PhD Defence

Zesheng Xu will defend his doctoral thesis, "Hamiltonian Engineering in Integrated Photonics: From Static Lattices to Reconfigurable Physical Simulation," on Friday 2 October 2026 at 09:00 in lecture hall F3, Lindstedtsvägen 26 & 28. The defence is open to all.

The thesis asks how far an optical chip can be pushed as a simulator of wave physics. Zesheng worked across two platforms: passive silicon nitride lattices, where light propagation in space stands in for evolution in time, and programmable silicon-on-insulator meshes of Mach-Zehnder interferometers, where couplings and onsite potentials can be tuned at will. On the passive circuits he obtained experimental evidence for the topological Anderson phase and for a reentrant metal-insulator transition driven by correlated disorder. On the programmable mesh he emulated the Bloch-Siegert shift, turning bidirectional Rabi oscillations into unidirectional transport.

The later chapters move beyond Hermitian physics. Using unitary dilation, ancillary modes on the mesh introduce controlled loss, which allowed the realization of a non-Hermitian Hamiltonian on a Klein bottle parameter space with signatures of paired exceptional points, and of dissipation-induced synchronization across many modes at a rate set by the spectral gap rather than by the absolute attenuation. 

Zesheng joined the group as a master's student in 2021 and continued as a doctoral student. He is the second doctoral student to graduate from the Q-Photon Laboratory.

Opponent: Associate Professor Max Yan, Umeå University. Grading committee: Guilherme Xavier (Linköping University), Anna Delin (KTH) and Mohamed Bourennane (Stockholm University). Chair: Professor Vladislav Korenivski, KTH. Supervisor: Associate Professor Ali W. Elshaari.

Thesis in DiVA: https://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-388167