1. Periodic Report Summary - 40CACQED (Entanglement with trapped ions in an optical cavity)

Abstract: The stated research objectives of the Marie Curie project were 'to demonstrate entanglement and quantum state mapping in an ion-trap CQED (cavity quantum electrodynamics) system, to investigate underlying mechanisms for decoherence through coupling to the environment, and to lay the groundwork for strong coupling between ions and cavities.' These goals represent important but technically challenging steps toward a quantum network. Scalable quantum networks are currently an outstanding challenge for the quantum information science community; a quantum network could link quantum computing sites over long distances and enable capabilities such as quantum key distribution and teleportation.

We have recently demonstrated entanglement for the first time in an ion-trap cavity experiment, and we are currently preparing these results for publication. We have all the necessary tools at hand to carry out quantum state mapping experiments, which are planned in the lab for September 2011. Ongoing investigations of decoherence have been crucial to the implementation of both of these experiments. Finally, we have made significant progress in constructing a new ion-trap experiment integrated with a fibre-based cavity, with which we expect to access the strong coupling regime.

The demonstration of entanglement is the result of a series of important steps forward in the lab over the duration of the Marie Curie project. The emission of single photons from the ion-cavity system was characterised and simulations were developed that allow us to understand the dynamics of this process. An additional laser at 729 nm was implemented in order to address the calcium quadrupole (qubit) transition, with which we now coherently manipulate and read out quantum information stored in the ions. We have carried out Raman spectroscopy of the ion-cavity system and have developed new techniques to optimize coupling of the ion to the cavity. Finally, individual addressing and camera detection have been implemented, essential tools for future experiments with two or more ions in the cavity.

The development of the fibre-based cavity represents a collaboration with the group of Prof. J. Reichel at ENS, Paris. Construction and testing of these cavities drew on the researcher's background in neutral-atom CQED in the group of Prof. H. J. Kimble (Pasadena, USA), while the design of a miniaturised ion trap required expertise present in the host group in Innsbruck. We have successfully trapped ions using this new trap under ultra-high vacuum. In parallel, we have demonstrated that a high-finesse cavity can be constructed with more than twice the fibre-to-fibre separation previously used in neutral atom experiments; by increasing the cavity length, perturbations of the trap potential from the dielectric fibres are suppressed.

After extensive testing, we have found that our initial design for integrating cavity and trap was not sufficiently robust. We have subsequently developed a new design, which has the advantage that we will be able to adjust the position of each fibre separately in the vacuum environment. Testing of this design will begin in the next month. Coupling ions to fibre cavities is a research topic of significant international interest, and similar projects are underway in research groups in Germany, the United Kingdom, and the United States. We are currently well-positioned to make important contributions to this field.

The Marie Curie researcher has accepted a position to lead the cavity project in Innsbruck as a university assistant. She is currently supervising a team of three PhD students and two master's students.

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