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

TL;DR

This paper demonstrates how external electromagnetic pulses can convert virtual photons into real photons in ultrastrong cavity QED, enabling experimental exploration of quantum vacuum fluctuations and the relationship between bare and physical particles.

Contribution

It introduces a method to convert virtual photons into real photons using external pulses, providing a new way to test quantum vacuum effects in cavity QED.

Findings

Virtual photons can be converted into real photons with external pulses.

The process allows experimental probing of virtual particles in cavity QED.

Feynman diagrams illustrate the virtual-physical photon conversion mechanism.

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 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.