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Physica Scripta

Quantum feature analysis in double qubit–cavity coupling system under dephasing

Taher Hassan, S. I. Ali, Mouataz Billah Mesmouli, Hasan Nihal Zaidi, Rabie Α. Ramadan

Published May 7, 2026• 0 citations
Publisher

Abstract

Abstract We investigate the dynamics of entanglement, steerability, and Bell non-locality in a circuit quantum electrodynamics (QED) system of two identical superconducting qubits coupled to a cavity, modeled via a rigorous global master equation to ensure validity in the ultrastrong coupling (USC) regime. Subject to dephasing, spontaneous emission, and cavity loss, our analysis reveals several key findings. First, we observe a consistent numerical ordering of robustness using generalized measures for arbitrary quantum states, where entanglement persists significantly longer than steerability, and Bell non-locality is the most fragile resource. We emphasise that this observed ordering reflects the specific circuit-QED model and Markovian noise parameterisation studied here, and should not be interpreted as a proven general hierarchy for arbitrary states or measures. Second, comparing pure and mixed Werner initial states, we demonstrate that operating in the USC regime is highly advantageous, inducing pronounced, coherent revivals that preserve quantum correlations. Third, we clarify the nuanced role of the qubit mixing angle, which modulates the dynamics of initially entangled states and decisively suppresses the generation of correlations from the ground state at a key symmetry point. Our results underscore that preparing a maximally entangled initial state is a crucial prerequisite for robustness, offering quantitative guidance for engineering resilient quantum states in noisy circuit QED systems.

DOI
10.1088/1402-4896/ae6a48

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Abstract We investigate the dynamics of entanglement, steerability, and Bell non-locality in a circuit quantum electrodynamics (QED) system of two identical superconducting qubits coupled to a cavity, modeled via a rigorous global master equation to ensure validity in the ultrastrong coupling (USC) regime. Subject to dephasing, spontaneous emission, and cavity loss, our analysis reveals several key findings. First, we observe a consistent numerical ordering of robustness using generalized measures for arbitrary quantum states, where entanglement persists significantly longer than steerability, and Bell non-locality is the most fragile resource. We emphasise that this observed ordering reflects the specific circuit-QED model and Markovian noise parameterisation studied here, and should not be interpreted as a proven general hierarchy for arbitrary states or measures. Second, comparing pure and mixed Werner initial states, we demonstrate that operating in the USC regime is highly advantageous, inducing pronounced, coherent revivals that preserve quantum correlations. Third, we clarify the nuanced role of the qubit mixing angle, which modulates the dynamics of initially entangled states and decisively suppresses the generation of correlations from the ground state at a key symmetry point. Our results underscore that preparing a maximally entangled initial state is a crucial prerequisite for robustness, offering quantitative guidance for engineering resilient quantum states in noisy circuit QED systems.
Abstract We investigate the non-Gaussian feature of radiation in a circuit quantum electrodynamics (QED) system where two qubits are strongly coupled to a single-mode cavity. In the regime of ultrastrong coupling (USC), the rotating-wave approximation is not valid, and the Rabi Hamiltonian contains counter-rotating wave terms, leading to level crossing and avoided crossings in the energy spectrum. We further analyze the intensity-amplitude correlation of the output field in these two novel scenarios. In the USC regime, the creation and annihilation operators in the correlation function are replaced, allowing for the identification of non-Gaussian features in the output field. Our findings reveal that despite the absence of squeezing effects in the output light, significant non-Gaussian characteristics are present. Additionally, we demonstrate that as the driving or coupling strength increases, the non-Gaussian features of the output field become more pronounced. This suggests that USC systems hold broad potential applications in the realms of nonlinear optics and the generation of non-Gaussian states.
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Hybrid quantum systems integrating superconducting qubits and nitrogen-vacancy (NV) center qutrits are promising for quantum information processing but are vulnerable to environmental dephasing. This study investigates the distinct dynamical behaviors of three facets of quantumness — total nonclassical correlations (Local Quantum Uncertainty (LQU)), quantum superposition ([Formula: see text] norm of coherence), and entanglement (Negativity) — in such a qubit–qutrit hybrid system subjected to non-Markovian dephasing noise. We reveal a clear hierarchy of robustness: coherence exhibits remarkably persistent oscillations, LQU decays with damped oscillations to a stable non-zero baseline, while entanglement proves most fragile, suffering periodic sudden death. Our central finding is the uniquely constructive role of the direct qubit–qutrit coupling strength, which robustly enhances all three quantum resources. In contrast, most other system parameters either accelerate decoherence or have complex, non-monotonic effects. These results clarify the different resilience levels of quantum resources and identify key parameters for preserving quantumness in noisy hybrid systems.
The robustness of quantum synchronization induced by the dynamical Casimir effect (DCE) is analyzed within a circuit quantum electrodynamics (cQED) architecture consisting of two superconducting qubits coupled to a shared, parametrically driven cavity. Using a Lindblad master equation approach, this study evaluates the impact of various decoherence channels (including photon loss, relaxation, thermal excitation, and pure dephasing) on the resulting synchronization dynamics. The results demonstrate that DCE-induced synchronization persists under dissipation rates achievable in state-of-the-art superconducting platforms, confirming its experimental feasibility. Among the studied mechanisms, pure dephasing is identified as the dominant factor limiting the fidelity of the synchronized state. Furthermore, it is shown that in strongly dissipative regimes, the Pearson correlation coefficient may overestimate the degree of quantum synchronization, as environmental relaxation can generate classical correlations that mimic synchronized behavior. These findings establish the parameter regimes necessary for observing genuine DCE-induced quantum synchronization and provide practical guidelines for future cQED experiments.
Abstract We investigate dissipation-induced trade-offs between nonlocal entanglement and local quantum coherence in a two-qubit system coupled to an open superconducting cavity. Using concurrence C , intrinsic concurrence, and the total first-order coherence ( FOC ) of the reduced two-qubit state, we examine how the initial distribution angle, coherent-field intensity, and spontaneous-emission damping redistribute quantum resources during the system evolution. We find that increasing the damping rate and coherent-field intensity suppresses entanglement generation and revivals, whereas the FOC can remain finite over a wide range of parameters. Within this open-system description, the purity of the two-qubit state (or equivalently its mixedness) provides an effective budget that governs the competition between coherence and entanglement in the dynamics. Our results provide a clear physical picture of how dissipation reshapes the balance between local coherence and nonlocal quantum correlations in realistic qubit-cavity platforms, highlighting the relative robustness of coherence compared to entanglement under environmental interactions.
Cavity quantum electrodynamics (QED) investigates the interaction between resonator-confined radiation and natural atoms. Similar phenomena can be explored using superconducting artificial atoms coupled to microwave resonators. Here, we examine incoherent and coherent spectra in a circuit QED system consisting of a flux qubit coupled to an LC resonator, considering different coupling configurations to the output transmission line. Although such systems can be effectively described by the quantum Rabi model, we demonstrate that the choice of the coupling between the resonator and the input-output channel can significantly affect the emission properties, particularly in the ultrastrong coupling regime. Specifically, we show that when the resonator is capacitively coupled with the input-output transmission line, the spectral features at zero flux offset can be mapped onto those of the cavity QED model. However, this correspondence breaks down for the mutual inductive coupling case. Extending our analysis to the parity-symmetry broken regime of the flux qubit, we further highlight how capacitive and inductive coupling mechanisms lead to distinct spectral features, emphasizing the necessity of a careful input-output treatment for an accurate description of the emission processes.