PhD defence

Tropical Tensor Networks and Bell Inequalities: From Multipartite Nonlocality to Quantum Device Certification

• M. Hu

Date

Friday 5 June 2026

Time

11:30 - 12:30 hour

Address

Academy Building
Rapenburg 73
2311 GJ Leiden

Supervisor(s)

• dr. J. Tura Brugues

Summary

Quantum computers promise to solve problems that are far beyond the reach of classical computers. But this raises a question: how can we check whether a quantum computer is truly working in a quantum way, when its answers are too complex for any classical computer to verify?

In this thesis, I study how to certify the “quantumness” of large quantum systems. The main tool is Bell nonlocality: a special kind of correlation between particles that cannot be explained by classical local models. Observing such correlations provides strong evidence that a device is genuinely exploiting quantum mechanics.

The difficulty is that standard Bell tests work well for small systems, but quickly become computationally impossible when many particles are involved. To overcome this, I developed new mathematical and computational methods that combine ideas from geometry, optimization, and tensor networks. I also investigated how the shape of a quantum system, such as particles arranged on a donut-like surface or a Klein bottle, affects our ability to detect quantum correlations.

I then applied these methods to real experiments on programmable superconducting quantum processors. The results show that energy measurements can serve as practical and reliable tools for certifying non-classical behaviour in systems with many qubits.

These findings matter because they connect a deep question about the nature of reality with concrete tools for testing future quantum technologies. They bring us closer to answering a simple but important question: how quantum is your quantum computer?

PhD dissertations

Approximately one week after the defence, PhD dissertations by Leiden PhD students are available digitally through the Leiden Repository, that offers free access to these PhD dissertations. Please note that in some cases a dissertation may be under embargo temporarily and access to its full-text version will only be granted later.

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