The impact of memory effect on space fractional strong quantum couplers with tunable decay behavior and its numerical simulation

The nontrivial behavior of wave packets in the space fractional coupled nonlinear Schrödinger equation has received considerable theoretical attention. The difficulty comes from the fact that the Riesz fractional derivative is inherently a prehistorical operator. In contrast, nonlinear Schrödinger equation with both time and space nonlocal operators, which is the cornerstone in the modeling of a new type of fractional quantum couplers, is still in high demand of attention. This paper is devoted to numerically study the propagation of solitons through a new type of quantum couplers which can be called time-space fractional quantum couplers. The numerical methodology is based on the finite-difference/Galerkin Legendre spectral method with an easy to implement numerical algorithm. The time-fractional derivative is considered to describe the decay behavior and the nonlocal memory of the model. We conduct numerical simulations to observe the performance of the tunable decay and the sharpness behavior of the time-space fractional strongly coupled nonlinear Schrödinger model as well as the performance of the numerical algorithm. Numerical simulations show that the time and space fractional-order operators control the decay behavior or the memory and the sharpness of the interface and undergo a seamless transition of the fractional-order parameters.

www.nature.com/scientificreports/ is based on making their cores more close to each other to have the opportunity to act as a directional coupler. Armed by that, the light can make a jump between the two cores which are separated only by a few nanometers. Some recent applications are based on that new technology. An example of these applications is optical buffering which can enhance processing, routing, and switching properties over long distances. Owing to potential applications in optical communication, propagation of solitons through optical fibers in most optical communication systems became a wide area of research. The integrable nonlinear Schrödinger's equation (NLSE) 8,9 is used to model these dynamics. A more generalized form 10 , known as the generalized nonlinear Schrödinger's equation (GNLSE), is given by where F is a real-valued algebraic function which can be considered as the refractive index of the fibers. Earlier work 10 extended the GNLSE to the couplers case. Fast switching and signal coupling in optical communication links can be allowed by optical nonlinear couplers. For twin-core couplers, wave propagation at relatively high field intensities is described by coupled nonlinear equations. In the dimensionless form, they are: The coupling coefficient between the cores of the fiber is represented by the constant K . The system (2) and (3) has been derived and studied before in the context of Kerr law nonlinearity 11 . The dimensionless forms of the optical fields in the respective cores of the optical fibers are given by u and ν . This system of equations is known as the generalized vector NLSE. Intensity dependent switches and devices for separating a compressed soliton from its broad 'pedestal' are examples of applications of (2) and (3). Its integrals of motion can be respectively given by energy (E), linear momentum (M) and the Hamiltonian (H) namely: where we define F(I) = I 0 F (ζ )dζ , and the intensity I is given by I = |u| 2 or I = |ν| 2 depending on the core. The energy E is commonly known as the wave energy and in the context of fiber optics it is known as the wave power.
The theory which used to discuss quantum phenomena in fractal environments is called fractional quantum mechanics. The similarity between the classical diffusion equation and the Schrödinger equation motivated the generalization of the nonlinear Schrödinger equation in the light of non-Brownian motion in a path integral formulation. This generalization leads to the space-fractional, the time-fractional, and the time-space-fractional Schrödinger equations (FSEs). The Riesz space FSE has been introduced by Laskin in quantum physics by replacing the Brownian paths in the Feynman path integrals by Lévy flights 12,13 . Similar to the conventional Schrödinger equation, the Riesz space FSE satisfies the Markovian evolution law. Stickler 14 discussed the Lévy crystal in a condensed matter environment as a possible realization of the space-fractional quantum mechanics by introducing a tight binding infinite range chain. Longhi 15 discussed an optical realization of the space FSE based on transverse light dynamics in a cavity by exploiting the Fourier optics properties. Other physical applications of the