We discuss the design and optimisation of two types of junctions between surface-electrode radiofrequency ion-trap arrays that enable the integration of experiments with sympathetically cooled molecular ions on a monolithic chip device. A detailed description of a multi-objective optimisation procedure applicable to an arbitrary planar junction is presented, and the results for a cross junction between four quadrupoles as well as a quadrupole-to-octupole junction are discussed. Based on these optimised functional elements, we propose a multi-functional ion-trap chip for experiments with translationally cold molecular ions at temperatures in the millikelvin range. This study extends complex chip-based trapping techniques to Coulomb-crystallised molecular ions with potential applications in mass spectrometry, spectroscopy, controlled chemistry and quantum technology.

We introduce a scheme to entangle Rydberg ions in a linear ion crystal, using the high electric polarizability of the Rydberg electronic states in combination with mutual Coulomb coupling of ions that establishes common modes of motion. After laser-initialization of ions to a superposition of ground-and Rydberg-state, the entanglement operation is driven purely by applying a voltage pulse that shuttles the ion crystal back and forth. This operation can achieve entanglement on a sub-µs timescale, more than two orders of magnitude faster than typical gate operations driven by continuous-wave lasers. Our analysis shows that the fidelity achieved with this protocol can exceed 99.9% with experimentally achievable parameters.

We present an experimental investigation of the Rydberg 23 P 1/2 state of single, laser-cooled 40 Ca + ions in a radiofrequency ion trap. Using micromotion sideband spectroscopy on a narrow quadrupole transition, the oscillating electric field at the ion position was precisely characterised, and the modulation of the Ryd-berg transition due to this field was minimised. From a correlated fit to this P line and previously measured P and F level energies of Ca II, we have determined the ionization energy of 95 751.916(32) cm −1 , in agreement with the accepted value, and the quantum defect for the n P 1/2 states.

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We report the sympathetic cooling and Coulomb crystallization of molecular ions above the surface of an ion-trap chip. N + 2 and CaH + ions were confined in a surface-electrode radiofrequency ion trap and cooled by the interaction with laser-cooled Ca + ions to secular translational temperatures in the millikelvin range. The configuration of trapping potentials generated by the surface electrodes enabled the formation of planar bicomponent Coulomb crystals and the spatial separation of the molecular from the atomic ions on the chip. The structural and thermal properties of the Coulomb crystals were characterized using molecular dynamics simulations. The present study extends chipbased trapping techniques to Coulomb-crystallized molecular ions with potential applications in mass spectrometry, cold chemistry, quantum information science and spectroscopy. The recent development of miniaturized trapping devices providing tightly confining, highly flexible trapping potentials has paved the way for new schemes for the precise control of neutral atoms and atomic ions. The ability to trap and cool neutral atoms on the surface of microstructured chips has enabled new experiments in the realms of, e. g., quantum optics, quantum interferometry and metrology [1][2][3]. Similarly, the development of radiofrequency (RF) ion-trap chips [4][5][6] has laid the basis for improved protocols for the manipulation, addressing and shuttling of ions which is of importance for, e.g., quantum information processing [7,8] and quantum simulation [9][10][11].While chip techniques for atomic species are by now well established, their extension to molecules has proven challenging. Translationally cold molecules and molecular ions are currently of great interest for applications in precision spectroscopy [12,13], cold chemistry and collision studies [14][15][16][17], quantum optics [18] and quantum information science [19,20]. However, significant difficulties arise because the complex molecular energy level structure precludes the implementation of closed optical transitions in most cases. Therefore, standard techniques of atomic physics like laser cooling and fluorescence detection of single particles are at best only applicable to a very restricted number of molecular systems [21]. For polar neutral molecules such as CO and Rydberg atoms, the deceleration and trapping on a chip has only recently been achieved using their interaction with time-varying inhomogeneous electric fields generated by microstructured electrodes on a surface [22][23][24].Here, we report the generation of Coulomb crystals, i.e., ordered structures of translationally cold and spatially localized molecular ions, above the surface of an ion-trap chip. The molecular ions were cooled sympathetically by the interaction with simultaneously trapped laser-cooled atomic ions [25,26]. The trapping potential configuration generated by our chip enabled the formation of two-and three-dimensional atomic-molecular bicomponent Coulomb crystals as well as the spatial separation of both...