Fiber Fabry-Perot cavities, formed by micro-machined mirrors on the end-facets of optical fibers, are used in an increasing number of technical and scientific applications, where they typically require precise stabilization of their optical resonances. Here, we study two different approaches to construct fiber Fabry-Perot resonators and stabilize their length for experiments in cavity quantum electrodynamics with neutral atoms. A piezo-mechanically actuated cavity with feedback based on the Pound-Drever-Hall locking technique is compared to a novel rigid cavity design that makes use of the high passive stability of a monolithic cavity spacer and employs thermal self-locking and external temperature tuning. Furthermore, we present a general analysis of the mode matching problem in fiber Fabry-Perot cavities, which explains the asymmetry in their reflective line shapes and has important implications for the optimal alignment of the fiber resonators. Finally, we discuss the issue of fiber-generated background photons. We expect that our results contribute towards the integration of high-finesse fiber Fabry-Perot cavities into compact and robust quantum-enabled devices in the future. I. INTRODUCTIONIn recent years optical high-finesse resonators with small mode volumes have become powerful tools for enhancing the interaction between light and matter. Resonator-enhanced interaction in Cavity Quantum Electrodynamics (CQED), for instance, provides the basis for the realization of efficient single photon interfaces in quantum communication and information [1, 2] and the study of quantum opto-mechanical systems [3].Among the geometries of optical micro-cavities that are currently investigated, optical Fiber Fabry-Perot Cavities (FFPCs) [4] are particularly attractive for CQED experiments because they combine several desirable features. Formed by dielectric mirrors on the endfacets of opposing optical glass fibers, FFPCs provide small mode volumes, high optical Q factors, direct access to the cavity mode and intrinsic fiber coupling of the mode field. Details of the fabrication [5,6] and optical characterization of fiber mirrors and cavities [7,8], including the effects of thermo-optical bistability [7] and cavity polarization mode splitting [9, 10], have been described in several recent studies. To date fiber Fabry-Perot cavities have been successfully applied in experiments interfacing single photons with a wide range of quantum systems, including cold atoms [4], ions [11], and solid state emitters [12][13][14] as well as quantum optomechanical experiments [15].The resonator-enhanced light-matter interaction in CQED experiments relies on the precise tuning of a cavity resonance to an optical transition of the quantum system under investigation. In ever more miniaturized and integrated experimental setups the task of stabilizing highfinesse FFPCs to within a small fraction of their optical linewidth (corresponding to mirror displacements of or- *

We observe a sixfold Purcell broadening of the D2 line of an optically trapped 87 Rb atom strongly coupled to a fiber cavity. Under external illumination by a near-resonant laser, up to 90% of the atom's fluorescence is emitted into the resonant cavity mode. The sub-Poissonian statistics of the cavity output and the Purcell enhancement of the atomic decay rate are confirmed by the observation of a strongly narrowed antibunching dip in the photon autocorrelation function. The photon leakage through the higher-transmission mirror of the single-sided resonator is the dominant contribution to the field decay (κ ≈ 2π×50 MHz), thus offering a high-bandwidth, fiber-coupled channel for photonic interfaces such as quantum memories and single-photon sources.

We demonstrate the parallel and nondestructive readout of the hyperfine state for optically trapped ^{87}Rb atoms. The scheme is based on state-selective fluorescence imaging and achieves detection fidelities >98% within 10 ms, while keeping 99% of the atoms trapped. For the readout of dense arrays of neutral atoms in optical lattices, where the fluorescence images of neighboring atoms overlap, we apply a novel image analysis technique using Bayesian inference to determine the internal state of multiple atoms. Our method is scalable to large neutral atom registers relevant for future quantum information processing tasks requiring fast and nondestructive readout and can also be used for the simultaneous readout of quantum information stored in internal qubit states and in the atoms' positions.

Recently we have demonstrated scalable, non-destructive, and high-fidelity detection of the internal state of 87 Rb neutral atoms in optical dipole traps using state-dependent fluorescence imaging [M. Martinez-Dorantes et al., PRL, 2017]. In this article we provide experimental procedures and interpretations to overcome the detrimental effects of heating-induced trap losses and state leakage. We present models for the dynamics of optically trapped atoms during state-dependent fluorescence imaging and verify our results by comparing Monte Carlo simulations with experimental data. Our systematic study of dipole force fluctuations heating in optical traps during near-resonant illumination shows that off-resonant light is preferable for state detection in tightly confining optical potentials.

Fabry–Perot interferometers have stimulated numerous scientific and technical applications ranging from high-resolution spectroscopy over metrology, optical filters, to interfaces of light and matter at the quantum limit and more. End facet machining of optical fibers has enabled the miniaturization of optical Fabry–Perot cavities. Integration with fiber wave guide technology allows for small yet open devices with favorable scaling properties including mechanical stability and compact mode geometry. These fiber Fabry–Perot cavities (FFPCs) are stimulating extended applications in many fields including cavity quantum electrodynamics, optomechanics, sensing, nonlinear optics and more. Here we summarize the state of the art of devices based on FFPCs, provide an overview of applications and conclude with expected further research activities.