Effective and Scalable Control of Quantum Processors – Insight and Perspective

To achieve the dream of useful quantum computation, there are two main thrusts to make improvements: quantum system design and control. Now at the junction of realizing effective quantum error correction, the boundaries of quantum system control become an essential piece of information. Establishing the limits of efficient, scalable, and accurate quantum control can inform the efficacy and choice of different kinds of error correction. The effort of quantum system control, specifically in terms of measurements during calibration, must be sufficiently low to compensate for system parameter drift or fast enough to enable periodic re-calibration. The effectiveness of different control routines can be evaluated using theoretical and numerical studies. While theoretical models help inform on general structures of quantum control landscapes, the controllability, or the complexity of the computational effort, they are often insufficient to represent the real quantum system. Complete system characterization can lead to accurate numerical models representing the entire quantum system but are incredibly cumbersome. Model-free learning control, a laboratory-costly approach, represents the other extreme. These methods tend to be highly laborious and measurement-intensive as systems scale in size. Here, we discuss our insights and perspective on the limits and capabilities of quantum control, the role of theoretical studies, and the methods available for experimental quantum control from a measurement point of view. Finally, we explore the design of quantum control schemes that are as robust as possible to system uncertainties while remaining resource-efficient.