Fundamental limits of repeaterless quantum communications

Quantum communications promise reliable transmission of quantum information, efficient distribution of entanglement and generation of completely secure cryptographic keys. However, for all these tasks, it was an open problem for about 20 years to determine the optimal point-to-point rates that are achievable by two remote parties at the ends of a quantum channel, even with unlimited two-way classical communication. Indeed, these optimal rates are very difficult to calculate because they involve an optimization over all the adaptive local operations, based on feedback from remote parties.

Recently, Pirandola, Laurenza, Ottaviani and Banchi (PLOB), in their work “Fundamental limits of repeaterless quantum communications”, found a solution to this open problem, by determining the optimal rates for many quantum channels that are of experimental interests, such as photonic lossy channels, quantum-limited amplifiers and qubit dephasing. This was made possible by developing new reduction techniques, based on quantum teleportation, that allowed them to simplify the most general protocols for quantum and private communication.

In this way, PLOB were able to establish exact formulas for the two-way quantum and private capacities of several fundamental channels and finally determine the fundamental rate-loss scaling for optical quantum communications as 1.44 bits per channel use.  This is currently used as a general benchmark to test the performance of quantum repeaters, i.e., to check when they are actually useful or not. The article was published on “Nature Communications”.

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• Read the full article here.