  The ability to harness light-matter interactions at the few-photon level
plays a pivotal role in quantum technologies. Single photons - the most
elementary states of light - can be generated on-demand in atomic and solid
state emitters. Two-photon states are also key quantum assets, but achieving
them in individual emitters is challenging because their generation rate is
much slower than competing one-photon processes. We demonstrate that atomically
thin plasmonic nanostructures can harness two-photon spontaneous emission,
resulting in giant far-field two-photon production, a wealth of resonant modes
enabling tailored photonic and plasmonic entangled states, and plasmon-assisted
single-photon creation orders of magnitude more efficient than standard
one-photon emission. We unravel the two-photon spontaneous emission channels
and show that their spectral line-shapes emerge from an intricate interplay
between Fano and Lorentzian resonances. Enhanced two-photon spontaneous
emission in two-dimensional nanostructures paves the way to an alternative
efficient source of light-matter entanglement for on-chip quantum information
processing and free-space quantum communications.
