QUANSIP · Quantum Networking with Single Ions and Photons

Final Activity Report Summary - QUANSIP (Quantum Networking With Single Ions and Photons)

Strings of laser-cooled ions confined in radio-frequency traps, presently are the most advanced system for quantum information processing. Qubits are stored and manipulated in electronic states of the ions. The power of quantum computation would be greatly extended by distributing it among multiple processing nodes, connected in a quantum network. The obvious choice for linking the nodes are photons, which are already used as qubit-carriers in quantum communication. The principal task of the project QUANSIP was to develop the foundations of a coherent interface between the quantum states of photons and ions to map quantum information with high fidelity. As atom-light coupling in free space is weak, it must be enhanced using cavity quantum-electrodynamics. An ion placed in an optical cavity interacts with light more strongly, the smaller the cavity volume is. For microscopic cavities, the enhancement can be many orders of magnitude. The technological challenge is to tightly integrate these miniature cavities in radio-frequency ion traps without disturbing the trapping field. The cavity sizes and hence the achievable ion-photon coupling strengths crucially depend on the chosen geometry. In the course of the project, we have investigated three different geometries: The first setup developed has a cavity collinear with the axis of a linear trap. At a mirror separation of 7mm, only moderately strong ion-photon coupling is achieved. Nevertheless, the entanglement of ions and photons emitted from the cavity can be used for probabilistic processes. During the QUANSIP project, we have developed a scheme to generate highly entangled states of ions in the cavity. Simultaneous measurement of two photons emitted from the cavity with orthogonal polarization can be used to entangle pairs of ions in a string. The experimental setup is complete. We have trapped large laser-cooled strings of ions, aligned them with the cavity axis, locked the cavity at an arbitrary detuning and generated photons from the cavity, meeting all requirements to produce ion-ion entanglement probabilistically. To realise a deterministic ion-photon interface, stronger coupling is necessary. To this end, we have developed a system with a cavity oriented transverse to the axis of the trap. In this case, deterministically controlled single-photon generation and quantum state mapping are possible. By analogy with ions coupled to a collective phonon mode, ions simultaneously interacting with the cavity mode may even be entangled through the exchange of photons. We attempted different approaches to miniaturize the transverse dimensions of the trap electrodes, with four gold-plated thin alumina-substrates being the most successful one. In this system, we have established interaction between the cavity and up to seven ions stored in a linear string. We expect to implement quantum interfaces in the coming months. For even stronger interaction, we have constructed a microscopic endcap trap for a single ion, in which optical fibres are tightly integrated, separated from the ion by only 200µm. We could demonstrate the robustness and efficiency of the system by capturing the quantum light emitted by a single calcium ion. Under cw-excitation, we have observed strong antibunching in the g(2) function of fluorescent light, while for pulsed excitation fibre-guided single photons on demand were generated. No adjustments were required for the injection and extraction of photons via the optical fibres. To convert the system to a cavity-QED setup, at least one of the fibres' end-facets must have a small radius of curvature. We have developed a CO2-laser system to machine the end-facets to a radius of curvature as small as150µm. After application of a high-reflectivity coating, the fibre ends form a microscopic cavity around the ion, replacing the conventional mirrors used in the experiments mentioned above. In this way, ultra-strong coupling conditions can be achieved.

Data: CORDIS, © European Union