Routing single photons with quantum emitters coupled to nanostructures

TL;DR

This review surveys how quantum emitters coupled to nanostructures can act as controllable single-photon switches, enabling routing in photonic quantum networks. It juxtaposes theoretical frameworks—real-space, input-output, and discrete-coordinate methods—with experimental demonstrations across quantum dots, neutral atoms, superconducting qubits, and solid-state defects. Key insights include interference between photon pathways as the central mechanism for routing, the pivotal role of emitter–cavity interactions, and the potential of chiral and multi-port architectures to enhance performance. While microwave and all-optical switches show outstanding speed and extinction in various platforms, challenges remain in achieving high fidelity, telecom compatibility, and scalable, deterministic multiport operation for practical quantum networks.

Abstract

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.

Figures (18)

• Figure 1: Diagram of a simple three-node network. Alice sends photons into the network, and uses a switch to route photons to one of the two receivers (Bob or Charlie). The red arrows indicate the direction of photon flow, and the black arrows indicate a control mechanism used by Alice to flip the switch.
• Figure 2: A two-level quantum emitter coupled to (a) a continuous waveguide and (b) a discrete coupled-resonator waveguide. In (a), the emitter couples to the right- and left-moving modes of the waveguide with coupling rates $V_R$ and $V_L$, respectively. In (b), the emitter couples to one resonator with coupling rate $g$, and the nearest-neighbor hopping rate between the resonators is $\xi$.
• Figure 3: Linearization of the waveguide dispersion $\omega(k)$ for the system in Fig. \ref{['fig:two_level_emitter_waveguide']}(a). The waveguide dispersion (solid blue curve) is approximated as being linear (solid red lines) near the wave numbers $\pm k_0$ corresponding to the frequency $\omega_0$.
• Figure 4: Single-photon transmission [dashed red curve, Eq. (\ref{['eq:T']})] and reflection [solid blue curve, Eq. (\ref{['eq:R']})] as a function of the photon-emitter detuning $\Delta$ for the system in Fig. \ref{['fig:two_level_emitter_waveguide']}(a). Photon loss is neglected in these results.
• Figure 5: Single-photon transmission [dashed red curve, Eq. (\ref{['eq:T_loss']})] and reflection [solid blue curve, Eq. (\ref{['eq:R_loss']})] as a function of the photon-emitter detuning $\Delta$ for the system in Fig. \ref{['fig:two_level_emitter_waveguide']}(a). The emitter loss rate $\gamma$ is included in these results, and was chosen such that ${\gamma = (2/9)\Gamma}$.