Nonlocality and the Rotating Wave Approximation

Bruce J. West, Katja Lindenberg

Zi-Yu Xiong, Yongjun Xiao, Ye-Qi Zhang +1 more

We investigate the effect of counter-rotating-wave terms on nonlocality and entanglement for three qubits coupled to a common bath for strong- and ultrastrong-coupling regimes beyond the traditional treatment of Born-Markovian, perturbative, and rotating-wave approximations by employing the numerical hierarchical equations-of-motion approach. Our findings are as follows: (1) In the strong-coupling regime, the counter-rotating terms accelerate the decay of genuine three-party correlations, and the obvious sudden birth of Bell nonlocality is found. (2) In the ultrastrong-coupling regime, we observe a phenomenon in which nonlocality is consistently transferred between a three-qubit system and its subsystem. In addition, the inclusion of counter-rotating wave terms obviously enhances genuine tripartite nonlocality. (3) These counter-rotating terms cannot effectively generate genuine three-party correlations in zero-excitation cases, which differs from previous studies involving only two qubits. (4) A significant reduction in steady-state correlations for three- and two-party systems due to counter-rotating-wave terms is observed, which is not detectable under the rotating-wave approximation.

Mohd Asad Siddiqui, Tabish Qureshi

Victor V. Albert, Gregory D. Scholes, Paul W. Brumer

The single-mode spin-boson model exhibits behavior not included in the rotating-wave approximation (RWA) in the ultra and deep-strong coupling regimes, where counter-rotating contributions become important. We introduce a symmetric rotating-wave approximation that treats rotating and counter-rotating terms equally, preserves the invariances of the Hamiltonian with respect to its parameters, and reproduces several qualitative features of the spin-boson spectrum not present in the original rotating-wave approximation both off-resonance and at deep-strong coupling. The symmetric rotating-wave approximation allows for the treatment of certain ultra- and deep-strong coupling regimes with similar accuracy and mathematical simplicity as does the RWA in the weak-coupling regime. Additionally, we symmetrize the generalized form of the rotating-wave approximation to obtain the same qualitative correspondence with the addition of improved quantitative agreement with the exact numerical results. The method is readily extended to higher accuracy if needed. Finally, we introduce the two-photon parity operator for the two-photon Rabi Hamiltonian and obtain its generalized symmetric rotating-wave approximation. The existence of this operator reveals a parity symmetry similar to that in the Rabi Hamiltonian as well as another symmetry that is unique to the two-photon case, providing insight into the mathematical structure of the two-photon spectrum, significantly simplifying the numerics, and revealing some interesting dynamical properties.

Daniel Zeuch, Fabian Hassler, Jesse J. Slim +1 more

Yuyu Zhang, Qing‐Hu Chen

The generalized rotating-wave approximation (GRWA) is presented for the two-qubit quantum Rabi model. The analytical expressions in the zeroth-order approximation recover the previous adiabatic ones. The counter-rotating-wave terms can be eliminated by performing the first-order corrections. An effective solvable Hamiltonian with the same form as the ordinary RWA one is then obtained, giving very accurate eigenvalues and eigenstates. Energy levels in the present GRWA are in accordance with the numerical exact diagonalization ones in a wide range of coupling strengths. The population dynamics in the GRWA are in quantitative agreement with the numerical results and exhibit the absence of collapses clearly, revealing the effects of the counter-rotating wave.