Asymmetric excitation of left- vs right-handed photons in accelerating waveguides

TL;DR

This paper demonstrates that duality symmetry in Maxwell's equations can be broken in non-inertial waveguides, leading to observable asymmetries in photon helicities and vacuum excitations due to acceleration and frame-dragging effects.

Contribution

It shows that duality symmetry is violated in accelerating waveguides, resulting in spectral asymmetries and photon-pair creation, extending previous gravitational findings to flat spacetime systems.

Findings

Frame-dragging induces spectral asymmetry between photon helicities.

Accelerated detectors observe photon-pair excitations from the vacuum.

Duality symmetry breaking occurs in non-inertial, flat spacetime systems.

Abstract

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 normal-ordered operator $\hat Q$ represents the difference between the number operators of right- and left-handed photons. Previous studies have shown that its expectation value is not conserved for observers propagating in a gravitational field. Here, we show that this Noether symmetry can also be realized in empty waveguides with duality-preserving boundary conditions, and we quantize the source-free Maxwell theory inside a long, cylindrical waveguide undergoing both linear and rotational acceleration from rest. In the vacuum $|0\rangle$ associated to inertial observers, we find that the expectation value $\langle 0| \hat Q |0\rangle $ fails to be conserved for observers co-moving with the waveguide. In particular, frame-dragging effects induce a spectral asymmetry between the right- and left-handed field modes at late times. As a consequence, accelerated detectors co-moving with the rotating waveguide can detect photon-pair excitations from the quantum vacuum, exhibiting an imbalance between opposite helicity modes. This is a relativistic quantum effect, which shows that the classical conservation law associated with duality symmetry is broken in the quantum theory even in flat spacetime, provided we work with non-inertial systems. Our analysis provides a concrete proof of concept for testing this effect in analogue gravity platforms.