Riesz space FSE have been discussed by Guo and Xu 16 and its solution for a free particle and an infinite square potential well have also been introduced. The existence and uniqueness of the solution to the space FSE have been investigated by Guo et al. 17 using the energy method. Moreover, the existence and uniqueness of the solution to systems of the space FSEs have been proved by Hu et al. 18 using the Faedo-Galerkin method. Cho et al. 19 studied the low regularity well-posedness of the space FSE with cubic nonlinearity in periodic and non periodic settings. Following Laskin and similar to deriving the time-fractional diffusion equation by considering non-Markovian evolution 20 , Naber 21 used the Caputo temporal fractional derivative 22 as a generalization of the integer-order derivative in the conventional Schrödinger equation to study non-Markovian evolution in quantum mechanics and constructed the temporal FSE. More recently, Dong and Xu 23 , and Wang and Xu 24 combined Laskin's work with Naber's work to construct space-time FSEs. A detailed derivation and numerical simulation of coupled system of nonlinear Schrödinger equations for pulses of polarized electromagnetic waves in cylindrical fibers was investigated 25 . Ghalandari and Solaimani 26 investigated the numerical treatment of the fractional Young double-slit experiment with incident Gaussian wavepackets using a split step Fourier method. Liangwei Zeng and Jianhua Zeng 27 proposed a coupled system of space FSEs considering two arrays of quantum waveguides. They considered the following system with the wave function u, ν: www.nature.com/scientificreports/ where represents the Planck constant, the differentiation order parameter α represents the Lévy index, τ is the time, and K is the linear coupling parameter. The parameter D α = 1/m is constant with m denoting the mass of the atom. When F (S) = S , then the nonlinear term is of Kerr style and G 1,2 > 0 are the Kerr coefficients. These coefficients can make a characterization of atoms collisions strengths in Bose-Einstein condensates. A kind of normalization can be applied on (7) by defining The fractional Laplacian is defined as . Accordingly, the system (7) should have the following dimensionless form: Based on (8), two arrays of quantum waveguides composition represent a new kind of space-fractional quantum couplers was considered in 27 . It is called space-fractional quantum couplers 27 . This can be understood in the sense of two branches polariton condensates in solid state physics 28  where � = (a, b) ⊂ R and J = (0, T] ⊂ R are the time and the space domains, ∂� is the boundary of , the parameter ε > 0 is the group velocity dispersion, γ denotes the normalized birefringence constant, ψ and φ are complex functions defined in × J , ψ 0 (x) and φ 0 (x) are known sufficiently smooth functions. Of particular concern for the present work is the linear coupling parameter ξ . It accounts for effects that result from the twisting and elliptic deformation of the fiber. The term proportional to k 1 explains the self-focusing of a signal for pulses in birefringent media whereas k 1 + 2k 2 represents the cross phase modulation describing the integrability of the nonlinear system. The operator ∂ β ∂t β (0 < β < 1) is the Caputo fractional derivative, and (−�) α/2 is the fractional order Laplacian operator of order 1 < α ≤ 2 defined in Riesz form ∂ α ∂|x| α (1 < α < 2) . We refer to seminal works 30 for more details on fractional order differential operators. We here numerically solve model (9) and provide an easy to implement algorithm in the next section.
In this paper, we numerically investigate the nontrivial behavior of wave packets in the time-space fractional model (9) in one space dimension. In this model, we choose the form of the fractional Laplacian to be the Riesz fractional operator. The time-fractional derivative is considered to describe the decay behavior and the non-local memory of the model. The numerical methodology is based on the finite difference/Galerkin-Legendre spectral method with an easy to implement numerical algorithm. We conduct numerical simulations to demonstrate the performance of the tunable decay and the sharpness behavior of the time-space fractional strongly coupled nonlinear Schrödinger model as well as the the performance of the numerical algorithm. Numerical simulations show that the time and space fractional order operators control the decay behavior or the memory and the sharpness of the interface and undergo a seamless transition of the fractional order parameters. Of particular interest for this work also is the linear coupling parameter ξ . We test the effect of this parameter on the collision of solitary waves.

