Entanglement over global distances via quantum repeaters with satellite links

We study entanglement creation over global distances based on a quantum repeater architecture that uses low-Earth-orbit satellites equipped with entangled photon sources, as well as ground stations equipped with quantum nondemolition detectors and quantum memories. We show that this approach allows entanglement creation at viable rates over distances that are inaccessible via direct transmission through optical fibers or even from very distant satellites.

Figure 1

Proposed quantum repeater architecture with satellite links. Each elementary link (of length $L_0$) consists of an entangled photon pair source on a low-Earth-orbit satellite (at height $h$) and two ground stations consisting of quantum nondemolition (QND) measurement devices and quantum memories (QM). The successful transmission of entangled photons to each ground station is heralded by the QND devices, which detect the presence of a photon nondestructively and without revealing its quantum state. The entanglement is then stored in the memories until information about successful entanglement creation in two neighboring links is received. Then the entanglement can be extended by entanglement swapping based on a Bell state measurement (BSM). Figure 2 shows that four to eight such links are sufficient for spanning global distances.

Figure 2

Rates of entangled pairs created per day as a function of ground distance for quantum repeaters with LEO satellites (solid lines) at heights $h=500$, 1000, and 1500 km, compared to direct transmission (dotted lines) from satellites at heights $h=2000$ and 10 000 km and from a geostationary satellite.

Figure 3

Impact of inefficiencies in various elements on the entanglement distribution rate over 20 000 km for the repeater protocols shown in Fig. 2. (a) Effect of the memory read efficiency. The detector efficiency has the same effect. (b) Effect of QND detector efficiency. Memory write efficiency and source efficiency have similar effects (see also the Supplemental Material [22]). The repeater protocol is more sensitive to memory read and detector efficiency than to that of the other components, because the former efficiencies intervene in each entanglement swapping step, whereas the latter only intervene in the entanglement creation in the elementary links.