  We demonstrate a plasmonic route to control the collective spontaneous
emission of two-level quantum emitters. Superradiance and subradiance effects
are observed over distances comparable to the operating wavelength inside
plasmonic nanochannels. These plasmonic waveguides can provide an effective
epsilon-near-zero operation in their cut-off frequency and Fabry-Perot
resonances at higher frequencies. The related plasmonic resonant modes are
found to efficiently enhance the constructive (superradiance) or destructive
(subradiance) interference between different quantum emitters located inside
the waveguides. By increasing the number of emitters located in the elongated
plasmonic channel, the superradiance effect is enhanced at the
epsilon-near-zero operation, leading to a strong coherent increase in the
collective spontaneous emission rate. In addition, the separation distance
between neighboring emitters and their emission wavelengths can be changed to
dynamically control the collective emission properties of the plasmonic system.
It is envisioned that the dynamic modification between quantum superradiant and
subradiant modes will find applications in quantum entanglement of qubits,
low-threshold nanolasers and efficient sensors.
