Robust Quantum Memory in a Trapped-Ion Quantum Network Node

Quantum networks can revolutionise the way in which we distribute and process information. Applications in the fields of cryptography, quantum computing, metrology, and fundamental physics will require the ability to store entangled states while further entanglement is generated across the network. Trapped-ion nodes connected via photonic links are an excellent candidate for realising such networks.

Here, we integrate a long-lived memory qubit into a mixed-species trapped-ion quantum network node [1]. Ion-photon entanglement, first generated with a network qubit in ⁸⁸Sr⁺, is transferred to ⁴³Ca⁺ with 0.977(7) fidelity and mapped to a robust memory qubit. We then entangle the network qubit with another photon, which does not affect the memory qubit. We perform quantum state tomography to show that the fidelity of ion-photon entanglement decays ~ 70 times slower on the memory qubit. Dynamical decoupling and sympathetic cooling further extends the storage time; we measure an ion-photon entanglement fidelity of 0.81(4) after 10 s.

[1] P. Drmota et al., arXiv:2210.11447 (2022)