  Sources of entangled multiphotons are not only essential for fundamental
tests of quantum foundations, but are also the cornerstone of a variety of
optical quantum technologies today. Over 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, for the first time, the observation of
continuous-mode time-energy-entangled W-class triphotons with an unprecedented
generation rate directly through the process of spontaneous six-wave mixing
(SSWM) in a four-level triple-Lambda 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. Most importantly, our
work paves a way for the development of a reliable and efficient genuine
triphoton source, thus making the research on multiphoton entanglement within
easy reach.
