Two different studies published in Nature-affiliated journals by CiViQ’s consortium partner Stefano Pirandola, from University of York, and colleagues, prove further advancement in the field of quantum communications, by overcoming limitations that condition the fully integration of these systems into classical telecom networks.

The first study reported by Pirandola focuses on understanding how end-to-end quantum communications can, firstly, aim to achieve long distances while, secondly, maintaining high rates. So far, in quantum mechanics, a fundamental law (known as Pirandola-Laurenza-Ottaviani-Banchi “PLOB” bound) prevents quantum communications to simultaneously achieve both parameters. This limitation is well known for a point-to-point connection that remains within the same quantum channel, but little is known about what happens when you insert a, or several, quantum repeaters in the communication line.

Cited in the Second Year Anniversary Collection of the Nature journal Communications Physics, the work identifies the optimal network protocols that are able to overcome the maximum point-to-point rate at which two remote parties can distribute quantum bits, entangled bits or secret bits. The study derives upper bounds and exact formulas for the ultimate end-to-end capacities achievable by the most general (adaptive) protocols of quantum private communication, and establishes these results when using a single repeater on the line or multiple repeaters in an arbitrarily complex quantum network, where systems may be routed through single or multiple paths.

Assuming arbitrary quantum channels between the nodes, Pirandola shows that the end-to-end capacities are expressed by simple single-letter quantities based on an entanglement measure known as the relative entropy of entanglement.

The corresponding results provide the ultimate benchmarks for testing the optimal performance of repeater-assisted quantum communications. In particular, the results also show how the parallel or “broadband” use of a lossy quantum network via multi-path routing may greatly improve the communication rates between two remote end-users.

In the second study published in Nature Communications, Stefano Pirandola and colleagues report a method to achieve higher communication rates in a quantum communication channel when the amount of energy at its input is limited.

Limited input power clearly restricts the achievable rates of various quantum communication tasks. It is therefore crucial to identify or bound the quantum capacities of a quantum channel assuming this important constraint.

In this work, the researchers consider Gaussian channels that model energy loss and thermal noise errors in realistic optical and microwave communication channels and study their various quantum capacities in the energy-constrained scenario. They demonstrate that one can boost the transmission rates of quantum information and private classical information by using a suitably-correlated multi-mode thermal state instead of single-mode thermal states of the same energy. In this way they effectively mitigate one of the limits that may affect low-energy secure quantum communications.

Read End-to-end capacities of a quantum communication network

Read Enhanced energy-constrained quantum communication over bosonic Gaussian channels