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Coupling of trapped ions via normal metal and superconductor transmission lines

Final Report Summary - QOSC2007 (Coupling of trapped ions via normal metal and superconductor transmission lines)

Introduction:
Trapped ions offer a unique system for the study of quantum information and constitute one of the most promising candidates for the implementation of a quantum computer. Extreme control of their quantum state, quantum gates and elementary algorithms have been demonstrated. Currently, several different schemes for the transfer of quantum information, the information encoded in the electronic state of a trapped ion, are being explored, with the aim of constructing a scalable quantum computer. Our motivation in this project was to provide a solid-state quantum bus for trapped ions, in order to couple the motional states of distant trapped ions. This route is of great technological and fundamental importance, extending beyond the scope of ion-trap quantum computation. It will allow the coupling of atomic systems to mesoscopic solid-state systems, a very active field of research. In addition, it can give rise to a new set of experimental tools for studying electronic transport and noise in normal metals and superconductors.

Trap fabrication:
One key requirement in ion trap technology is the availability of micro-fabricated ion traps. A decrease in the size of ion traps is necessary, as it will lead to increased speed of the quantum-logical operations one can achieve. In addition, ion-trap chips lithographically fabricated in a 'clean room' micro-fabrication environment offer the great promise of ultimate scalability, very much like the paradigm of semiconductor integrated circuits. In collaboration with the group of A. Wallraff in Zurich, we have developed such a micro-trap fabrication process.

Current technology limitations:
To date, ion trap miniaturisation is impeded by two major problems: excessive electric field noise originating from trap electrodes, and electrostatic charging of the traps. The former problem exceeds the boundaries of the ion trap community. In fact, an environment with minimal electric field noise is important in many fields in science, for example nanomechanics, single spin detection, and the measurement of weak forces. Experiments with single ions trapped above metallic surfaces reveal that electric noise is several orders of magnitude stronger than that expected from Brownian motion of electrons in the trap electrodes. A known way to reduce electronic noise on trap electrodes is by operating the trap at cryogenic temperatures. Thus, construction of cryogenic ion-trap systems is a direction which many ion-trap groups are taking.

Advances in the noise problem:
In the course of our work, we used single trapped ions to address both problems mentioned above: we used a single trapped ion as a movable and extremely sensitive electronic noise sensor, and as a movable electrostatic field sensor. These advances allowed us to shed new light into the possible mechanism of anomalous heating in ion traps, and to correlate trap charging with trap operation. Especially with regard to the noise problem, we established a possible connection between electric field noise and surface contamination of trap electrodes. As a result, the possibility of using surface cleaning and characterization tools to attack the problem of electric field noise is starting to be accepted by the ion trap community as a promising solution that could be used to achieve outstanding ion trap performance. This has culminated in a new direction for ion trap groups: the combination of the standard atomic phycisist's toolbox with those of the surface scientist to try and bring modern ion traps to their full potential. We anticipate this to turn into a rather active field of research within, and perhaps even beyond, the ion trap community.

Electrostatic field sensing:
In various modern ion trap experiments it is necessary to optimise the trap parameters in the presence of unknown, and possibly changing, quantities, such as stray charges on the experimental setup.

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