In a study published in Physical Review Letters, physicists at the University of Oxford have demonstrated a heterogeneous quantum network that generates multipartite entangled states across separate trapped-ion modules using two distinct atomic elements: strontium (88Sr+) and calcium (43Ca+).
Detailed in the paper (“Multipartite Mixed-Species Entanglement over a Quantum Network”, Phys. Rev. Lett. 137, 160801), the team generated up to 4-qubit Greenberger-Horne-Zeilinger (GHZ) states across macroscopic distances and stored the distributed entanglement in calcium memory qubits for more than 10 seconds—extending entanglement lifetimes by more than 100x over the creation time.
| [ Summary of Mixed-Species Remotely Entangled States (Oxford Network) ] | |||
|---|---|---|---|
| Entangled State / Target | Component Qubits (Alice – Bob) | State Fidelity (Full Tomography) | Generation & Storage Performance |
| • Sr+ – Sr+ Bell State | • 88Sr+ – 88Sr+ | • 96.94 ± 0.09% | • 1.24% success prob. / 39.3 Hz net generation rate. |
| • Ca+ – Ca+ Remote Bell State | • 43Ca+ – 43Ca+ | • 93.10 ± 0.70% | • Error-detected iSWAP quantum state transfer. |
| • 3-Qubit GHZ State | • (88Sr+, 43Ca+) – 88Sr+ | • 93.10 ± 0.70% | • Local mixed-species CNOT entanglement. |
| • 4-Qubit GHZ State | • (88Sr+, 43Ca+) – (88Sr+, 43Ca+) | • 91.90 ± 0.80% | • 4-particle distributed GHZ state across modules. |
| • Stored Ca+ – Ca+ State | • 43Ca+ – 43Ca+ (after 10s storage) | • 69.00 ± 4.00% | • Exponential decay time constant τ = 14 ± 4s. |
In trapped-ion quantum networks, using a single atomic species for both photonic coupling and quantum storage creates trade-offs: laser pulses and photon emissions required for networking induce decoherence and resonant light scattering in nearby memory qubits. The Oxford team resolved this by co-trapping one 88Sr+ ion (optical network interface) and one 43Ca+ ion (hyperfine ground memory qubit) in microfabricated surface-electrode Paul traps at each node. Photons spontaneously emitted at 422 nm from the strontium ions interfere at a central heralding station to generate remote matter-matter entanglement. Local 402-nm Raman laser pulses then drive geometric phase gates and error-detected iSWAP operations to transfer the entangled state onto the calcium memory qubits.
Because the ground hyperfine manifold of 43Ca+ possesses first-order magnetic field insensitivity at 0.5 mT (~2 orders of magnitude lower sensitivity than strontium), the calcium memory qubits preserved remote entanglement for more than 10 seconds (τ = 14 ± 4 s)—exceeding the average entanglement generation time (~100 ms) by two orders of magnitude and clearing a major hurdle for distributed quantum computing and quantum sensing networks.
Review the research paper in Physical Review Letters here and examine the institutional release on the University of Oxford Department of Physics Portal here.
October 17, 2026
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