Description
Berry phases are geometric phases acquired by a system carried adiabatically around a closed loop, fixed entirely by the loop's geometry, as in the Aharonov-Bohm effect, where a charged particle's wavefunction shifts measurably on encircling a magnetic flux it never samples. We realise this physics in a tabletop platform: a free-space microwave cavity formed by twisting a triangular waveguide into a closed loop, analogous to a Möbius strip, with conducting walls forming all boundaries [1].
In an untwisted toroidal cavity, every resonant mode returns to itself, in phase, after one winding. In our Möbius cavity, the cross-section rotates as you traverse the loop, so modes lacking its rotational symmetry meet up out of phase after one winding, producing fractional azimuthal mode numbers and an accumulated Berry phase. Its magnitude is fixed by the resonator's total twist angle, in turn set by the cross-section's rotational symmetry: for our three-fold-symmetric triangular geometry, it takes integer multiples of $\pm2\pi/3$.
Its sign is tied to the mode's electromagnetic helicity: the degree to which its electric and magnetic fields run parallel or anti-parallel. Frequency shifts measured between 3D-printed aluminium Möbius and mirror-symmetric cavities confirm the predicted $\pm2\pi/3$ phases.
The same chiral, twisted boundary supports bulk modes with intrinsic, tunable electromagnetic helicity, via magnetoelectric coupling between near-degenerate modes [2,3]. We propose harnessing this for ultralight axion dark matter detection [4], where helicity, not an external magnetic field, drives the axion-photon coupling, enabling superconducting, low-loss detector cavities
[1] E.C.I. Paterson, M.E. Tobar, M. Goryachev, J. Bourhill, Phys. Rev. A 113, 043501 (2026).
[2] E.C.I. Paterson, J. Bourhill, M.E. Tobar, M. Goryachev, Phys. Rev. A 112, 013530 (2025).
[3] E.C.I. Paterson, J.Bourhill, M.E. Tobar, M. Goryachev, Phys. Rev. A 113, 033506 (2026).
[4]J. Bourhill, E.C.I. Paterson, M. Goryachev, M.E. Tobar, Phys. Rev. D 108, 052014 (2023).
| I am the presenting author | Yes |
|---|