Atomic memory keeps distant ions entangled for about 10 seconds

Long‑Lived Entanglement in a Two‑Node Trapped‑Ion Quantum Network

Quantum networks promise ultra‑secure communication and distributed quantum computing, but they hinge on the ability to create and preserve entanglement between distant qubits. A recent experiment from the University of Oxford showcases a practical step forward: using mixed‑species trapped ions to generate, transfer, and store entanglement across two separate nodes for more than ten seconds.

Mixed‑Species Nodes: Strontium for Links, Calcium for Memory

The researchers built a miniature network consisting of two nodes linked by optical fibers. Each node housed a pair of ions—one strontium (Sr) ion and one calcium (Ca) ion. The Sr ions served as the communication qubits: they emitted photons that traveled through the fibers to a central station, where a Bell‑state measurement projected the two distant Sr ions into an entangled state. This photon‑mediated link is the standard approach for establishing remote entanglement.

Once the Sr ions were entangled, local quantum operations transferred the shared quantum state onto the co‑located Ca ions. Calcium’s internal states are far less sensitive to ambient magnetic‑field noise, making them ideal memory qubits. By swapping the entanglement onto Ca, the team could preserve the quantum correlation far longer than the millisecond‑scale coherence of the communication qubits.

Demonstrating Multi‑Ion, Multi‑Node Entanglement

Using this hybrid strategy, the Oxford team generated entangled states involving up to four ions—two Sr and two Ca—distributed across the two nodes. The process unfolded in three stages:

1. Remote entanglement of the Sr ions via photon detection.
2. Local entanglement swapping to extend the correlation to the Ca ions within each node.
3. Storage of the resulting mixed‑species entangled state in the Ca memory qubits.

The key metric was the lifetime of the stored entanglement. Measurements showed that the Ca‑ion memory retained the quantum correlation for roughly 10 seconds, a dramatic improvement over the sub‑millisecond coherence of the Sr‑ion link qubits.

Why Ten Seconds Matters

A ten‑second entanglement window opens practical avenues for quantum technologies:

• Distributed quantum computing: Separate quantum processors could synchronize operations over a timescale that allows for error‑correction cycles and intermediate classical processing.
• Quantum‑enhanced sensing: Entangled remote sensors could perform joint measurements with a coherence time long enough to integrate weak signals.
• Clock comparison and secure communication: Long‑lived entanglement enables high‑precision synchronization of distant atomic clocks and robust quantum key distribution over fiber links.

These capabilities hinge on the ability to store entanglement, not merely generate it fleetingly. The mixed‑species approach leverages the strengths of each ion: Sr for efficient photon emission and Ca for stable internal states.

Looking Ahead: Scaling and Protocol Complexity

The Oxford experiment is a proof‑of‑concept that demonstrates both the feasibility of mixed‑species entanglement swapping and the practical benefit of long‑lived quantum memory. Future work will likely focus on:

• Increasing node count to build larger, multi‑hop networks.
• Integrating error‑correction protocols that exploit the extended memory time.
• Optimizing photon‑collection efficiency to raise entanglement generation rates.
• Exploring other ion species that may offer complementary advantages for specific applications.

By combining local processing, long‑term storage, and photonic links, such networks could become the backbone of a future quantum internet.

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This article synthesizes the findings of the Oxford team’s recent publication in Physical Review Letters (2026) and places them in the broader context of quantum networking research.