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Abstract:

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.

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.

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 %.

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.

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.

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.