| Summary: | <p>Hybrid quantum systems, combining the advantages of matter-based carriers of quantum information with those of light, have potential applications across many domains of quantum science and technology. In this thesis, we present a high-fidelity, high-rate interface between trapped ions and polarisation-encoded photonic qubits, based on the spontaneous emission of 422 nm photons from ⁸⁸Sr⁺, entangled in polarisation with the resulting electronic state of the ion.</p>
<p>We show that photons can be efficiently collected perpendicular to the ambient magnetic field without loss of polarisation purity by exploiting the symmetry properties of single-mode optical fibres, and analyse the impact of a number of common experimental imperfections, including in the heralded entanglement swapping step used to probabilistically generate entanglement between remote ion qubits.</p>
<p>Our experimental platform consists of two ⁸⁸Sr⁺–⁴³Ca⁺ mixed-species quantum network nodes, linked by 2 × 1.75 m of single-mode optical fibre. We measure an ion–photon entanglement fidelity of 97.7(1) %, generated at an attempt rate of 1 MHz and up to 2.3 % overall collection/detection efficiency. Bell states between remote ⁸⁸Sr⁺ ions are generated at a fidelity of 96.0(1) % and rate of 100 s⁻¹. This is the highest fidelity for optically mediated entanglement between distant qubits reported across all matter qubit platforms, and the highest rate among those with fidelities >70 %.</p>
<p>To compensate stray electric fields that would cause a periodic modulation of the ion position, we introduce a versatile method which relies on the synchronous detection of parametrically excited motion through time-stamped detection of photons scattered during laser cooling. Crucially, only a single laser beam is required to resolve fields in multiple directions; we achieve a stray field sensitivity of 0.1 V m⁻¹ / √Hz.</p>
<p>Finally, we present the first experimental demonstration of device-independent quantum key distribution, by which two distant parties can share an information-theoretically secure private key even in the presence of an arbitrarily powerful eavesdropper without placing any trust in the quantum behaviour of their devices. This is enabled by a record-high detection-loophole-free CHSH inequality violation of 2.677(6) and low quantum bit error rate of 1.44(2) %, stable across millions of Bell pairs, and an improved security analysis and post-processing pipeline. We implement the complete end-to-end protocol in a realistic setting, allowing Alice and Bob to obtain a 95 884-bit key across 8.5 hours that is secure against the most general quantum attacks.</p>
<p>Our results establish trapped ions as a state-of-the-art platform for photonic entanglement distribution at algorithmically relevant speeds and error rates. The link performance nevertheless remains far from fundamental limits; further improvements are discussed from the perspective of large-scale modular quantum computation as well as from that of long-distance quantum networking applications.</p>
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