Simple variational ground state and pure-cat-state generation in the quantum Rabi model

Abstract

We introduce a simple, physically-motivated variational ground state for the quantum Rabi model, and demonstrate that it provides a high-fidelity approximation of the true ground state in all parameter regimes (including intermediate and strong coupling regimes). Our variational state is constructed using Gaussian cavity states and nonorthogonal qubit pointer states, and contains only three variational parameters. We use our state to develop a heuristic understanding of how the ground state evolves with increasing coupling, and find a previously unexplored regime where the ground state corresponds to the cavity being in a nearly pure Schrödinger cat state.

Enhancing Cavity Quantum Electrodynamics via Antisqueezing: Synthetic Ultrastrong Coupling

“…More challenging is reaching the so called ultra-strong coupling (USC) regime, where the coupling strength is comparable to the qubit/cavity frequency, or the deep-strong coupling (DSC) regime [17], where it exceeds the frequencies. Here, counter-rotating terms cannot be ignored, and the system is best described by the quantum Rabi model [18,19], which is known to exhibit a wide range of interesting phenomena, including strongly entangled and nonclassical eigenstates [20][21][22][23][24]. To date, only specially designed architectures have reached USC experimentally [25][26][27][28][29], though simulations of USC have also been considered [20,30,31].…”

Generalized adiabatic approximation to the quantum Rabi model

“…Therefore, the GAA brings no improvement to the ground state and the first excited state. For completeness, we note that these states can be well described by the non-orthogonal qubit states in all parameter regimes [27,38]. More generally, the description of the lowest k pairs of energy levels cannot be well improved by the GAA for ∆ in the range 2k < ∆/ω < 2(k + 1).…”

Nonorthogonal-qubit-state expansion for the asymmetric quantum Rabi model

“…In this paper, we introduce a variational wave function to describe the ground state of the AQRM. We start from physical observations of the system evolution with respect to the coupling strength, and propose a trial wave function consisting of a weighted superposition of squeezed coherent states entangled with non-orthogonal qubit states [41,42]. The resulting trial function, depending on only three variational parameters, describes the system ground state remarkably well in almost all parameter regimes.…”