“…They enable efficient coupling of electromagnetic waves from free space to nanoscale systems [5][6][7][8][9], and have therefore found a broad range of applications, including spectroscopy, non-linear optics [10,11], photodetection [12][13][14][15], and solar energy harvesting [16,17]. Additionally, novel phenomena continue to be discovered in the field of plasmonics, such as quantum interference of plasmons [18][19][20], quantum coupling of plasmons to single-particle excitations, and quantum confinement of plasmons in single-nm scale plasmonic particles [21][22][23]. Experimental developments in quantum plasmonics have shown, remarkably, the ability of surface plasmons to preserve many key quantum mechanical properties of the photons used to excite them, notably entanglement, interferometry and sub-Poissonian statistics.…”