A qubit strongly coupled to a resonant cavity: asymmetry of the spontaneous emission spectrum beyond the rotating wave approximation

Abstract

We investigate the spontaneous emission spectrum of a qubit in a lossy resonant cavity. We use neither the rotating-wave approximation nor the Markov approximation. The qubit-cavity coupling strength is varied from weak, to strong, even to lower bound of the ultra-strong. For the weak-coupling case, the spontaneous emission spectrum of the qubit is a single peak, with its location depending on the spectral density of the qubit environment. Increasing the qubit-cavity coupling increases the asymmetry (the positions about the qubit energy spacing and heights of the two peaks) of the two spontaneous emission peaks (which are related to the vacuum Rabi splitting) more. Explicitly, for a qubit in a low-frequency intrinsic bath, the height asymmetry of the splitting peaks becomes larger, when the qubit-cavity coupling strength is increased. However, for a qubit in an Ohmic bath, the height asymmetry of the spectral peaks is inverted from the same case of the low-frequency bath, when the qubit is strongly coupled to the cavity. Increasing the qubit-cavity coupling to the lower bound of the ultra-strong regime, the height asymmetry of the left and right peak heights are inverted, which is consistent with the same case of low-frequency bath, only relatively weak. Therefore, our results explicitly show how the height asymmetry in the spontaneous emission spectrum peaks depends not only on the qubit-cavity coupling, but also on the type of intrinsic noise experienced by the qubit.

Output field-quadrature measurements and squeezing in ultrastrong cavity-QED

“…Recently, a new regime of cavity quantum electrodynamics (QED) has been experimentally reached in different solid state systems and spectral ranges [1][2][3][4][5][6][7][8]. In this so-called ultrastrong coupling (USC) regime, where the light-matter coupling rate becomes an appreciable fraction of the unperturbed resonance frequency of the system, the routinely invoked rotating wave approximation (RWA) is no longer applicable and the antiresonant terms significantly change the standard cavity-QED scenarios [9][10][11][12][13][14][15][16][17][18].…”

Generalized rotating-wave approximation for the two-qubit quantum Rabi model

“…In this regime, the ubiquitous rotating-wave approximation (RWA) [6] is expected to break down, leading to a mass of unexplored physics and giving rise to fascinating quantum phenomena, such as the asymmetry of vacuum Rabi splitting [8,9], collapse and revival dynamics [10,11], a Bloch-Siegert shift [2], super-radiance transition [12][13][14], and radiation processes based on virtual photons [15][16][17]. It is highly desirable to understand the behavior of the qubit-oscillator in the whole coupling regime.…”

Enhancement of the two-photon blockade in a strong-coupling qubit-cavity system

“…For instance, the qubit and the quantum harmonic oscillator can repeatedly exchange excitations before the dissipation becomes effective; the ground state of the system is no longer the standard vacuum, but a quasidegenerate with entangled structure; photons can be created from the quantum vacuum [35]; the Purcell effect is reversed [36]; etc. These novel features can lead to the asymmetry of vacuum Rabi splitting [37], superradiance transition [38], nonclassical photon statistics [39,40], and virtual photons [41,42] because the number of excitations in the cavity-transmission system is no longer conserved. In the meantime, energy levels [43] in the regime of the strong interaction may intercross.…”