Quantum technologies

In this work I present the experimental realization of a versatile platform for the interplay between light and matter at the single-quanta level. In particular, I demonstrate the high cooperativity of small ensembles of rubidium atoms strongly coupled to a state-of-the-art, open fiber-based microcavity, emphasizing the capabilities of the system as an efficient source and storage device for single photons.

The first part of this thesis focusses on the construction and characterization of the microresonator, which is composed of two dielectric mirrors machined on the end-facets of optical glass fibers. Through the implementation of an in-house facility, a large number of fiber-based mirrors are manufactured and precisely characterized. The intrinsic properties of this particular type of resonator are then analyzed and discussed. I present a theoretical model that explains, for the first time, the asymmetry in their reflective line shape and that has important implications for the optimal alignment of fiber-based cavities.

In the following chapter, I introduce the main experimental apparatus, which contains a miniaturized fiber cavity — with small mode volume and a linewidth of ϰ=2π×25 MHz — that is actively stabilized and integrated in a compact assembly. The monolithic structure features several high–numerical aperture (NA) lenses that provide the necessary tools for the trapping, manipulation and high-resolution imaging of atoms inside the resonator. Neutral rubidium atoms are delivered by an optical conveyor belt from the cooling region into the cavity mode, where their deterministic coupling to the resonator is ensured by the tight confinement of a 3D optical lattice. The high linewidth of our open cavity also prevents the manifestation of cavity-heating mechanisms, enabling a constant monitoring of the atom’s presence by probing the cavity field without increased trap losses. This atom detection method allows us to perform real-time optical feedback in the transport scheme and to observe the characteristic vacuum Rabi splitting for individual atoms in a non-destructive manner.

The rest of this work focusses on the interplay between atomic and photonic excitations inside the resonator. Due to the small mode volume of the microcavity, coupling strengths up to g=2π×100 MHz are observed for single atoms, corresponding to light–matter interaction in the strong coupling regime. The system’s cooperativity is collectively enhanced more than five times when placing a small atomic ensemble inside the resonator. Such a fast interaction rate — along with the relatively high transmission of the input cavity mirror — provides a rapid, non-destructive readout of the internal hyperfine state of a coupled atom when probing the cavity field. The state-detection method yields fidelities of 99.8% in 5 ms with less than 1% population transfer and negligible atom losses.

The last part of the thesis is dedicated to the study of the influence of the cavity on the emission properties of an atom. I show how, despite the small solid angle covered by the cavity mode, the resonator alters the radiation pattern of an externally pumped single atom and increases its emission rate by a factor of 20 due to the process known as cavity back-action. More than 85% of the emitted photons are collected by the single cavity-mode — as a result of the strong Purcell enhancement — and subsequently channeled out by one of the fiber mirrors. A characterization of the photon statistics of the cavity output shows a clear antibunching dip, confirming that the emission corresponds to a single quantum emitter and that our system can be used as a readily fiber-coupled, efficient single-photon source. The geometry of our fiber-based resonator provides wide optical access that — in combination with the high-NA lenses — allows us to study the free-space emission rate of an atom coupled to the cavity. The various coupling strengths associated to different positions of the atom in the cavity mode lead to a clear visualization of the cavity back-action for all cooperativity regimes.

The high cooperativity, intrinsic fiber coupling and scalability properties of our system make it suitable for the realization of an efficient, high-bandwidth quantum memory and its implementation in quantum networks. Additionally, the ability to couple an ensemble of indistinguishable atoms to the same cavity mode provides a versatile platform for the study of multipartite entangled states.

We demonstrate the parallel and nondestructive readout of the hyperfine state for optically trapped ⁸⁷Rb 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.

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.