Weak Coupling Conditions Research Articles

Generalized Quantum Fluctuation Theorem for Energy Exchange.

The nonequilibrium fluctuation relation is a cornerstone of quantum thermodynamics. It is widely believed that the system-bath heat exchange obeys the famous Jarzynski-Wójcik fluctuation theorem. However, this theorem is established in the Born-Markovian approximation under the weak-coupling condition. Via studying the energy exchange between a harmonic oscillator and its coupled bath in the non-Markovian dynamics, we establish a generalized quantum fluctuation theorem for energy exchange being valid for arbitrary coupling strength. The Jarzynski-Wójcik fluctuation theorem is recovered in the weak-coupling limit. We also find the average energy exchange exhibits rich nonequilibrium characteristics when different numbers of system-bath bound states are formed, which suggests a useful way to control the quantum heat. Deepening our understanding of the fluctuation relation in quantum thermodynamics, our result lays the foundation to design high-efficiency quantum heat engines.

Orthogonal multi-peak solitons from the coupled fractional nonlinear Schrödinger equation

In this paper, we investigate the dynamics and stability of multi-peak solitons from the coupled nonlinear Schrödinger equation with the fractional dimension based on Lévy random flights. By implementing linear stability analysis and direct simulations, we demonstrate regions where the single and multi-peak modes are stable. Analysis of perturbed coupled solitons confirms the stability of higher-order modes compared to lower-order modes under the same configurations. The stability diagrams show that the force coupling, Lévy index, power, and the nonlinear intensity significantly influence the stability of high-order modes. Our findings indicate that stability is favored in self-defocusing systems with high Lévy indices under weak coupling conditions, with higher-order states exhibiting smaller stability regions.

Semi-explicit integration of second order for weakly coupled poroelasticity

We introduce a semi-explicit time-stepping scheme of second order for linear poroelasticity satisfying a weak coupling condition. Here, semi-explicit means that the system, which needs to be solved in each step, decouples and hence improves the computational efficiency. The construction and the convergence proof are based on the connection to a differential equation with two time delays, namely one and two times the step size. Numerical experiments confirm the theoretical results and indicate the applicability to higher-order schemes.

Quantum nonlinear effect in a dissipatively coupled optomechanical system.

A full-quantum approach is used to study the quantum nonlinear properties of a compound Michelson-Sagnac interferometer optomechanical system. By deriving the effective Hamiltonian, we find that the reduced system exhibits a Kerr nonlinear term with a complex coefficient, entirely induced by the dissipative and dispersive couplings. Unexpectedly, the nonlinearities resulting from the dissipative coupling possess non-Hermitian Hamiltonian-like properties preserving the quantum nature of the dispersive coupling beyond the traditional system dissipation. This protective mechanism allows the system to exhibit strong quantum nonlinear effects when the detuning (the compound cavity detuning Δc and the auxiliary cavity detuning Δe) and the tunneling coupling strength (J) of two cavities satisfy the relation J2 = ΔcΔe. Moreover, the additive effects of dispersive and dissipative couplings can produce strong anti-bunching effects, which exist in both strong and weak coupling conditions. Our work may provide a new way to study and produce strong quantum nonlinear effects in dissipatively coupled optomechanical systems.

A Phase Model of the Bio-Inspired NbOx Local Active Memristor under Weak Coupling Conditions.

For some so-called computationally difficult problems, using the method of Boolean logic is fundamentally inefficient. For example, the vertex coloring problem looks very simple, but the number of possible solutions increases sharply with the increase of graph vertices. This is the difficulty of the problem. This complexity has been widely studied because of its wide applications in the fields of data science, life science, social science, and engineering technology. Consequently, it has inspired the use of alternative and more effective non-Boolean methods for obtaining solutions to similar problems. In this paper, we explore the research on a new generation of computers that use local active memristors coupling. First, we study the dynamics of the memristor coupling network. Then, the simplified system phase model is obtained. This research not only clarifies a physics-based calculation method but also provides a foundation for the construction of customized analog computers to effectively solve NP-hard problems.

