A quantum computer in the lab resembles an orchestra: lasers play the role of bows, and ions are the strings. To play the desired melody, powerful radio signals are sent to the lasers: they set the rhythm and pitch. But part of this "music" escapes the stage.
Researchers noticed that the radio signals controlling the lasers for ion cooling, operations, and readout slightly leak from the enclosure. Using an ordinary radio receiver, they captured these waves and, from their shape, determined which gates (basic operations) the processor was executing. This way, one can spy on someone else's algorithm without touching the device.
This is the first demonstration of such an attack on an ion-based quantum processor. The authors also propose countermeasures — for instance, shielding the device or adding noise to the radio signals to confuse the eavesdropper.
Side-channel leaks also exist in classical computers: from voltage fluctuations or electromagnetic emissions, one can recover cryptographic keys.
In the novel "The Quantum Thief" by Hannu Rajaniemi, stealing quantum states is commonplace, and the protagonists are constantly on the lookout for leaks.