Tuning a piano is delicate work. Strike one string, and its neighbors resonate. A silicon chip with crystal defects, being prepared for quantum computing, behaves similarly.
In a silicon carbide (SiC) crystal, there are special spots — silicon vacancies. There, an electron behaves like a tiny magnet with four states. Instead of an ordinary bit (0 or 1), such a system stores more information: it's a qudit. But it's trickier to control: a short radio pulse aimed at one transition slightly nudges others. This creates crosstalk, as if a pianist wanted to play one note, but the instrument added a couple more.
The authors approached it like tapping a wall: they sent a sequence of pulses and watched how the system responded. The response spectrum revealed six extra lines — one for each pair of levels that the pulse accidentally touches. Each line corresponds to the energy difference between two states of the qudit. The researchers derived formulas that predict the strength of each disturbance.
Numerical simulations matched the experiment without any frequency fitting. This means the disturbances can not only be suppressed but also put to use — for instance, calibrating pulses right during device operation.
Now engineers have a map of spurious transitions. With it, they can shape pulses to avoid hitting the extra strings of the quantum piano, or, conversely, turn them to their advantage.
A qudit from a silicon vacancy has four states, so one such element replaces two ordinary qubits.