Direct generation of reliable genuine W-class triphotons in atomic vapor

Sources of entangled multiphotons are essential for fundamental tests of quantum foundations and a variety of optical quantum technologies today. Over the past three decades, tremendous efforts have been devoted to creating multiphoton entanglement by multiplexing existing biphoton sources with linear optics and postselections. Different from all previous protocols, here we report the first observation of genuine reliable W-class triphoton generation with an unprecedented production rate directly through spontaneous six-wave mixing (SSWM) in a four-level triple-Λ atomic vapor cell. Facilitated by electromagnetically induced transparency and coherence control, our SSWM scheme enables versatile narrowband triphoton generation with many intriguing properties including long temporal coherence and controllable waveforms, ideal for implementing long-distance quantum communications, networking, and information processing by interfacing photons and atoms. Importantly, our work paves a way for the realization of the first dependable and efficient genuine triphoton source, making the research and applications based on multiphoton entanglement within easy reach.