Near-Unity Coupling Efficiency of a Quantum Emitter to a Photonic Crystal Waveguide

TLDR

Efficient coupling of a quantum emitter to a nanophotonic waveguide enables single‑photon transistors, quantum‑logic gates, and high‑bit‑rate deterministic single‑photon sources, with the β‑factor—the probability that an emitted photon is channeled into the desired waveguide mode—being the key figure of merit. We achieved a β‑factor of 98.43 ± 0.04 % for a quantum dot in a photonic‑crystal waveguide, corresponding to a cooperativity of 62.7 ± 1.5, demonstrating a nearly ideal photon‑matter interface where the dot interacts almost exclusively with a single propagating mode, and the β‑factor is robust to position and wavelength variations, underscoring the potential for highly efficient single‑photon generation and on‑chip photon‑photon interaction.

Abstract

A quantum emitter efficiently coupled to a nanophotonic waveguide constitutes a promising system for the realization of single-photon transistors, quantum-logic gates based on giant single-photon nonlinearities, and high bit-rate deterministic single-photon sources. The key figure of merit for such devices is the $\beta$-factor, which is the probability for an emitted single photon to be channeled into a desired waveguide mode. We report on the experimental achievement of $\beta = 98.43 \pm 0.04\%$ for a quantum dot coupled to a photonic-crystal waveguide, corresponding to a single-emitter cooperativity of $\eta = 62.7 \pm 1.5$. This constitutes a nearly ideal photon-matter interface where the quantum dot acts effectively as a 1D "artificial" atom, since it interacts almost exclusively with just a single propagating optical mode. The $\beta$-factor is found to be remarkably robust to variations in position and emission wavelength of the quantum dots. Our work demonstrates the extraordinary potential of photonic-crystal waveguides for highly efficient single-photon generation and on-chip photon-photon interaction.