microwave control pulse

A microwave control pulse is the little burst of radio-frequency energy that actually performs a gate on a superconducting qubit. Each qubit has a natural tone it likes to absorb, usually somewhere between 4 and 8 gigahertz. Send a precisely shaped pulse at that tone down a wire to the qubit, and you can rotate its state by exactly the amount you want. In plain terms, this pulse is the keystroke: it is how a control rack at room temperature reaches into the cold chip and nudges a single qubit.

Three knobs on the pulse set what the gate does. The amplitude (how loud) and the duration (how long) together decide how far the qubit rotates, so a calibrated area gives you a half-turn (an X gate) or a quarter-turn. The phase (the timing of the wave's crests) decides the axis of rotation, which lets you choose between, say, an X and a Y gate. The pulses are built by an arbitrary waveform generator that paints a smooth low-frequency envelope, then an IQ mixer lifts that envelope up onto the qubit's gigahertz carrier so the qubit will respond to it.

The honest catch is that the qubit lives at about 10 millikelvin and is extremely sensitive, so the pulse cannot arrive at full room-temperature strength. It is attenuated by tens of decibels on the way down the fridge, with attenuators bolted to each cold plate to strip away the room's thermal noise along with the signal. Pulses must also be shaped carefully (see DRAG) to avoid accidentally exciting the qubit's neighboring energy levels, and every qubit needs its own calibrated pulse, which is part of why scaling the wiring and the calibration is so hard today.