Feynman-diagrams approach to the quantum Rabi model for ultrastrong cavity QED: stimulated emission and reabsorption of virtual particles dressing a physical excitation

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.

Deterministic quantum nonlinear optics with single atoms and virtual photons

“…The experimental progress in USC physics has motivated many theoretical studies of the interesting new effects that occur in this regime [40][41][42][43][44][45][46][47][48][49][50][51]. Some previous [52][53][54] and concurrent [55,56] works explore processes in the USC regime where the number of excitations is not conserved, such as multiphoton Rabi oscillations [53] and a single photon exciting multiple atoms [54].…”

Two-photon quantum Rabi model with superconducting circuits

“…Together with experimental advances, the theoretical interest in the USC regime has been steadily growing, concerning fundamental properties of ultrastrong light-matter interactions [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31], potential applications in quantum information [32][33][34][35][36][37] and effective implementations [38][39][40][41][42][43][44][45][46][47]. Such results have prompted a number of theoretical studies on generalizations of the QRM, including multiqubit [48] and multimode [49] cases, as well as anisotropic couplings [50,51] and two-photon interactions [52][53][54][55].…”

Hybrid Rabi interactions with traveling states of light

“…Recently, a sequence of RIs between a qubit and an oscillator has been proposed to induce the deterministic Kerr, cubic or arbitrary-order nonlinear phase gates [56,57]. It was also studied for exhibition of universal phase transition properties [58][59][60], multi-photon exchange [61], stimulated emission [62], microwave-to-optical conversion [63], generation of non-Gaussian states [64][65][66], and decomposition of arbitrary unitary dynamics [67]. This power enabling synthesis of various types of nonlinearity and observation of consequent nonlinear properties is the reason why achieving RI on continuous variable platforms is important.…”