Quantum networks, in which information is shared via quantum bits (qubits), have been heralded as revolutionary for computational and communications-related tasks given their theoretical security. However, the practical quantum devices that make up such networks might be intrinsically insecure—realistic detectors are susceptible to hacking attacks. Here, we propose a new method to achieve scalable high-rate quantum networks with untrusted relays, where the relay hosting the detectors can even be operated by the hacker herself. This method can be applied directly to networks with users at arbitrary distances from the relay, thereby mitigating a major limitation of previous methods.

Most quantum networks constructed to date have been based on trusted relays, but quantum networks with untrusted relays have also been proposed using the concept of measurement-device-independent quantum key distribution (MDI-QKD). However, MDI-QKD has worked well up until now only for nearly symmetric-length channels (i.e., the two channels from the two parties, Alice and Bob, leading to the untrusted relay, Charlie, have nearly the same loss). In real-life situations, however, quantum channels can have vastly different losses. Therefore, existing MDI-QKD methods are cumbersome and have low key rates (i.e., the rate of secure bits generated per second). Our new method removes the requirement of nearly equal channel loss completely by allowing the two users to employ quantum optical signals with vastly different intensities. This technique leads to dramatically higher key rates and greatly extends the applicability of MDI-QKD networks. Moreover, it easily allows users to be added or deleted in real time, at arbitrary locations.

We expect that our work will open up new directions of research into asymmetric channels in quantum networks with untrusted relays and inspire more theoretical and experimental work.