We present the realization of a combined trapped-ion and optical cavity system, in which a single Yb + ion is confined by a micron-scale ion trap inside a 230 µm-long optical fiber cavity. We characterize the spatial ion-cavity coupling and measure the ion-cavity coupling strength using a cavity-stimulated Λ-transition. Owing to the small mode volume of the fiber resonator, the coherent coupling strength between the ion and a single photon exceeds the natural decay rate of the dipole moment. This system can be integrated into ion-photon quantum networks and is a step towards cavity quantum-electrodynamics (cavity-QED) based quantum information processing with trapped ions.Trapped atomic ions play an important role in studies of small, isolated quantum systems, for example in quantum information processing and precision metrology. Aside from the long achievable coherence times, their success is largely based on the excellent manipulation and interrogation possibilities of their internal quantum states, which are usually performed by optical means. In order to employ the outstanding properties of trapped ions for future applications such as cavity-QED based quantum computers [1] or quantum network nodes [2][3][4], strong coupling between a single ion and a single photon is a prerequisite, i.e., the coherent coupling strength must exceed the decoherence rate of the atomic dipole moment. Unlike for neutral atoms [5,6] and solid-state emitters, such as quantum dots [7] or Cooper pairs [8], this strong coupling regime has not yet been reached for a single trapped ion despite decade-long efforts [9][10][11][12][13][14][15].The route to achieve strong light-matter coupling employs the principles of cavity-QED; a resonator changes the mode structure of the vacuum electromagnetic field in order to strongly enhance coupling to one photon mode. The coupling strength g between a single emitter and a single-photon mode depends on the mode-volume V of the cavity and on the electric dipole moment d of the transition, g ∝ d/ √ V . Owing to the large mode volumes of the cavities used in previous experiments [9][10][11][12][13][14][15], the coherent single-photon coupling rate g has been inferior to the decay rate Γ of the atomic dipole moment. The main restriction has been that the crucial ingredient to achieve strong coupling, namely placing the ion near dielectric mirror surfaces which are necessary to form an optical cavity, has been found to severely compromise the performance of a Paul trap [16]. For sizing down both the mode-volume and the amount of dielectric material, the development of cavities based on optical fibers [17] has opened a new perspective, also with respect to the integration of optical elements into microchip-based ion traps. Optical fiber cavities offer significantly smaller radii of curvature of the mirrors, which lead to a small waist of the field mode inside the optical cavity. Recently, significant experimental efforts have been devoted to integrating optical fibers with ion traps for efficient light c...

Time-resolved photoemission with ultrafast pump and probe pulses is an emerging technique with wide application potential. Real-time recording of nonequilibrium electronic processes, transient states in chemical reactions, or the interplay of electronic and structural dynamics offers fascinating opportunities for future research. Combining valence-band and core-level spectroscopy with photoelectron diffraction for electronic, chemical, and structural analyses requires few 10 fs soft X-ray pulses with some 10 meV spectral resolution, which are currently available at high repetition rate free-electron lasers. We have constructed and optimized a versatile setup commissioned at FLASH/PG2 that combines free-electron laser capabilities together with a multidimensional recording scheme for photoemission studies. We use a full-field imaging momentum microscope with time-of-flight energy recording as the detector for mapping of 3D band structures in (kx, ky, E) parameter space with unprecedented efficiency. Our instrument can image full surface Brillouin zones with up to 7 Å−1 diameter in a binding-energy range of several eV, resolving about 2.5 × 105 data voxels simultaneously. Using the ultrafast excited state dynamics in the van der Waals semiconductor WSe2 measured at photon energies of 36.5 eV and 109.5 eV, we demonstrate an experimental energy resolution of 130 meV, a momentum resolution of 0.06 Å−1, and a system response function of 150 fs.

Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantum dot and a trapped ion and we demonstrate that single photons generated by a quantum dot controllably change the internal state of an Yb + ion. We ameliorate the effect of the sixty-fold mismatch of the radiative linewidths with coherent photon generation and a high-finesse fiber-based optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the σz-projection of the quantum-dot spin and the internal state of the ion provides a promising step towards quantum state-transfer in a hybrid photonic network.