The electromagnetic duality symmetry of Maxwell’s equations in vacuum implies that the circular polarization $Q$ of classical electromagnetic waves is conserved. In quantum field theory, the corresponding operator $\widehat{Q}$ represents the difference between the number operators of right- and left-handed photons. Previous studies have shown that its renormalized expectation value is not conserved in gravitational fields. Here, we show that this Noether symmetry can also be realized in empty cylindrical waveguides with duality-preserving boundary conditions, and we quantize the electromagnetic field in this setting. In the vacuum $|0⟩$ associated with inertial observers, we find that the expectation value $⟨0|\widehat{Q}|0⟩$ fails to be conserved in time when the field is quantized with respect to observers undergoing relativistic helical motion inside the waveguide. In particular, frame-dragging effects induce a spectral asymmetry between right- and left-handed field modes at late times. As a consequence, such noninertial observers perceive photon-pair excitations from the quantum vacuum with an imbalance between opposite helicity modes. This relativistic quantum effect shows that the classical conservation law associated with duality symmetry is broken in the quantum theory even in flat spacetime, provided the field is quantized with respect to noninertial observers. Our analysis provides a concrete proof of concept for testing this effect in analog gravity platforms.

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• Quantum fields in curved spacetime
• Unruh effect