Thermal transport mechanism of 4H–SiC/SiO2 heterostructures: a molecular dynamics study

Silicon carbide (SiC) is widely used in high-frequency, high-speed, and high-power applications such as power electronics, rail transportation, new energy vehicles, and aerospace. However, the thermal properties of the SiC/SiO2 interface, which is commonly found in SiC-based devices, are not yet fully understood. This study aims to investigate the influence of temperature and interface coupling strength on the interface thermal resistance (ITR) of 4H-SiC/SiO2 using non-equilibrium molecular dynamics simulations. Both crystalline and amorphous SiO2, as well as two interface contact modes (Si-terminated and C-terminated), have also been considered. The results reveal that the ITR of 4H-SiC/SiO2 is significantly affected by the interface coupling strength and contact modes. Under strong interface coupling conditions, the ITR for Si-terminated and C-terminated contacts modes of 4H-SiC/SiO2 interfaces are 8.077 × 10−10 m2KW−1 and 6.835 × 10−10 m2KW−1, respectively. However, under weak interface coupling conditions, these values increase to 10.142 × 10−10 m2KW−1 and 7.785 × 10−10 m2KW−1, respectively. Regardless of whether SiO2 is crystalline or amorphous, the ITR of the 4H-SiC/SiO2 interface exhibits a similar trend with increasing temperature (from 300 to 700 K). Additionally, the ITR of the amorphous SiO2 interface is smaller than that of the crystalline SiO2 interface under both strong and weak coupling conditions. To gain insights into the heat transport mechanism, the phonon density of states was analyzed to examine the phonon spectral characteristics under varying coupling strengths. These findings have implications for enhancing the thermal management and heat dissipation of SiC devices, providing a framework for controlling interface phonon scattering, and informing the thermal design of nanodevices.

Effects of Non-Adiabatic Electromagnetic Vacuum Fluctuations on Internal Conversion.

To explore non-adiabatic effects caused by electromagnetic (EM) vacuum fluctuations in molecules, we develop a general theory of internal conversion (IC) in the framework of quantum electrodynamics and propose a new mechanism, "quantum electrodynamic internal conversion" (QED-IC). The theory allows us to compute the rates of the conventional IC and QED-IC processes at the first-principles level. Our simulations manifest that, under experimentally feasible weak light-matter coupling conditions, EM vacuum fluctuations can significantly affect IC rates by an order of magnitude. Moreover, our theory elucidates three key factors in the QED-IC mechanism: the effective mode volume, coupling-weighted normal mode alignment, and molecular rigidity. The theory successfully captures the nucleus-photon interaction in the factor "coupling-weighted normal mode alignment". In addition, we find that molecular rigidity plays a totally different role in conventional IC versus QED-IC rates. Our study provides applicable design principles for exploiting QED effects on IC processes.

A Dual-Frequency-Detuning Method for Improving the Coupling Tolerance of Wireless Power Transfer

This letter proposes a dual-frequency-detuning method for wireless power transfer systems to gain a smooth power profile in large coupling variation ranges. The application of the detuning method in the dual-frequency resonant network is explored to adjust the equivalent impedance of each frequency channel at different coupling conditions. Accordingly, the proposed method can freely regulate the voltage gain and output power curves even under a square-wave voltage excitation with high-order harmonic components. By designing the system properly, both the fundamental and 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> frequency channels alternately serve as the main power transfer channel under strong and weak coupling conditions, so less power fluctuation over a large coupling variation is realized. A 130W prototype is developed, and the experimental results show that the maximum power fluctuation is less than 6.8% against coupling coefficient variations from 0.35 to 0.74.

A broadband metamaterial damper design based on synergetic coupling among multi-cells

This paper proposes a broadband damping method to achieve broadband vibration absorption by superposition of the working bands of several damping units. The device composes of several local resonant metamaterial dampers with gradient elastic wave band gaps via weak coupling. By integrating the units with band gaps in different frequency ranges in parallel, a linear superposition of the absorption bandwidth can be achieved. This property overcomes the shortcomings of the traditional dynamic vibration absorbers (DVAs) or metamaterial dampers with narrow working bands, and provides a solution for broadband low-frequency vibration attenuation. Compared to conventional multi-degree-of-freedom (MDOF) DVAs, this design belongs to a special metamaterial-based MDOF damper that could satisfy both low damping ratio and weak coupling conditions. As the working bandwidth of each sub-unit could be optimized by changing structural parameters and structural forms, the available design parameters are significantly increased. Moreover, the sizes of the damper device and each sub-unit are sub-wavelength, so the additional mass is greatly lower than that of the traditional DVA. The design method and device proposed in this paper are expected to be widely used in the broadband low-frequency vibration suppression of various types of machinery and equipment.

