Sensitive magnetic field detection via scattering of light from a gyrotropic dimer.

We present an approach for sensitive detection of weak magnetic fields based on electromagnetic scattering from a gyrotropic dimer-a pair of identical point-like dipoles with a polarizability tensor containing magnetically induced gyrotropic components. For plane-wave excitation, dimer resonances enhance the magnetic field-dependent scattering response. For equilibrium systems satisfying the optical theorem, including Drude metallic nanoparticles, exact resonance conditions are inaccessible, and no enhancement over the single-particle response occurs. In contrast, resonant atomic dipoles can provide substantial enhancement, reaching sensitivities of order 103 G-1 for Rb atoms near the D2 transition. While equilibrium systems allow only finite near-resonant enhancement, scattering from nonequilibrium systems can access exact resonance condition, yielding formally unbounded enhancement of the magnetic-field response.