Long-lived entanglement memory in a trapped-ion quantum network node

Trapped ions that are connected over a photonic network can be useful for quantum computing, cryptography and metrology. Mapping the remote entanglement to a long-lived memory qubit is a prerequisite for entanglement distillation, interactive client-server protocols, and entanglement-enhanced remote sensing applications.

Here, we present progress towards extending ion-photon entanglement to a memory qubit with a coherence time of >1s. As described in previous works [1], the electronic state of ⁸⁸Sr⁺ is entangled with the polarisation of a spontaneously emitted photon. The qubit decoheres predominantly due to magnetic field noise, limiting the coherence time to <1ms. Using local mixed-species entangling operations we are able to map the state onto a long-lived magnetic field insensitive "clock" qubit in co-trapped ⁴³Ca⁺. We report the fidelity of the mixed-species iswap operation and characterise the joint ⁴³Ca⁺-photon state using tomography after a free evolution time. Compared to ⁸⁸Sr⁺, we show an enhancement of the entangled qubit memory lifetime. [1] PRL 124, 110501 (2020)