Group seminar at LMU and Zoom: Two-dimensional realisation of parity anomaly in an ultracold Dysprosium experiment

 

Nehal Mittal, Collège de France Paris, France
Group seminar at LMU seminar room and Zoom
Tuesday, 31 March 2026, 09:00 am (MEZ)

 

Symmetry is a central organising principle of modern physics, shaping the laws of nature through invariance structures that constrain dynamics, enforce conservation laws, and unify interactions from classical mechanics to quantum field theory. Yet not all symmetries survive quantisation: in field theory, a symmetry is said to be anomalous when it cannot be consistently preserved at the quantum level. Quantum anomalies leave measurable fingerprints across physics, governing classically forbidden processes such as the rapid decay of the neutral pion into two photons and placing powerful constraints on low-energy effective field theories through ’t Hooft anomaly matching. In many-body systems with boundaries, anomalies give rise to phenomena such as bulk-edge correspondence and quantised topological response via anomaly inflow.

In this talk, I will report the observation of a parity-anomalous Hall response at the critical point of a quantum Hall topological phase transition in a synthetic two-dimensional system of ultracold dysprosium atoms. By coupling a continuous spatial dimension to a finite synthetic dimension encoded in atomic spin states, we engineer tunable Chern bands with C = 0 and 1. At the transition, the bulk gap closes at a single Dirac point, where we observe a robust half-quantised Hall drift despite strong non-adiabatic excitations. We
show that this response originates from the global structure of the band topology, is protected by an emergent parity symmetry at criticality, and disappears when parity is explicitly broken.