Well-Posedness of the Four-Derivative Scalar-Tensor Theory of Gravity in Singularity Avoiding Coordinates.

We show that the most general scalar-tensor theory of gravity up to four derivatives in 3+1 dimensions is well-posed in a modified version of the CCZ4 formulation of the Einstein equations in singularity-avoiding coordinates. We demonstrate the robustness of our new formulation in practice by studying equal mass black hole binary mergers for different values of the coupling constants. Although our analysis of well-posedness is restricted to cases in which the couplings are small, we find that in simulations we are able to push the couplings to larger values, so that a certain weak coupling condition is order one, without instabilities developing. Our Letter provides the means for such simulations to be undertaken by the many numerical relativity codes that rely on the moving puncture gauge to evolve black hole singularities.

Generation of Perfectly Entangled Two and Three Qubits States by Classical Random Interaction

AbstractThis study examines the possibility of finding perfect entanglers for a Hamiltonian which corresponds to several quantum information platforms of interest at the present time. However, in this study, a superconducting circuit is used that stands out from other quantum‐computing devices, especially because transmon qubits can be coupled via capacitors or microwave cavities, which enables to combine high coherence, fast gates, and high flexibility in its design parameters. There are currently two factors limiting the performance of superconducting processors: timing mismatch and the limitation of entangling gates to two qubits. In this work, a two‐qubit SWAP and a three‐qubit Fredkin gate is presented, additionally, a perfect adiabatic entanglement generation between two and three programmable superconducting qubits is also demonstrated. Furthermore, the impact of random dephasing, emission, and absorption noises on the quantum gates and entanglement is also demonstrated in this study. It is demonstrated by numerical simulation that CSWAP gate and W‐state generation can be achieved perfectly in one step with high reliability under weak coupling conditions. Hence, this scheme could contribute to quantum teleportation, quantum communication, and some other areas of quantum information processing.

Building Networks with a New Cross-Bubble Transition Entropy for Quantitative Assessment of Mental Arithmetic Electroencephalogram

The complex network nature of human brains has led an increasing number of researchers to adopt a complex network to assess the cognitive load. The method of constructing complex networks has a direct impact on assessment results. During the process of using the cross-permutation entropy (CPE) method to construct complex networks for cognitive load assessment, it is found that the CPE method has the shortcomings of ignoring the transition relationship between symbols and the analysis results are vulnerable to parameter settings. In order to address this issue, a new method based on the CPE principle is proposed by combining the advantages of the transition networks and the bubble entropy. From an interaction perspective, this method suggested that the node-wise out-link transition entropy of the cross-transition network between two time series is used as the edge weight to build a complex network. The proposed method was tested on the unidirectional coupled Henon model and the results demonstrated its suitability for the analysis of short time series by decreasing the influence of the embedding dimension and improving the reliability under the weak coupling conditions. The proposed method was further tested on the publicly available EEG dataset and showed significant superiority compared with the conventional CPE method.

Empirical Low-Dimensional Dynamics of Atmospheric Stable Boundary Layer Temperature Inversions

