Skip to main content Skip to navigation

Department Events

The department runs a variety of seminars, workshops and colloquia. See upcoming events below. You are also welcome to sign up to the seminar mailing list.

For visiting the department, see the map of campus, directions, and accommodation recommendations.
(Be reminded that the University of Warwick is not, surprisingly, located in the town of Warwick.)

Show all calendar items

CS Colloquium: Matthias Caro (FU Berlin)

- Export as iCalendar
Location: CS1.04

Speaker: Matthias Caro (FU Berlin)

Title: Learning quantum states and unitaries of bounded gate complexity


While quantum state tomography is notoriously hard, most states hold little interest to practically-minded tomographers. Given that states and unitaries appearing in Nature are of bounded gate complexity, it is natural to ask if efficient learning becomes possible. In this work, we prove that to learn a state generated by a quantum circuit with G two-qubit gates to a small trace distance, a sample complexity scaling linearly in G is necessary and sufficient. We also prove that the optimal query complexity to learn a unitary generated by G gates to a small average-case error scales linearly in G. While sample-efficient learning can be achieved, we show that under reasonable cryptographic conjectures, the computational complexity for learning states and unitaries of gate complexity G must scale exponentially in G. We illustrate how these results establish fundamental limitations on the expressivity of quantum machine learning models and provide new perspectives on no-free-lunch theorems in unitary learning. Together, our results answer how the complexity of learning quantum states and unitaries relate to the complexity of creating these states and unitaries. This is joint work with Haimeng Zhao, Laura Lewis, Ishaan Kannan, Yihui Quek and Hsin-Yuan (Robert) Huang, see


Show all calendar items