ATTESTER project sheds new light on the limits of quantum communication

Quantum communication could one day enable networks whose security is guaranteed by the laws of physics. Before that can happen, researchers need to understand how real, imperfect channels behave. The ATTESTER project at the University of Malta has spent the past two years on this question, and it has now concluded with three new results.

The research brought together Prof. Tony J. G. Apollaro from the Department of Physics and Prof. Ing. Gianluca Valentino, Prof. Johann Briffa and Dr Ing. Trevor Spiteri from the Department of Communications and Computer Engineering. Research Support Officers Mr Hayden Zammit, Mr Goncalo Rocha De Macedo Barreto, and, Dr Roberto Benjamin Salazar Vargas worked with them.

• A limit on reuse: Channels can keep a "memory" of earlier transmissions. In a paper in Journal of Physics A: Mathematical and Theoretical (Zammit et al., 2026, 59, 295305), the team measured exactly how this memory lowers the efficiency of quantum state transfer. The result is a practical limit on how many times a channel can be used reliably.
• Machine learning as a diagnostic tool: In the paper Inferring the Number of Quantum State Transfers with Neural Networks (arXiv:2610.38844), the researchers developed a machine-learning algorithm that infers how many times a channel has already been used. This helps with security checks and with forecasting how good later transmissions will be.
• When entanglement can no longer be shared: In the paper Memory-Induced Limits on Entanglement Distribution (arXiv:2610.38817), the team used semidefinite programming to set bounds on the quantum capacity of channels with memory. The bounds show the trade-off between channel length and read-out timing errors, beyond which the channel can no longer distribute entanglement.

ATTESTER was funded by Xjenza Malta and the Malta Digital Innovation Authority through the R&I Thematic Programmes: Digital Technologies Programme.

“Information is physical” says Prof. Tony Apollaro, quoting the physicist Rolf Landauer: every bit is stored, sent and processed by a physical system, so its limits are set by the laws of physics.

ATTESTER shows how much physicists and engineers can achieve when they explore these limits together, and the two departments look forward to building on this collaboration in future projects. Stay tuned.