Abstract
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 realised 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. We consider a single three-level quantum system coupled to an optical resonator. 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 can be unravelled and becomes experimentally testable. The conversion between virtual and physical photons can be clearly pictured using Feynman diagrams with cut loops.
Highlights
In quantum field theory, the creation and annihilation operators in the Lagrangian describe the creation and destruction of bare particles which, cannot be directly observed in experiments
The influence of virtual particles is prominent in the ultrastrongcoupling regime of cavity quantum electrodynamics (QED), which has recently been realized in several condensed-matter systems
The results presented here show that the ultrastrong coupling (USC) regime of cavity QED can be exploited to observe, in a direct way, how interactions dress observed particles by a cloud of virtual particles
Summary
The creation and annihilation operators in the Lagrangian describe the creation and destruction of bare particles which, cannot be directly observed in experiments (see, e.g., Refs. [1, 2]). The energy corrections to the ground state and to the excited states of a cavity-QED system in the USC regime are described by loop diagrams corresponding to the emission and reabsorption of virtual photons. An interesting feature of these condensed-matter systems is that the effective interaction between atom-like transitions and the cavity field can be switched on and off by applying external drives This offers the opportunity to convert the virtual excitations into real particles which can be detected. Both spontaneous [7] and stimulated [34] conversion of virtual photons from the ground state of a cavity QED system in the USC regime have recently been analyzed. The proposed scheme, does not need ultrafast modulation of boundary conditions and it can give rise to a conversion probability close to one
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