  In quantum field theory, bare particles are dressed by a cloud of virtual
particles to form physical particles. The virtual particles affect properties
such as the mass and charge of the physical particles, and it is only these
modified properties that can be measured in experiments, not the properties of
the bare particles. The influence of virtual particles is prominent in the
ultrastrong-coupling regime of cavity quantum electrodynamics (QED), which has
recently been realized in several condensed-matter systems. In some of these
systems, the effective interaction between atom-like transitions and the cavity
photons can be switched on or off by external control pulses. This offers
unprecedented possibilities for exploring quantum vacuum fluctuations and the
relation between physical and bare particles. Here we show that, by applying
external electromagnetic pulses of suitable amplitude and frequency, each
virtual photon dressing a physical excitation in cavity-QED systems can be
converted into a physical observable photon, and back again. In this way, the
hidden relationship between the bare and the physical excitations becomes
experimentally testable. The conversion between virtual and physical photons
can be clearly pictured using Feynman diagrams with cut loops.
