Ultrastrong-coupling quantum-phase-transition phenomena in a few-qubit circuit QED system

Abstract

We study ultrastrong-coupling quantum-phase-transition phenomena in a few-qubit system. In the one-qubit case, three second-order transitions occur and the Goldstone mode emerges under the condition of ultrastrong-coupling strength. Moreover, a first-order phase transition occurs between two different superradiant phases. In the two-qubit case, a two-qubit Hamiltonian with qubit-qubit interactions is analyzed fully quantum mechanically. We show that the quantum phase transition is inhibited even in the ultrastrong-coupling regime in this model. In addition, in the three-qubit model, the superradiant quantum phase transition is retrieved in the ultrastrong-coupling regime. Furthermore, the N -qubit model with U (1) symmetry is studied and we find that the superradiant phase transition is inhibited or restored with the qubit-number parity.

Dynamic sensitivity of quantum Rabi model with quantum criticality

“…1(a), the ground-state energy obtained by these three methods in the normal phase are consistent with each other in the large η case. As the ratio η decreases, the deviation between the approximate result and the exact result in the finite 19), (21), and (24) (crosses).…”

“…Consequently, an interesting question is whether quantum phase transition can take place in a finite-component system. It has recently been shown that quantum phase transition can take place in simple systems [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. An advantage of this kind of systems is that the systems have less degrees of freedom, and hence those previous mentioned difficulties in infinite-component systems can be improved [22].…”

Effects of counter-rotating-wave terms on the noisy frequency estimation

“…First, in our work, the strength of the counter-rotatingwave terms in the interaction Hamiltonian, namely the parameter χ, is continuously tunable for the entire range of χ ∈ [0, 1], which can build a bridge connecting two particular limits: totally with and without RWA. In fact, such a controllable strength of the counterrotating-wave terms in the quantum Rabi model is called anisotropy [59][60][61], which leads to a much richer groundstate phase diagram [60,61], and can be used to explain the anomalous Bloch-Siegert shift in the ultrastrongcoupling regime [62]. These previous studies of the anisotropic quantum Rabi model motivate us to explore the influence of the counter-rotating-wave terms on the metrological precision by introducing the engineered tunable parameter χ.…”

Simulating Anisotropic quantum Rabi model via frequency modulation

“…Due to such interesting characteristics, the AQRM has been utilized to study various theoretical issues, e.g., quantum phase transitions 67 , 68 , quantum state engineering 69 , quantum fisher information 70 , and so on. To date, people have proposed several methods to realize AQRM, which include the natural implementations of AQRM in quantum optics in a cross-electric and magnetic field 64 , electrons in semiconductors with spin-orbit coupling 70 , 71 , and superconducting circuits systems 72 , 73 . Meanwhile, quantum simulation methods with superconducting circuits 74 and trapped ions 75 have also been proposed.…”