Microwave Techniques for Quantum Computers: State-of-the-Art Control Systems for Quantum Processors

Let's boot up a quantum computer. Far from being a simple push of a button, initializing a prototype quantum computer requires the precise tuning and calibration of many different parameters. Rather than switching transistors on and off, the controller of a quantum processor emits combinations of analog waveforms, each with a characteristic shape, frequency, and duration. These waveforms are used to either manipulate or read out the states of quantum bits (qubits), the basic units of quantum information in the processor. The analog nature of the system inherits many of the complexities of analog computing, including device parameter drift and offsets, system component tolerances and variabilities, and other well-known analog-circuit intricacies. Further, de livering these signals to the target qubits necessitates coordination between multiple low-noise and low-jitter instruments. In spite of these challenges, the quantum computing community has made tremendous progress toward useful quantum machines. We hope to provide an instructive introduction to the control, signal generation, and distribution principles currently used in small quantum systems that operate in the microwave frequency regime.