Thomas Doherty

Abstract:

We characterise an efficient optically-heated neutral atom source for ion trapping. We observe loading rates of up to 24(3) s−1 with heating powers below 85 mW, and demonstrate loading of a single ion in under 30 s with 41.4(4) mW of optical power in a room-temperature ion trap system with an ionisation probability of 1.50(5) × 10−5 . We calibrate a thermal model for the source’s internal temperature by imaging the fluorescence of a collimated flux of neutral calcium that effuses from the source at various optical heating powers. We show that the thermal performance of this source is mainly limited by radiative losses. We explore the effect of second-stage photo-ionisation laser power on the loading rate, and identify a path beyond the loading rates reported in this study. We predict that this source is also well-suited to a wide range of metals used in ion trapping.

Abstract:

The transverse field structure and diffraction loss of the resonant modes of Fabry-Pérot optical cavities are acutely sensitive to the alignment and shape of the mirror substrates. We develop extensions to the mode-mixing method applicable to arbitrary mirror shapes, which both facilitate fast calculation of the modes of cavities with transversely misaligned mirrors and enable the determination and transformation of the geometric properties of these modes. We show how these methods extend previous capabilities by including the practically motivated case of transverse mirror misalignment, presenting the ability to study the rich and complex structure of the resonant modes.

Abstract:

We present a study on the optical losses of Fabry-Pérot cavities subject to realistic transverse mirror misalignment. We consider mirrors of the two most prevalent surface forms: idealised spherical depressions, and Gaussian profiles generated by laser ablation. We first describe the mode mixing phenomena seen in the spherical mirror case and compare to the frequently-used clipping model, observing close agreement in the predicted diffraction loss, but with the addition of protective mode mixing at transverse degeneracies. We then discuss the Gaussian mirror case, detailing how the varying surface curvature across the mirror leads to complex variations in round trip loss and mode profile. In light of the severe mode distortion and strongly elevated loss predicted for many cavity lengths and transverse alignments when using Gaussian mirrors, we suggest that the consequences of mirror surface profile are carefully considered when designing cavity experiments.

Abstract:

We report the realisation of a high-finesse open-access cavity array, tailored towards the creation of multiple coherent light-matter interfaces within a compact environment. We describe the key technical developments put in place to fabricate such a system, comprising the creation of tapered pyramidal substrates and an in-house laser machining setup. Cavities made from these mirrors are characterised, by laser spectroscopy, to possess similar optical properties to state-of-the-art fibre-tip cavities, but offer a compelling route towards improved performance, even when used to support only a single mode. The implementation of a 2×2 cavity array and the independent frequency tuning between three neighbouring sites are demonstrated.

Abstract:

In the design optimization of ion-cavity interfaces for quantum networking applications, difficulties occur due to the many competing figures of merit and highly interdependent design constraints, many of which present “soft limits,” which are amenable to improvement at the cost of engineering time. In this work, we present a systematic approach to this problem that offers a means to identify efficient and robust operating regimes and to elucidate the trade-offs involved in the design process, allowing engineering efforts to be focused on the most sensitive and critical parameters. We show that in many relevant cases it is possible to approximately separate the geometric aspects of the cooperativity from those associated with the atomic system and the mirror surfaces themselves, greatly simplifying the optimization procedure. Although our approach to optimization can be applied to most operating regimes, here we consider cavities suitable for typical ion-trapping experiments and with substantial transverse misalignment of the mirrors. We find that cavities with mirror misalignments of many micrometers can still offer very high photon extraction efficiencies, offering an appealing route to the scalable production of ion-cavity interfaces for large-scale quantum networks.