Optimised surface-electrode ion-trap junctions for experiments with cold molecular ions

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 opens the door to extending complex chip-based trapping techniques to Coulomb-crystallised molecular ions with potential applications in mass spectrometry, spectroscopy, controlled chemistry and quantum technology.Comment: 19 pages, 10 figure

Scalable microchip ion traps and guides for cold molecular ions

Sympathetic cooling and Coulomb crystallisation of molecular ions above the surface of an ion-trap chip were demonstrated. N2+ 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. The structural and thermal properties of the crystals were characterized using molecular dynamics simulations. The effects of trap anharmoncities on the shape and energy of bicomponent crystals were theoretically investigated. It was shown that the trapping potentials can also deliberately be engineered to spatially separate ion species in bicomponent crystals. Furthermore, a multi-functional surface-electrode radiofrequency ion-trap chip has been developed to enable experiments with cold molecular ions using a monolithic device. The chip was designed to combine various tasks such as loading and preparation of ions, mass spectrometry, spectroscopy, reaction studies, and manipulation of ion crystals in a miniaturised device. This chip features carefully engineered ion channel intersections that enable transporting sympathetically cooled molecular ions in the form of bicomponent crystals. A detailed description of the fabrication and simulation of the two chips are presented. The present study extends chipbased trapping techniques to Coulomb-crystallised molecular ions with potential applications in mass spectrometry, cold chemistry, quantum technology, and spectroscopy

Shuttling of Rydberg ions for fast entangling operations

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 states, 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 2 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

Rydberg ions in coherent motional states: a new method for determining the polarizability of Rydberg ions

We present a method for measuring the polarizability of Rydberg ions confined in the harmonic potential of a Paul trap. For a highly excited electronic state, the coupling between the electronic wave function and the trapping field modifies the excitation spectra depending on the motional state of the ion. This interaction strongly depends on the polarizability of the excited state and manifests itself in the state-dependent secular frequencies of the ion. We initialize a single trapped ^40 Ca ^+ ion from the motional ground state into coherent states with $|\alpha|$ up to 12 using electric voltages on the trap segments. The internal state, firstly initialised in the long-lived 3D $_{5/2}$ state, is excited to a Rydberg S $_{1/2}$ -state via the 5P $_{3/2}$ state in a two-photon process. We probe the depletion of the 3D $_{5/2}$ state owing to the Rydberg excitation followed by a decay into the internal ground 4S $_{1/2}$ state. By analysing the obtained spectra we extract the polarizability of Rydberg states which agree with numerical calculations. The method is applicable to different Rydberg states regardless of their principal or angular quantum numbers. An accurate value of the state polarizability is needed for quantum gate operations with Rydberg ion crystals