The former has been demonstrated over two 150 km arms, 17 while the latter has been demonstrated over a single arm consisting of a 100 km fibre spool. The distribution of entanglement through fibre-optic cables has been predominantly focused on time-bin entanglement, 14, 15, 16, 17, 18 where the entangled degree of freedom between the two photons is temporal, i.e., related to the order in which the photons arrive, as opposed to their polarisation. 13 Nevertheless, for links of moderate length, fibre-optic connections are often the more suitable solution, in terms of the amount of key generated per year. If the distance were to be bridged by a fibre-optic cable connecting the two end-points directly, the attenuation would be at least 170 dB, assuming world-record low-loss fibres having a loss of 0.142 dB/km. The same satellite has also been used to demonstrate the longest distance quantum entanglement has ever been deployed over, 12 bridging a geographical distance of 1200 km while yielding coincident counts at a rate of 1.1 s −1 over a total loss in the dual link of 64–82 dB. Using a satellite, a distance record has been achieved both for QKD with a trusted node 10 as well as for entanglement-based QKD, establishing a secure key between a satellite and a ground station bridging a distance range of 530–1000 km. Thus in the long run, we believe that entanglement distribution will play a key role in future quantum communication techniques.
This will enable applications such as QKD and distributed quantum computation over distances beyond the metropolitan length scale. Since even an ideal quantum repeater would be susceptible to loss, it is of paramount importance to demonstrate entanglement distribution over the longest distance possible. 9 Further, the distribution of entanglement allows entanglement purification, which is a fundamental part of the implementation of quantum repeaters. Although its implementation has its own challenges, MDI QKD has been demonstrated over 404 km of optical fibre. 5, 6 Measurement-device independent QKD 7, 8 (MDI QKD) avoids side-channels in the detectors and promises a similar scaling of the key rates with increased loss as entanglement-based QKD. 3, 4 Entanglement also facilitates device independent quantum key distribution (QKD), where a secure key can be generated even if the devices used are provided by an adversary. Notably, entanglement provides the potential for being able to generate a secure key, over longer distance than decoy based quantum cryptography. 2 Studying quantum communication over long-distance links marks the next step in the advancement of this technology.
The maturity of quantum communication and the information theoretic security it provides have already found multiple applications in metropolitan fibre networks 1 including some elections.
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