Photon-linked entanglement of remote trapped ions in three experiments

Short answer: Remote entanglement links two trapped-ion qubits by having each ion emit a photon, interfering the photons, and treating a detection as a signal that the ions are entangled. Three experiments cover this from 2007 (ions one metre apart), 2020 (a fibre link with heralded Bell pairs at 94 percent fidelity) and 2023 (two ions in different buildings, 230 metres apart). All are laboratory demonstrations.

How does photon-mediated entanglement work?

The 2007 Nature abstract sets out the motivation. Large-scale quantum information processors need stable, addressable quantum memories, usually fixed qubits, and a way to transfer and entangle information between memories that may be separated by macroscopic or even geographic distances. The authors say atomic systems are excellent memories because internal electronic states can store qubits coherently for very long times, while photons are the natural carriers between remote qubits. They also note that single-atom memories, unlike atomic ensembles, allow conditional quantum gates through photonic channels.

What did the 2007 experiment show?

The authors demonstrate the entanglement of two fixed single-atom quantum memories separated by one metre. Two remotely located trapped atomic ions each emit a single photon, and the interference and detection of those photons signals the entanglement of the atomic qubits. They characterise the entangled pair by directly measuring qubit correlations with near-perfect detection efficiency. They say the method is probabilistic, but is still in principle useful for later quantum operations and scalable quantum information applications.

What did the 2020 fibre link show?

The 2020 Physical Review Letters paper demonstrates remote entanglement of trapped-ion qubits through a quantum-optical fibre link, with fidelity and rate that the authors say approach those of local operations. Two qubits are entangled through the polarization of two spontaneously emitted 422 nm photons, which are coupled by high-numerical-aperture lenses into single-mode optical fibres and interfere on a beam splitter. A novel geometry allows high-efficiency photon collection while maintaining unit fidelity for ion-photon entanglement. The authors generate heralded Bell pairs with a fidelity of 94 percent. The average rate figure did not render in the page text we read, so it is not quoted.

What did the 2023 network experiment show?

The 2023 paper reports an elementary quantum network of two atomic ions separated by 230 metres. The ions are trapped in different buildings and connected with 520(2) metres of optical fibre. At each node the electronic state of an ion is entangled with the polarization of a single cavity photon. After the photons interfere at a beam splitter, photon detection heralds entanglement between the two ions. The fidelity and success probability values did not render in the text we read, so they are not quoted. The authors say they analyse routes to improve these metrics, paving the way for long-distance networks of entangled quantum processors, which is their stated aim. A summary published by the journal describes it as long-distance entanglement between two calcium ions in different buildings.

How to read this

The three papers use different ions, photon wavelengths and geometries, so rates and fidelities are not directly comparable. Heralded means the detection event announces success, so most attempts fail and are discarded. This post is based on abstracts and visible summary text, and full texts were not checked. It makes no claim about operating networks or products.

FAQ

What does heralded entanglement mean?

A photon detection signals that the two ions are entangled, as in the 2007 and 2023 abstracts.

How far apart were the ions?

One metre in 2007, and 230 metres in different buildings in 2023.

What fidelity was reported in 2020?

94 percent for heralded Bell pairs.

Is the method deterministic?

The 2007 authors describe it as probabilistic.