An international research team has devised and demonstrated that it is possible to perform calculations remotely on quantum processors while maintaining the privacy of all users involved. The results of the experiment, conducted at the Quantum Lab of Sapienza University of Rome, were published in the journal Nature Communications and represent a fundamental step towards the realisation of secure quantum networks

A growing number of companies and laboratories around the world are making different types of prototype quantum processors available to users. Indeed, with current technologies, the costs of purchasing and maintaining these devices are unaffordable for ordinary users. Instead, through a cloud computing approach, anyone can ‘queue up’ to reserve the use of a small processor and conduct their own quantum computing experiment. The problem of maintaining the privacy of these users is therefore a major challenge that needs to be addressed.

Although it was already known how to maintain the privacy of a single user connected to a remote server, the problem of protecting the privacy of a group of users collaborating on the same calculation remained open. This could be the case, for instance, of a group of banks that aim to jointly process their customers’ data in order to develop a common financial model, but without either the other participating banks or the operators of the remote processor being able to steal any information about their customers’ data.

In a new study, published in the journal Nature Communications, a cryptographic protocol adaptable to platforms of increasing complexity and size was demonstrated, allowing multiple users to carry out a joint calculation while keeping their data secure and protecting all details of the calculation.

This was the result of a scientific collaboration in the field of computing protocols and quantum cryptography between Sapienza University of Rome, the Sorbonne University of Paris, the Centre National de la Recherche Scientifique (CNRS) and VeriQloud company.

Platforms based on quantum light states are among the main candidates for the realisation of densely interconnected quantum networks that can link multiple users, either to each other or to servers with computing power. Its physical properties make it a very promising system for transmitting information over long distances, as experiments on quantum communication between ground stations and orbiting satellites have shown.

The experiment led by Fabio Sciarrino and conducted at the Quantum Lab of the Physics Department of Sapienza University of Rome demonstrated, for the first time, a protocol in which two users perform a quantum calculation on a distant server while ensuring the total security of the calculation data.The experimental platform uses optical fibres to connect the clients to each other and to the server, demonstrating the security and effectiveness of the protocol even if the protocol participants are at a distance.

The protocol and its security were devised and demonstrated by research teams led by Elham Kashefi and Marc Kaplan and affiliated with the Université Sorbonne de Paris and the company VeriQloud, respectively.

“Our work”, says Beatrice Polacchi, a PhD student in the Quantum Lab team, “is the first experimental demonstration of a secure quantum computing delegation protocol involving more than one client, and therefore constitutes a building block for the construction of larger and more secure quantum networks.”

Another important outcome of this collaboration is the possibility of continuing along this path to demonstrate increasingly secure computing protocols and investigate quantum networks of increasing size and connectivity.

“Our results”, concludes Fabio Sciarrino, head of the Quantum Lab, “motivate research aimed at identifying new secure delegated quantum computing protocols and new modular architectures for quantum networks. We expect that this work will provide a significant stimulus for research on the future realisation of a quantum cloud”.

This research line is supported by the European Union’s Horizon 2020 research and innovation programme through the FET project ‘PHOQUSING’: www.phoqusing.eu.