Abstract:

The efficient distribution of quantum states over global distances constitutes a key challenge in the implementation of quantum communication protocols. While direct transmission through optical fibres allows for quantum key distribution up to a few hundred kilometres , achieving truly global distances remains impossible due to the exponential decay of photon transmission in fibres. To overcome this, satellite links have been proposed, and demonstrated to distribute entangled pairs of photons between two parties separated by more than 1100 km [1, 2]. However, the two photon count rate at the ground stations is limited due to the two-photon transmission being 56 to 71 dB. To overcome this, we study the inclusion of quantum memories to the satellite, which will make the rate depend on the one-photon transmission instead, thereby potentially increasing the rate by three orders of magnitude. For quantum key distribution, we find that the implementation of state-of-the- art quantum memories allows us to reach the same rate as the current satellites [1, 2]. We suggest an uplink protocol requiring two memory qubits in the satellite with a coherence time of 0.2 s, in order to reach a two photon count rate of 1.1 Hz.

[1] J. Yin et al. Science 356, 1140-1144 (2017) [2] J. Yin et al. Nature 582, 501-505 (2020)