Photonic quantum Hall effect

Because photons, unlike electrons, have no charge, they don’t couple to applied magnetic fields. Yet it’s possible to make them behave as if they do—the key is to manipulate the photons’ quantum mechanical phase. (See the Quick Study by Mohammed Hafezi and Jake Taylor, Physics Today, May 2014, page 68 .) One way to do that is to use an orderly arrangement of discrete optical resonators to synthesize an artificial magnetic field. Now Jonathan Simon and colleagues at the University of Chicago have demonstrated a new, lattice-free approach. The researchers exploit a powerful analogy: Quantum mechanically, photons bouncing back and forth between curved mirrors exhibit the same transverse behavior as massive particles in a two-dimensional harmonic oscillator. Routing the light using four mirrors instead of just two, the team made its path nonplanar. That caused photons to pick up an additional phase proportional to their angular momentum, just as if they were charged particles in a magnetic field. Through careful tuning of the resonator’s 78 mm optical path, the researchers induced photons from a 780 nm laser into an integer quantum Hall state. The Chicago team further engineered their resonator to induce the photons to behave as if constrained to the surface of a cone, and provided the first experimental validation of a 1992 theory describing the response of quantum Hall particle density to spatial curvature. The researchers expect the approach to allow further exploration of the interplay between geometry and topology and to lead to the creation of photonic fractional quantum Hall fluids. (N. Schine et al., Nature 534, 671, 2016, doi:10.1038/nature17943 )