Abstract The atmospheric stable boundary layer (SBL) is observed to display multiple regimes of stratification, flow, and turbulence. Transitions between weakly stable regimes of sustained turbulence and very stable regimes of weak turbulence are observed to occur abruptly. The understanding and predictability of turbulent recovery remains limited, reducing the accuracy of numerical weather prediction and climate projections. Idealized SBL models have related regimes to dynamically stable equilibria. Under conditions of weak energetic surface coupling, two stable branches separated by an unstable branch are predicted by these models. Such bifurcation structures are associated with rapid transitions. This work investigates the extent to which observed temperature inversion variability can be described by an empirical one-dimensional stochastic differential equation (SDE). The drift and diffusion coefficients of the SDE of observed inversion strength are approximated from statistics of their averaged time tendencies, conditioned on wind speed. Functional forms of the state dependence of these coefficients are estimated using Gaussian process regression. Probabilistic estimates of the system’s deterministic equilibria are found and used to create empirical bifurcation diagrams of inversion strength as a function of wind speed. These data-driven bifurcation structures are first obtained from idealized model simulations, then repeated for observations from several meteorological towers. It is found that the effective low-dimensional dynamics of observed temperature inversions is similar to that of the idealized model. Evidence of multiple equilibria and hysteresis is found at a single site, Dome C, Antarctica, but is not robust to variations in the analysis. Evidence of state-dependent noise consistent with intermittent turbulence under very stably stratified conditions is presented.

Echo effect in brain networks

Echo behavior has been found in many systems and takes roles in different aspects. By checking the time series of terminating processes of dynamics events, we find that echo behavior may also exist in brain networks but its mechanism remains unclear. We here study the echo effect on real brain networks by presenting a new framework, which replaces the two perturbations in previous studies by only one perturbation and is thus convenient for theoretical analysis. We interestingly find that echo effect depends on both the phase-resetting parameter and coupling strength, and there is an optimal area of echo effect in phase diagram where the optimal phase-resetting parameter and weak coupling strength are the two necessary conditions. We further reveal that echo comes from a new mechanism of perturbation selected correlation, i.e. a reverse deduced correlation, which explains a more fundamental aspect of echo. Moreover, a theoretical analysis is provided to explain the echo effect, based on the uncoupled brain network. These findings uncover the underlying mechanism why the echoes in brain networks can only happen during the terminating processes, i.e. the condition of optimal phase-resetting or the needed perturbations is provided right before the terminating process while the condition of weak coupling is provided by the terminating process.

Analysis of frequency bifurcation and frequency splitting of IPT system under overcoupling

Wireless power transfer has gradually become a promising way for the charging of electric vehicles (EVs) and the design of a high efficiency and high stability charging system is inevitable. Inductive power transfer (IPT) systems, as one of the most important method to realize wireless power transfer, usually work under weak coupling conditions and obtain high power transmission efficiency through resonance compensation circuits. In the case of reduced power transmission distance, the IPT system will enter an over-coupled state, causing the system output power to change drastically and the system to detune, and frequency bifurcation and frequency splitting will also occur. This paper first theoretically deduces the root cause of the output power variation of several typical topological IPT systems under over-coupled conditions, and then analyzes their frequency bifurcation and frequency splitting characteristics by numerical analysis methods, which can provide a reference for the design of IPT systems. Finally, the theory of this paper is verified by simulation and a 2 kW test prototype.

Plasmon-exciton nanostructures, based on CdS quantum dots with exciton and trap state luminescence

The study is devoted to the establishment of plexciton coupling manifestations in the luminescence of colloidal CdS quantum dots (3.7 nm), passivated with Oleic acid (OA) in mixtures with spherical silver nanoparticles (Ag NPs), coated with Oleylamine (OAm) with size of 10–15 nm. Mixtures of CdS QDs and Ag NPs in toluene, in which the associates formation occurs as a result of the π-staking and dipole-dipole interaction have been realized. The case of the simultaneous effect of plasmonic nanoparticles on exciton and trap state luminescence is considered. Under conditions of maximum overlap of Ag NPs extinction spectrum (415 nm) and exciton luminescence band, it was found that the exciton luminescence quenching (430 nm), accompanied by an decrease in its lifetime from 7.4 ns to 2.5 nm due to resonance energy transfer with an efficiency of 0.66 is dominant effect. At the same time, under conditions of weak plexciton coupling, caused by an insignificant overlap of Ag NPs plasmon band, an increase in the intensity of trap state luminescence (603 nm) by a factor of 1.8 with simultaneous decrease in its luminescence lifetime from 75 ns to 25 ns was observed due to the manifestation of the Purcell effect. The obtained results demonstrate the possibility of increasing the quantum yield of QDs trap state luminescence due to the Purcell effect.

