A universal quantum processor is a device that takes as input a (quantum) program, containing an encoding of an arbitrary unitary gate, and a (quantum) data register, on which the encoded gate is applied. While no perfect universal quantum processor can exist, approximate processors have been proposed in the past two decades. A fundamental open question is how the size of the smallest quantum program scales with the approximation error.
In this talk, I will provide an answer to the question, by proving a bound on the size of the program and designing a concrete protocol that attains the bound in the asymptotic limit. The optimal construction highlights the usage of a new type of high-performance quantum reference frames, which can also be used to construct covariant quantum error correction codes.
This result witnesses a connection between optimal programming and the Heisenberg limit of quantum metrology, and establishes an asymptotic equivalence between the tasks of programming, learning, and estimating unitary gates.
 YY, R. Renner, G. Chiribella, PRL 125, 210501 (2020).
 YY, Y. Mo, J. Renes, G. Chiribella, M. Woods, arXiv 2007.09154.
 G. Chiribella, YY, R. Renner, PRX 11, 021014 (2021).
Bio: Yuxiang Yang （杨宇翔） is an assistant professor at Department of Computer Science, the University of Hong Kong. Before that he worked as a postdoctoral fellow at ETH Zürich. His research aims to identify quantum advantages in metrology and computation, and to design optimal protocols for the next generation of quantum computing devices. In 2017 he was awarded a Microsoft Research Asia Fellowship for his work in quantum information theory.