Integrated conversion and photodetection of virtual photons in an ultrastrongly coupled superconducting quantum circuit

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 favorable 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.