Efficient, High-Fidelity Single-Photon Switch Based on Waveguide-Coupled Cavities

We demonstrate theoretically that waveguide-coupled cavities with embedded two-level emitters can act as a highly efficient, high-fidelity single-photon switch. The photon switch is an optical router triggered by a classical signal -- the propagation direction of single input photons in the waveguide is controlled by changing the emitter-cavity coupling parameters in situ, for example using applied fields. The switch reflects photons in the weak emitter-cavity coupling regime and transmits photons in the strong coupling regime. By calculating transmission and reflection spectra using the input-output formalism of quantum optics and the transfer matrix approach, we obtain the fidelity and efficiency of the switch with a single-photon input in both regimes. We find that a single waveguide-coupled cavity can route input photon wave packets with near-unity efficiency and fidelity if the wave packet width is smaller than the cavity mode linewidth. We also find that using multiple waveguide-coupled cavities increases the switching bandwidth, allowing wider wave packets to be routed with high efficiency and fidelity. For example, an array of three waveguide-coupled cavities can reflect an input Gaussian wave packet with a full width at half-maximum of 1 nm (corresponding to a few-picosecond pulse) with an efficiency E_r = 96.4% and a fidelity F_r = 97.7%, or transmit the wave packet with an efficiency E_t = 99.7% and a fidelity F_t = 99.8%. Such efficient, high-fidelity single-photon routing is essential for scalable photonic quantum technologies.