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...

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