A river whose one branch is blocked and another opened in a fraction of a second does not calm down immediately. The fast currents have already settled into the new channel, but slow ripples still spread across the water, encoding the entire history of the switch. A Majorana particle behaves in much the same way when its wave packet is truncated not by a knife but by a rotation of its propagation channels.

The author of the work described this rotation with a simple rule: the output wave is obtained from the input by multiplying by a rotation matrix that changes over time. As long as the initial and final orientations of the channels coincide, everything is calm. But if they differ, a trace of pair creation remains in the vacuum, and this trace grows logarithmically at arbitrarily small frequencies. This means that the vacuum before the switch and the vacuum after the switch no longer overlap: the field remembers the change of channels.

However, if the switch is tuned so that no pairs are created at all, the gate becomes a perfect filter. Outside the switching region, the observer sees exactly one particle, with no memory traces. This separation between the slow global trace and the fast local picture is the main result of the work. Previously, it was unclear how to rigorously describe the truncation of such a particle, and now there is a benchmark for testing future quantum devices where such particles will be controlled over time.