Universal control of quantum processes using sector-preserving channels

Understand

No quantum circuit can turn a completely unknown unitary gate into its coherently controlled version.

• Yet, coherent control of unknown gates has been realised in experiments, making use of a different type of initial resources.
• Here, we formalise the task achieved by these experiments, extending it to the control of arbitrary noisy channels, and to more general types of control involving higher dimensional control systems.
• For the standard notion of coherent control, we identify the information-theoretic resource for controlling an arbitrary quantum channel on a $d$-dimensional system: specifically, the resource is an extended quantum channel acting as the original channel on a $d$-dimensional sector of a $(d+1)$-dimensional system.