Quantum Computing with black-box Subroutines

Understand

Modern programming relies on our ability to treat preprogrammed functions as black boxes - we can invoke them as subroutines without knowing their physical implementation.

• Here we show it is generally impossible to execute an unknown quantum subroutine.
• This, as a special case, forbids applying black-box subroutines conditioned on an ancillary qubit.
• We explore how this limits many quantum algorithms - forcing their circuit implementation to be individually tailored to specific inputs and inducing failure if these inputs are not known in advance.