Controlled-unitary is impossible in the quantum circuit model



The Hebrew University of Jerusalem

October 13, 2021 3:15 PM


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.

* Up to a certain relative phase.

The papers:



Zoom meeting details

Topic: Quantum Information and Quantum Computing Working Group
Time: Wednesday, October 13, 2021, 15:15 Warsaw (CET)

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Meeting ID: 96294497969
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