Speaker
Description
Maxwell's equations in vacuum exhibit a duality invariance under electric-magnetic rotations. This is a Noether symmetry of the source-free Maxwell theory in any curved spacetime, and implies that the circular polarization state (the Stokes V parameter) of classical electromagnetic waves is conserved during propagation, even in the presence of strong gravitational fields.
Remarkably, quantum vacuum fluctuations of the electromagnetic field, when enhanced by gravitational effects, can break this symmetry. In previous work, we showed that the vacuum expectation value of the Noether charge operator is no longer conserved, and its time evolution is governed by the spacetime geometry [arXiv: 1607.08879, 1810.08085]. This constitutes a quantum anomaly for spin-$1$ fields---a direct analogue of the Adler-Bell-Jackiw anomaly for spin-$1/2$ fermions in gauge fields, previously unknown for photons. It was further shown that this effect arises when the background geometry develops a chiral structure, for instance when the gravitational field carries a flux of circularly polarized gravitational waves [arXiv: 2002.01593, 2106.08350].
In this talk, I will present a tabletop realization of this gravitational phenomenon in a non-inertial setting. I will show that, when the electromagnetic field is probed by observers undergoing relativistic helical motion inside a cylindrical waveguide, the vacuum expectation value of the duality charge fails to be conserved in time. The underlying mechanism involves two distinct ingredients: (i) a mismatch between early- and late-time notions of positive frequency due to the observers’ acceleration, which leads to photon pair creation, and (ii) a chiral structure of the late-time observer frame, which induces a spectral asymmetry between right- and left-handed modes due to frame-dragging effects inside the waveguide. As a result, a net helicity imbalance is generated from the quantum vacuum [arXiv: 2505.20409, 2512.04188].
This photon helicity non-conservation reflects a relativistic quantum effect, providing a novel manifestation of a quantum anomaly in flat spacetime, and opening new avenues for exploring chiral quantum effects in controlled photonic systems.