effects and their nature is highly nonclassical. It has been recognised that these fundamental correlations between quantum states, known as entanglement, can be used for the purposes of quantum information processing and quantum communication technologies. The entanglement between quantum states, that is, the nonclassical part of the correlations that are developed between coupled quantum systems, is vulnerable to the noise present in the external world. In general, noise causes the system to lose its coherence and reduces the correlations between its parts to purely classical ones. The aim of this project is to investigate the dynamics of certain quantum many-body systems, and in particular, to study the generation of quantum correlations between systems and their propagation from one system to another. The quantum systems that will be studied include spin systems and superconducting (Josephson) devices, which have been used successfully as physical implementations of quantum units of information (qubits) in the laboratory. Central to the objectives of the project is the description of the effects of noise on the dynamics of entanglement.In the last few years, experimental efforts have resulted in the fabrication of elementary quantum gates with coupled solid-state qubits. The main obstacle to operating larger quantum gates and networks is indeed noise; its precise influence is not yet fully understood and its detailed description is an important open problem. Another experimental challenge is the fabrication of a chain of superconducting qubits and its operation as a quantum communication channel. Experimental work on chains of superconducting qubits is currently under way in several laboratories around the world (for example, in the groups of Hans Mooij at Delft, Yasunobu Nakamura at Tokyo, and others). Although theoretical, the project will keep in touch with the experimental state of affairs in the superconducting quantum computation community. The proposed work will develop the relevant theory by looking at the influence of noise and disorder on the dynamics of entanglement in chains of superconducting qubits. The project will also address other important topics, such as the entanglement of geometric phases of distant superconducting qubits. A geometric phase is an additional phase acquired when the parameters of certain quantum systems, such as spin qubits, are varied slowly around a loop and then brought back to their original state. One possible way to entangle the geometric phases of distant spins or superconducting qubits is to make them interact with entangled photons (indeed, the coherent interaction of a superconducting qubit with a photon has recently been achieved experimentally by the group of Andreas Wallraff at Yale). It is possible that the entanglement between geometric phases, instead of the entanglement of quantum states themselves, is more resilient to noise and imperfections in the initial preparation and the external control mechanisms.The fellowship will be held at the Science and Technology Research Institute in the University of Hertfordshire. Parts of the proposed work will be carried out in collaboration with researchers in the University of Hertfordshire, Imperial College London, University of Cambridge and elsewhere. Up to eight months of the fellowship will be spent for collaborative work in the groups of Prof. J.E. Mooij in Holland (TU-Delft) and Prof. R. Fazio in Italy (Scuola Normale Superiore, Pisa).