ABSTRACT Quantum electrodynamics (QED) accurately describes all known forms of modern optics and photonics regarding interactions between photons and matter. Although matter ranges widely from atoms and particles to solids, photons are predominantly confined within a physical space, such as between a pair of mirrors, for enhanced photon–matter interactions known as cavity QED. Because position and momentum are canonically conjugate variables governed by Heisenberg’s uncertainty principle, a fundamental question arises—what if light confinement is in the not‐so‐intuitive momentum or reciprocal space? The realization of photonic bound states in the continuum (BICs) has made this exotic scenario possible. Here, we summarize the most recent advancements at this research frontier in optics and photonics, covering weak coupling, strong coupling, and nonlinear optics. We can designate such photon–matter interactions enabled by reciprocal light confinement through BICs with truly open systems as reciprocal QED, which holds great promise to comprehend and extend cavity QED for optics, photonics, and related fields.
Shoufeng Lan (2026) studied this question.