Routing single photons with quantum emitters coupled to nanostructures

Quantum emitters coupled to nanophotonic structures are an excellent platform for controllable single-photon scattering. The tunable light-matter interaction enables the construction of a single-photon switch - a device that can route a single photon from an input port to a selected output port. Such single-photon switching devices can be integrated into reconfigurable photonic circuits to actively control the photon propagation direction in a quantum network. Ideally, a single-photon switch should be fast, efficient, scalable, compatible with existing technology, and should preserve the routed photon states with high fidelity. In this review, we focus on theoretical proposals and experimental demonstrations of single-photon switches based on quantum emitters coupled to solid-state nanostructures, including waveguide and cavity architectures. We also present the theoretical methods that are commonly used to model single-photon scattering in waveguide-based systems by applying them to the elementary system of a two-level emitter coupled to a waveguide. This review brings together key theoretical techniques from quantum optics, their applications to controllable single-photon transport, and the experimental realization of single-photon switching devices across different physical platforms, including semiconductor quantum dots, neutral atoms, superconducting qubits, and color centers.