Entangling single atoms over 33 km telecom fibre

Abstract

Quantum networks promise to provide the infrastructure for many disruptive applications, such as efficient long-distance quantum communication and distributed quantum computing1,2. Central to these networks is the ability to distribute entanglement between distant nodes using photonic channels. Initially developed for quantum teleportation3,4 and loophole-free tests of Bell’s inequality5,6, recently, entanglement distribution has also been achieved over telecom fibres and analysed retrospectively7,8. Yet, to fully use entanglement over long-distance quantum network links it is mandatory to know it is available at the nodes before the entangled state decays. Here we demonstrate heralded entanglement between two independently trapped single rubidium atoms generated over fibre links with a length up to 33 km. For this, we generate atom–photon entanglement in two nodes located in buildings 400 m line-of-sight apart and to overcome high-attenuation losses in the fibres convert the photons to telecom wavelength using polarization-preserving quantum frequency conversion9. The long fibres guide the photons to a Bell-state measurement setup in which a successful photonic projection measurement heralds the entanglement of the atoms10. Our results show the feasibility of entanglement distribution over telecom fibre links useful, for example, for device-independent quantum key distribution11–13 and quantum repeater protocols. The presented work represents an important step towards the realization of large-scale quantum network links.

A device-independent quantum key distribution system for distant users

“…To overcome losses in longer fibre links, a promising solution is to convert the entangled single photons to the low-loss telecom band by polarization-preserving quantum frequency conversion 32 . Recent results demonstrate extension of the QNL to 33 km fibre length 35 and show that high-quality entanglement over distances up to 100 km is achievable.…”

“…, a lower bound on the fidelity is given by ≥ 0.892 (23) F (ref. 35 ). results in an entangled atom-photon state 41 , where ↑ 1 / 2( ↑ + ↓ )…”

“…The CHSH value is found to be S = 2.578(75) using equation ( 1) with E 2,0 = −0.599 (41), E 3,0 = −0.664 (36), E 2,1 = 0.618 (39) and E 3,1 = −0.697 (35). The QBERs are given by the correlation data for X = Y, that is, Q 0 = 0.0781(127) and Q 1 = 0.0777(132), which gives an average error rate of Q = 0.078 (9).…”

“…Together with a duty cycle of approximately ½ and a repetition rate of the entanglement generation tries of 52 kHz, this results in an event rate of 1/82 s −1 . Note that for event-ready entanglement generation schemes the repetition rate of the experiment is limited by the communication times between the two devices and the BSM 35 . For DIQKD protocols, this results in a trade-off between the maximum separation of the users and the achieved secret key rate.…”

Requirements for a processing-node quantum repeater on a real-world fiber grid

“…In this work, we present for the first time a NetSquid model for trapped-ion nodes in quantum networks. The trapped-ion nodes we model are based on the state of the art for trapped ions in a cavity, which consists of 40 Ca + ions in a linear Paul trap [50,76,78,81,[94][95][96][97][98][99][100][101] (we note that promising results have also been achieved for trapped ions without cavities, these systems are however not considered in this work [102][103][104][105][106]. In our model they have all-to-all connectivity, their qubits all have the same coherence time and can all be used to generate light-matter entanglement.…”