CONQUEST · CONtrolled Quantum Coherence and Entanglement in Sets of Trapped particles

Final Activity Report Summary - CONQUEST (CONtrolled Quantum Coherence and Entanglement in Sets of Trapped particles)

The pioneering experimental success obtained in trapping individual quantum system, i.e. cold atoms, stimulated the growth of interest in quantum information processing and communication. Microscopic traps for atoms called atom chips, in analogy with computer chips, represent a powerful tool for the controlled engineering of the quantum state of ultracold atoms. In order to achieve efficient manipulation of atoms in atom chips, we investigated decoherence phenomena threatening the coherent control of the quantum state of an atom. The small separation between an atom and the hot environment of an atom chip raised the question of the smallest distance that could actually be achieved while maintaining a strong atom confinement. Three different scenarios were considered, namely a carbon nanotube, a dielectric surface and a superconducting atom chip, and the interaction of an atom with the substrate was studied in terms of the minimum trapping distance and the maximum trapping time. Moreover, a remarkable outcome of the project was the realisation of an ideal photon counting. We realised such a measurement for the field trapped in a modern equivalent of Einstein photon box, a very high quality cavity. This experiment illustrated all the basic postulates of quantum measurements and opened the way to many studies on non-classical field states and decoherence. We also realised the trapping of atoms on a superconducting atom chip for the first time. In addition, we worked on the production of indistinguishable single photons, emitted by two independently trapped atoms. The first step of this experiment, published in Science, consisted in the demonstration of coherent Rabi oscillations for a single trapped atom. The second step, published in Nature, was the observation of the ‘coalescence’, or Hong-Ou-Mandel, effect for two photons emitted by two independently trapped atoms. Another subject was about encoding and moving a qubit on a single trapped atom. This experiment was partly done on a new experimental setup involving an aspherical lens. Furthermore, possible ways to use these results for quantum logic at the single atom level were theoretically studied, as well as their extension to holographic trap arrays. Two experimental systems emerged as particularly promising embodiments for quantum information processing with neutral atoms. Optical lattices allowed for a large number of qubits because of their three-dimensional periodic structure. In magnetic microtraps, i.e. atom chips, individual addressing of the qubits was possible, complex potentials could be realised and lithographic fabrication techniques enabled scalability and modularity in analogy with microelectronics. We developed the main ingredients for a quantum gate on an atom chip. A suitable qubit was identified, for which very long coherence lifetimes were experimentally obtained. A new manipulation technique for ultracold atoms based on microwaves on a chip was introduced and was crucial for quantum gate operation. Topological quantum computation is naturally fault-tolerant, thereby circumventing the need for demanding engineering approaches to fault-tolerance. Natural fault-tolerance in this project came directly from quantum physics of strongly correlated two-dimensional many-body quantum systems. Quantum information was stored and processed within quantum states that were sensitive to solely global, topological, structure of these systems. Quantum systems with this property were said to be in a topological phase whose identification and physical realisation was a critical step in the implementation of topological quantum computation. Finally, we investigated topological phases as essential materials for topological quantum computing. This research was important for the identification of topological order in two-dimensional quantum systems and its application in the realisation of naturally fault-tolerant quantum information processing and particularly topological quantum computing.

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