Magnetic-flux-controlled exceptional curve in a SWCNT microtoroid

Abstract

We develop a microscopic theory for realizing flux-programmable exceptional-curve dynamics in a hybrid platform composed of a single-walled carbon nanotube (SWCNT) microtoroid and a nearby nitrogen-vacancy (NV) center. The Aharonov–Bohm flux shifts the two counterpropagating chiral cavity modes in opposite directions, while the chiral NV coupling generates directional mode conversion. By adiabatically eliminating the NV excited state in the dispersive regime, we derive flux-dependent nonreciprocal couplings \(J_{12}(\Phi )=J_0+\alpha _J\Phi /\Phi _0\) and \(J_{21}(\Phi )=J_0-\alpha _J\Phi /\Phi _0\), which lead to an effective non-Hermitian two-mode Hamiltonian supporting an exceptional point. We demonstrate that while the exact exceptional curve requires simultaneous tuning of the coherent coupling and magnetic flux, a minimum-splitting trajectory tracked via a single flux sweep closely approaches this non-Hermitian degeneracy, manifesting itself in the transmission spectrum through mode coalescence, linewidth bifurcation, and collapse of the doublet resonance. These results provide a concrete microscopic route for engineering and probing exceptional-curve dynamics in chiral SWCNT-based quantum photonic devices.

Magnetic-flux-controlled exceptional curve in a SWCNT microtoroidMagnetic-flux-controlled exceptional curve in a SWCNT microtoroid