Zuzana Gavorova (The Hebrew University of Jerusalem): Controlled-unitary is impossible in the quantum circuit model
Quantum Information and Quantum Computing Seminars CTP PAS 2021-10-13
Abstract: I will discuss the following task: given an unknown unitary gate U as a black box, implement the controlled-unitary* gate. Araújo et al. showed that a quantum circuit that makes one call to U cannot implement controlled-U. I will show that the task remains impossible even if the quantum circuit is allowed any number of calls to U. Our result also excludes circuits that use postselection and only approximate the task. Handling approximation and postselection simultaneously requires a new notion: diamond distance for the postselected setting.