  Plasmonic nanocavities are known for their extreme field enhancement and
sub-wavelength light confinement in gaps of just a few nanometers. Pairing this
with the ability to host quantum emitters, they form highly promising platforms
to control or engineer quantum states at room temperature. Here, we use the
lossy nature of plasmonic nanocavities to form sub-radiant entangled states
between two or more quantum emitters, that persist for $\sim 100$ times longer
than the plasmonic excitation. We develop a theoretical description that
directly links quantum variables to experimentally measurable quantities, such
as the extinction cross-section, and unlike previous studies includes plasmonic
excitations necessary to resonantly form subradiant states. This work paves the
way towards engineering quantum entangled states in ambient conditions with
plasmonic nanocavities, for potential applications such as rapid quantum
memories, quantum communications and sensors.
