Abstract
When shared between remote locations, entanglement opens up fundamentally new capabilities for science and technology. Envisioned quantum networks use light to distribute entanglement between their remote matter-based quantum nodes. Here we report on the observation of entanglement between matter (a trapped ion) and light (a photon) over 50 km of optical fibre: two orders of magnitude further than the state of the art and a practical distance to start building large-scale quantum networks. Our methods include an efficient source of ion–photon entanglement via cavity-QED techniques (0.5 probability on-demand fibre-coupled photon from the ion) and a single photon entanglement-preserving quantum frequency converter to the 1550 nm telecom C band (0.25 device efficiency). Modestly optimising and duplicating our system would already allow for 100 km-spaced ion–ion heralded entanglement at rates of over 1 Hz. We show therefore a direct path to entangling 100 km-spaced registers of quantum-logic capable trapped-ion qubits, and the optical atomic clock transitions that they contain.
Highlights
Envisioned quantum networks[1] consist of distributed matterbased quantum nodes, for the storage, manipulation and application of quantum information, which are interconnected with photonic links to establish entanglement between the nodes
While the most ambitious form of a quantum network is a collection of remote quantum computers, far simpler networks with a handful of qubits at each node could already enable powerful applications in quantum enhanced distributed sensing, timekeeping, cryptography and multiparty protocols.[2]
A current goal is to significantly scale up the distance over which quantum matter can be entangled to a 100 km or more, which are practical internode spacings to enable large-scale quantum networks
Summary
Entanglement has been achieved between two atoms in traps a few 10 m apart,[3] between two ions in traps a few metres apart[4] and recently between two nitrogen-vacancy centres 1.3 km apart.[5]. In these experiments, photon-matter entanglement is first generated, detection of one or two photons heralds remote matter-matter entanglement (entanglement is ‘swapped’ from matter-light to matter-matter). The use of photon conversion to extend the distance over which light-matter and matter-matter entanglement can be distributed has not previously been achieved
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