Optimization of gain region in mid-IR ( ≈ 5 μm) QCL.

Non-equilibrium Green's function (NEGF) formalism is used to optimize the gain region of a quantum cascade laser (QCL) tailored to emit radiation at ∼5 µm wavelength, originally designed by Evans et al. [Appl. Phys. Lett., 88,051105(2006)10.1063/1.2171476]. The optimization strategy uses electron-photon selfenergies to find characteristics of devices under the "operating conditions," i.e., interacting with the laser field. These conditions can be quite different from the one when the device is in no-lasing state and the unsaturated gain is being optimized. The saturation caused by the optical field can push the structure from strong to weak coupling conditions, what changes laser parameters in a non-linear manner. Moreover, the NEGF method does not require any phenomenological parameters (such as, e.g., the phase relaxation times), so the quantities dependent on these parameters are determined solely on physical grounds. The use of the above procedure for the structure under investigation shows that the increase of the quantum efficiency by 24% and the output power by 83% in comparison to the original design can be achieved when the widths of injection and extraction barriers are changed to their optimal values.

All-fiber high-power spatiotemporal mode-locked laser based on multimode interference filtering.

Multimode interference (MMI) has been considered to be critical and investigated extensively in mode-locked laser based on single transverse mode systems, whereas there are few researches related to three-dimensional nonlinear dynamics within lasers. In this paper, we demonstrate all-fiber high-power spatiotemporal mode-locked (STML) laser by optimizing MMI filtering, where we find that the MMI filtering plays an important role in counteracting the coupling of high-order modes and improving output power of STML laser. The results under weak coupling condition when the length of graded-index multimode fiber (GIMF) is integral multiple of beat length show that the oscillator generates dissipative soliton pulses at 1036.86 nm with pulse width of 5.65 ps, and the slope efficiency of pump-signal is up to 10.3% with average power/energy of 215 mW/6 nJ, which is the highest among all-fiber STML lasers in normal dispersion regime. Besides, the multiple-soliton of STML, including multiple pulses and harmonic mode-locking can be observed in the experiment. Our work significantly broadens the dimensions of design for all-fiber high-power STML and makes them much more accessible for being put into applications.

Provably non-stiff implementation of weak coupling conditions for hyperbolic problems

In the context of coupling hyperbolic problems, the maximum stable time step of an explicit numerical scheme may depend on the design of the coupling procedure. If this is the case, the coupling procedure is sensitive to changes in model parameters independent of the Courant–Friedrichs–Levy condition. This sensitivity can cause artificial stiffness that degrades the performance of a numerical scheme. To overcome this problem, we present a systematic and general procedure for weakly imposing coupling conditions via penalty terms in a provably non-stiff manner. The procedure can be used to construct both energy conservative and dissipative couplings, and the user is given control over the amount of dissipation desired. The resulting formulation is simple to implement and dual consistent. The penalty coefficients take the form of projection matrices based on the coupling conditions. Numerical experiments demonstrate that this procedure results in both optimal spectral radii and superconvergent linear functionals.

Bound states in the continuum in strong-coupling and weak-coupling regimes under the cylinder – ring transition

Abstract Bound states in the continuum (BIC) have been at the forefront of research in optics and photonics over the past decade. It is of great interest to study the effects associated with quasi-BICs in the simplest structures, where quasi-BICs are very pronounced. An example is a dielectric cylinder, and in a number of works, quasi-BICs have been studied both in single cylinders and in structures composed of cylinders. In this work, we studied the properties of quasi-BICs during the transition from a homogeneous dielectric cylinder in an air environment to a ring with narrow walls while increasing the diameter of the inner air cylinder gradually. The results demonstrate the quasi-BIC crossover from the strong-coupling to the weak-coupling regime, which manifests itself in the transition from the avoided crossing of branches to their intersection with the quasi-BIC being preserved on only one straight branch. In the regime of strong-coupling and quasi-BIC, three waves interfere in the far-field zone: two waves corresponding to the resonant modes of the structure and the wave scattered by the structure as a whole. The validity of the Fano resonance concept is discussed since it describes the interference of only two waves under weak coupling conditions.