Rydberg excited Calcium Ions for Quantum Interactions

Trapped cold ions are among the most advanced systems to implement quantum information processing. In current experiments entanglement of the qubits, represented by long lived internal atomic states, is achieved via quantum control of the (collective) motion of the ion crystal. Instead, we propose an unprecedented experimental program supported by theory, where the huge dipole moments associated with Rydberg excited ions are the basis of extremely strong spin-dependent long range interactions, and thus exceptionally fast entangling operations as basic building blocks for quantum computing and quantum simulation. While in the short term the fundamental questions to be explored are the understanding of Rydberg excitation and dynamics of single and multiple ions stored in linear Paul traps, and the various ways of manipulating this dynamics with external electromagnetic fields, the long term promise of this project is a potentially scalable very fast ion trap quantum processor, and in particular also a novel efficient quantum simulator of spin models, for Heisenberg type interactions to exotic matter with topological phases. A main experimental challenge is the requirement of a coherent light source near 122nm for the ion Rydberg excitation. Our consortium is in the remarkable and unique situation where in a single laboratory both these coherent light sources as well as advanced ion quantum computing setups are available, thus allowing us to explore this extremely promising new frontier of Rydberg ion quantum information processing on a comparatively short time scale. The planned experiments will be based on the well established techniques of ion trapping, quantum state detection and manipulation with laser fields. An adapted quantum shelving method is proposed to detect transitions to Rydberg states with unity detection efficiency on individual ions even in large crystals. Initially we will accurately determine energy levels and atomic properties of ion Rydberg states, and then we aim for mutual Rydberg state interactions of adjacent ions. Such gate interactions, Rydberg induced quantum phase transitions and a full tomography of the resulting quantum state benefit from the highly developed schemes in quantum information processing. In the future, beyond the experimental horizon of the three-year project, fast Rydberg ion quantum logic operations could possibly be combined with the conventional gate schemes and modern ion trap technology.

The planned research is part of a plethora of worldwide activities which share the common objective of turning quantum physics into useful technology. Although quantum physics is ubiquitous and is believed to underlie most natural phenomena, the control of systems on the quantum level is a difficult and thus very challenging task. One of the long term goals is to devise a quantum computer in which the classical bits are replaced by so-called qubits, which can be brought into superpositions of their logical states. This technology is still far (>10 years) from commercial products. However, en route it has given rise to spin-off applications which already today are about to enter the mass market. A prominent example is the utilization of quantum communication protocols for secure communication. On a longer timescale our research might yield insights into the physics of molecular reactions or energy transfer processes in biological cells. This could highlight new ways to perform chemical reactions, to devise highly efficient solar cells or to create entirely new materials with yet unknown properties. Each of these examples and, more generally, a Quantum Technology, has the potential to change the entire society and/or economy. The results of the research will be made available to a public audience. This will be done through the website that is created and maintained by the consortium. Postings on this page will contain short and easily understandable descriptions of the research projects and their outcome. All participants will provide postings about the results of the actual research and keep them up to date. In case of outstanding research results, which are for instance published in journals such as Nature and Science, press releases will be issued by the press offices of the participating universities which also liaise with the national media. All participants will assist in preparing these press releases, i.e. writing comprehensive summaries of the research results that are also understandable to a layman. The research described in this proposal will provide an excellent opportunity the employed PDRAs to develop his career and to build an international network. Being in embedded in a larger research group he will acquire and improve his skills in student supervision and dissemination of knowledge in local and international seminars and workshops.