  The ground-state of an artificial atom ultrastrongly coupled to quantized
modes is entangled and contains an arbitrary number of virtual photons. The
problem of their detection has been raised since the very birth of the field
but despite the theoretical efforts still awaits experimental demonstration.
Recently experimental problems have been addressed in detail showing that they
can be overcome by combining an unconventional design of the artificial atom
with advanced coherent control. In this work we study a simple scheme of
control-integrated continuous measurement which makes remarkably favourable the
tradeoff between measurement efficiency and backaction showing that the
unambiguous detection of virtual photons can be achieved within state-of-the
art quantum technologies.
