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Journal of the Optical Society of America B

First-order quantum phase transition in the two-qubit squeezed Rabi model

Xuan-Tong Pei, Zhi‐Cheng Shi, Li-Tuo Shen, Zhen‐Biao Yang

Published May 2, 2024• 0 citations
Publisher

Abstract

We study the ground state of the two-qubit squeezed Rabi model. Two special transformations are found to diagonalize the system Hamiltonian when each qubit’s frequency is close to the field frequency, where both the squeezing and counterrotating-wave interactions are removed, leading to an effective integrable Hamiltonian. The analytical ground state is determined and matches with numerical solutions well for a range of squeezing strengths and qubit-field detunings in the ultrastrong-coupling regime. We demonstrate that the ground state exhibits a first-order quantum phase transition at a phase boundary linearly induced by the squeezed light. We characterize the two-qubit negativity analytically and find that its two-qubit entanglement increases with the increasing squeezing strength nonlinearly. The average photon numbers of the field mode and variances of position and momentum quadratures are also analyzed and discovered to have a nonlinear relation with the squeezing strength. Finally, we discuss the experimental scheme and realization possibility of the predicted results.

DOI
10.1364/josab.519312

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Abstract Quantum phase transition and entanglement in the Rabi model with a squeezed light are investigated. We find a special unitary-transformation method that removes the nonintegrable squeezing and counterrotating-wave interactions when the qubit frequency is close to the field frequency. The analytical ground state agrees well with the numerical solution. We demonstrate that the ground state exhibits a first-order quantum phase transition at a critical point induced linearly by the squeezed light. This quantum phase transition requires neither multiple qubits nor an infinite ratio of qubit frequency to field frequency, which solves a critical problem for the theory and experiment in Rabi model. As the qubit-field coupling strength increases, the ground-state entanglement reaches its maximum value at the critical point.
Quantum phase transition and quench dynamics in the Jaynes-Cummings model with squeezed light are investigated. We find a special unitary transformation that removes the nonintegrable squeezing interaction when the qubit frequency far outweighs the field frequency. The eigenenergies and the eigenstates of the normal and superradiant phases are derived analytically. We demonstrate that the system exhibits a second-order quantum phase transition at a phase-boundary-induced nonlinearly by squeezed light. This phase boundary requires neither an ultrastrong coupling regime nor multiqubits, which lessens the difficulty of the experiment. The entanglement entropy is very close to its maximum value as the squeezing strength increases in the superradiant phase. In quench dynamics, the nonlinear relation between the residual energy and the squeezing strength is obtained analytically.
We study the quantum phase transition and entanglement in the Jaynes–Cummings model with squeezed light, utilize a special transformation method to obtain the analytical ground state of the model within the near-resonance regime, and numerically verify the validity of the analytical ground state. It is found that the ground state exhibits a first-order quantum phase transition at the critical point linearly induced by squeezed light, and the ground state entanglement reaches its maximum when the qubit-field coupling strength is large enough at the critical point.
By analytically solving the quantum Rabi model, we investigate the photonic properties of its ground eigenstate. In particular, we find that in the deep strong coupling regime, where the coupling strength ggg exceeds the mode frequency ωωω, the photonic state is effectively squeezed in one of its quadratures. The squeezing reaches its maximum at the curve corresponding to the quantum phase transition of the quantum Rabi system, and decreases rapidly on both sides of the phase transition. Notably, for g/ω≈3g/ω\approx 3g/ω≈3, which is experimentally testable in existing trapped-ion platforms, the achievable squeezing parameter can reach approximately r≈0.8r\approx 0.8r≈0.8. Intriguingly, the photonic state is squeezed while its number distribution follows a super-Poissonian distribution, with the largest deviation from Poissonian behavior occurring at the phase transition between the normal and superradiant phases. In other words, the ground state of the quantum Rabi model contains super-Poissonian quantum squeezed photons.
Recent experiments about the circuit cavity quantum electrodynamics have realized the ultrastrong couplings between the artificial atom and the photon, in which the coupling strengths have the same order of the photon frequency. In such a regime, the well-known rotating wave approximation is invalid, and the system dynamics is thus governed by the Rabi model. In this paper, we investigate the photon squeezing of the Rabi model. We find numerically that with the increase of the atom-photon coupling strength, the photon squeezing does not increase linearly, but displays a maximum in the ultrastrong coupling regime. In addition, we also reveal that the photonsqueezing can be enhanced by the counter-rotating terms of the Rabi model. Our results are of benefit to preparing the required squeezing state of the photon in experiment.
We revisit the analytic solution of the two-qubit quantum Rabi model with inhomogeneous coupling and transition frequencies using a displaced oscillator basis. This approach enables us to apply the same truncation rules and techniques adopted in the Rabi model to the two qubits system. The derived analytical spectra match perfectly with the numerical solutions in the parameter regime where the qubits’ transition frequencies are far off-resonance with the field frequency and the interaction strengths reach the ultrastrong coupling regime. We further explore the dynamical behavior of the two qubits as well as the evolution of entanglement. The analytical methods provide unexpectedly accurate results in describing the dynamics of the two qubits in the present experimentally accessible coupling regime. The time evolutions of the probability for the qubits show that the collapse-revival phenomena emerge, survive and finally disappear when one coupling strength increases from weak to strong coupling regimes and the other coupling strength is well into the ultrastrong coupling regime. The inhomogeneous coupling system exhibits new dynamics, which are different from the homogeneous coupling case.