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In quantum field theory, bare particles are dressed by a cloud of virtualparticles to form physical particles. The virtual particles affect propertiessuch as the mass and charge of the physical particles, and it is only thesemodified properties that can be measured in experiments, not the properties ofthe bare particles. The influence of virtual particles is prominent in theultrastrong-coupling regime of cavity quantum electrodynamics (QED), which hasrecently been realized in several condensed-matter systems. In some of thesesystems, the effective interaction between atom-like transitions and the cavityphotons can be switched on or off by external control pulses. This offersunprecedented possibilities for exploring quantum vacuum fluctuations and therelation between physical and bare particles. Here we show that, by applyingexternal electromagnetic pulses of suitable amplitude and frequency, eachvirtual photon dressing a physical excitation in cavity-QED systems can beconverted into a physical observable photon, and back again. In this way, thehidden relationship between the bare and the physical excitations becomesexperimentally testable. The conversion between virtual and physical photonscan be clearly pictured using Feynman diagrams with cut loops.

Cutting Feynman Loops in Ultrastrong Cavity QED: Stimulated Emission and Reabsorption of Virtual Particles Dressing a Physical Excitation