Electroluminescence, the emission of light in the presence of an electric
current, provides information on the allowed electronic transitions of a given
system. It is commonly used to investigate the physics of strongly-coupled
light-matter systems, whose eigenfrequencies are split by the strong coupling
with the photonic field of a cavity. Here we show that, together with the usual
electroluminescence, systems in the ultrastrong light-matter coupling regime
emit a uniquely quantum radiation when a flow of current is driven through
them. While standard electroluminescence relies on the population of excited
states followed by spontaneous emission, the process we describe herein
extracts bound photons by the dressed ground state and it has peculiar features
that unequivocally distinguish it from usual electroluminescence.Comment: 6 pages, 3 figure
