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Results in Physics

Coherence, purity and correlation for superconducting charge qubits

K. Berrada, S. Abdel‐Khalek, Mariam Algarni

Published Mar 30, 2023• 7 citations
Publisher

Abstract

In a model consisting of two superconducting charge qubits (SC-Qs), we study how the Josephson energy (JE) and temperature have an influence on the coherence, the degree of mixedness, and the nonclassical correlation. In this example, we show how the quantumness metrics change depending on the JE and the temperature. We also show that the number of quantifiers can be controlled by appropriate selection of the JE of the SC-Q and temperature effect. Additionally, we determine the optimal conditions required to enhance and preserve the coherence and maintain the level of coherence in the existence of temperature variations. We observe important quantum phenomena, i.e., coherence trapping and the quantum correlation trapping. Furthermore, we explain the variations in the coherence based on the degree of mixedness in the superconducting qubit state.

DOI
10.1016/j.rinp.2023.106414

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In a model consisting of two superconducting charge qubits (SC-Qs), we study how the Josephson energy (JE) and temperature have an influence on the coherence, the degree of mixedness, and the nonclassical correlation. In this example, we show how the quantumness metrics change depending on the JE and the temperature. We also show that the number of quantifiers can be controlled by appropriate selection of the JE of the SC-Q and temperature effect. Additionally, we determine the optimal conditions required to enhance and preserve the coherence and maintain the level of coherence in the existence of temperature variations. We observe important quantum phenomena, i.e., coherence trapping and the quantum correlation trapping. Furthermore, we explain the variations in the coherence based on the degree of mixedness in the superconducting qubit state.
Abstract Superconducting charge qubits represent a cutting-edge technology in the field of quantum computing, offering a promising platform for quantum processing. This study delves into the behaviors of thermal coherence and quantum correlations within a two-superconducting charge qubit system coupled by a fixed capacitance. Specifically, we investigate the effects of thermal noise on entanglement (measured by concurrence), nonclassical correlations (quantified by local quantum uncertainty), and quantum coherence (measured by correlated coherence) within the two-superconducting charge qubit capacitively coupled. Our analysis takes into account the interplay between the equilibrium temperature of the reservoir and various system parameters. Our findings demonstrate that an increase in temperature leads to a decrease in coherence and quantum correlations within the considered system. However, the behavior of these quantum resources is heavily dependent on the system parameters, and a careful selection of these parameters can help mitigate the negative effects of absolute temperature. Additionally, we observe that local quantum uncertainty and correlated coherence are more resilient than thermal entanglement to rising temperatures. These results provide insight into how a two-superconducting charge qubit system can be optimized for achieving quantum advantages.
We investigate the time evolutions of purity, coherence and entanglement in two superconducting qubits coupled through Josephson junctions. The system is modeled by coupling strength, energy, Josephson energy and phase difference. Meanwhile, the structure is under the action of thermal field and classical Ornstein–Uhlenbeck (OU) noise. According to our findings, quantum correlations are notably developed governed by the system parameters. Purity and entanglement generally increase for the energetic qubit regimes. The state’s entanglement and purity degrade with increased coupling strength and temperature. On the other hand, the optimal preservation of quantum correlations reqires the adjustment to intermediate Josephson energies ([Formula: see text]) and phase differences ([Formula: see text]). The coherence measurement shows completely opposite traits compared to the entanglement and purity in the system. These results reveal that quantum states need proper parametrization for the considered state while providing optimization for the higher performance of superconducting qubits in quantum information processing.
This study examines the influence of temperature and Josephson energy on the quantum properties of superconducting qubits, specifically Bell non-locality and quantum coherence. Our results demonstrate that increasing temperature induces a degradation of both of these essential properties for quantum computing. In contrast, Josephson energy is revealed to be an adjustable parameter allowing for the modulation of non-locality and coherence. However, we observe a trade-off in the simultaneous optimization of these two quantum resources via Josephson energy. This study underscores the crucial importance of thermal control and precise engineering of qubit parameters for the development of high-performance and robust superconducting quantum technologies.
Optimal procedures and appropriate designs of transmitting channels are required to preserve quantum correlations (QCs) in quantum teleportation schemes. In pursuit of this objective, a comprehensive investigation into the intricate dynamics of a quantum channel comprising two singular Cooper-pair charge qubits, mutually coupled and influenced by Josephson energy, has been undertaken. The use of the Gibbs density operator has provided valuable insights into the thermal quantum characteristics and teleportation fidelity of the system. Furthermore, this study underscores the critical interplay between the energy parameters and the utilization of quantum features to establish conducive conditions for the preservation of nonclassicality and the success of the teleportation scheme. Additionally, the study reveals that trace distance discord is the most resilient in the thermal output state, surpassing both concurrence and the degree of violation of Bell’s inequality. Notably, precise control of the input state amplitude and adept manipulation of the energy parameters have enhanced QCs and resulted in the teleportation fidelity exceeding the classical limit.
We investigate the energy and phase relaxation of a superconducting qubit caused by a single quasiparticle. In our model, the qubit is an isolated system consisting of a small island (Cooper-pair box) and a larger superconductor (reservoir) connected with each other by a tunable Josephson junction. If such a system contains an odd number of electrons, then even at lowest temperatures a single quasiparticle is present in the qubit. Tunneling of a quasiparticle between the reservoir and the Cooper-pair box results in the relaxation of the qubit. We derive master equations governing the evolution of the qubit coherences and populations. We find that the kinetics of the qubit can be characterized by two time scales---quasiparticle escape time from the reservoir to the box ${\ensuremath{\Gamma}}_{\text{in}}^{\ensuremath{-}1}$ and quasiparticle relaxation time $\ensuremath{\tau}$. The former is determined by the dimensionless normal-state conductance gT{g}_{T}gT​ of the Josephson junction and one-electron level spacing ${\ensuremath{\delta}}_{r}$ in the reservoir $({\ensuremath{\Gamma}}_{\text{in}}\ensuremath{\sim}{g}_{T}{\ensuremath{\delta}}_{r})$, and the latter is due to the electron-phonon interaction. We find that phase coherence is damped on the time scale of ${\ensuremath{\Gamma}}_{\text{in}}^{\ensuremath{-}1}$. The qubit energy relaxation depends on the ratio of the two characteristic times $\ensuremath{\tau}$ and ${\ensuremath{\Gamma}}_{\text{in}}^{\ensuremath{-}1}$ and also on the ratio of temperature TTT to the Josephson energy EJ{E}_{J}EJ​.
The influence of intrinsic decoherence on various correlations and dense coding in a model which consists of two identical superconducting charge qubits coupled by a fixed capacitor is investigated. The results show that, despite the intrinsic decoherence, the correlations as well as the dense coding channel capacity can be effectively increased via the combination of system parameters, i.e., the mutual coupling energy between the two charge qubits is larger than the Josephson energy of the qubit. The bigger the difference between them is, the better the effect is.