The spectral scheme
Here, the discretization of problem (9) is done by using the high order L2-1 σ approximation difference formula for the Caputo time fractional operator next to the spectral Legendre-Galerkin scheme for the Riesz spatialfractional operator. The time and space fractional Schrödinger equalion and its coupled system are fully analyzed numerically in 31,32 . The proposed numerical approaches there are designed by the use of L1 difference scheme for temporal approximations and Galerkin spectral scheme for spatial fractional order operators. Alternatively, Fourier spectral method can be used effectively for Riesz space fractional partial differential equations. For example, a fast and accurate method for numerical solutions of space fractional reaction-diffusion equations is proposed in 33 based on an exponential integrator scheme in time and the Fourier spectral method in space. By the www.nature.com/scientificreports/ Fourier spectral techniques and advance the resulting equation in time with both Strang splitting and exponential time-differencing methods, the dynamics of the time-dependent Riesz space fractional nonlinear Schrödinger equation in the presence of the harmonic potential have been considered in 34 . A detailed implementation of the constructed high accuracy algorithm for (9) will be given below.
, the Alikhanov L2-1 σ difference formula at the node t j+σ is defined as The next identity holds directly by Taylor's theorem.

Lemma 2.2 The following identity holds:
Starting from the L2-1 σ formula (16) for the discretization of the time Caputo fractional derivative, we obtain that Making use of Lemma 2.1 and Lemma 2.2, this scheme is of second order accuracy in time. Let us introduce the following parameters www.nature.com/scientificreports/ The semi-scheme (18) has the following equivalent form: And so, the full discrete Alikhanov L2-1 σ Galerkin spectral scheme for (18) where P N is the projection operator.
Algorithmic implementation. Via the hypergeometric function, Jacobi polynomials can be represented 36 , for α, β > −1 and x ∈ (−1, 1): where the notation (·) i represents the symbol of Pochhammer. Armed by (20), we get the equivalent three-term recurrence relation where The Legendre polynomial L i (x) is a special case of the Jacobi polynomial, namely . www.nature.com/scientificreports/ The weight function that ensures the orthogonality of Jacobi polynomials is given by where δ ij is the Kronecker Delta symbol, and We introduce the following rescale functions: and we write ∧(x) as x . The basis functions selected for the spatial discretization are given by 38,39 :  www.nature.com/scientificreports/ Lemma 2.4 [see 36,38 ] The elements of the stiffness matrix S in (31) are given by  www.nature.com/scientificreports/

Results and discussions
In this section, we conduct numerical simulations to investigate the modeling capability of the space-time fractional Schrödinger equation (36). Computationally, we consider N = 150 and M = 1500.

Example 1
We consider the following weakly coupled system: In the integer order case, i.e. for α = 2 and β = 1 , the wave propagates from the left to the right spatial domain with a fixed angle. The model is the Manakov system. The collision is elastic and the system is completely integrable, the two waves cross each other, and their velocity and shape are unchanged. It is well known that the standard integer-order Schrödinger equation generates very diffusive numerical solutions. This can be observed here when α is close to 2 and β is close to 1.
The numerical results in Figs. 1, 2, 3, 4 and 5 display the effect of the Lévy index 1 < α ≤ 2 and the temporal fractality 0 < β ≤ 1 on the shapes and stability of the soliton solutions. These results represent the performance of the space-time fractional Schrödinger equation (35). We find that when α = 2 and β = 1 , the collision is not elastic. The non-integer orders significantly affect the shape of the solitons. One can see in Figs. 1 and 2 that, as the value of the space fractional-order α is decreased, the shape of the solitons changes more quickly.
From Figs. 3, 4, 5 and 6, we note that the distinct selections of the fractional-order parameters β yield simulation results with different decay time or decay properties in the time direction. We observe that as the value of the time fractional-order β is decreased, the interface grows sharper and sharper at the beginning of time where-after the solution propagates straight. Here, we notice a new phenomenon that as β is decreased, the decay www.nature.com/scientificreports/ behavior is gradually reduced too. The situation tends to the conventional case when β approaches 1. These features can be employed in physics to tunable sharpness of the space-time fractional Schrödinger equation (36) with the different choices of the space fractional order α and the time fractional order β , without changing the nonlinearity and dispersion effects.

Example 2
We consider the following strongly coupled system: subject to the initial conditions where D = 25 and H 0 = 1.
It is well known that the linear coupling parameter γ dramatically affects the collision of solitary waves 40,41 . In Figs. 7 and 8 we study the effect of the linear coupling parameter γ , the spatial fractional diffraction order 1 < α ≤ 2 and the temporal fractality 0 < β ≤ 1 on the collision of solitary waves. We find when α tends to 2 and β tends to 1 that as the linear coupling parameter γ is increased, the jump behavior gets stronger. Moreover, for www.nature.com/scientificreports/ any 1 < α ≤ 2 and when β tends to 1, the collision is always elastic. The collision will occur earlier with increasing α and become closer to the classical integer-order case. For the space fractional Schrödinger equation with β = 1 , the two soliton waves collide and keep their shapes when moving away after the collision. This can be observed here when β tends to 1. For smaller values of β , this feature is absent, the waves do not even cross, and the evolution of the solution is pretty sharp during the first time steps. We note also that the distinct selections of the fractional-order parameters β yield simulation results with decay properties in the time direction, see Figs. 7 and 8 (second row). At the same time, some waves are created with oscillations corresponding to fluctuations and indicating the presence of decoherence phenomena that depends on β.

Conclusion
In this paper, we numerically investigated the nontrivial behavior of wave packets in a new type of time-space fractional quantum coupler. The model involving a coupled system of time and space fractional nonlinear Schrödinger equations with linear or nonlinear coupling, can be used to uncover a wealth of information about an extended variety of phenomena, such as modeling the Bose-Einstein condensates, the interaction between pulses in nonlinear optics, or signals in nonlinear acoustic media. The time-fractional derivative was considered to describe the nonlocal memory or decay behavior of the model. The numerical simulation for modelling such kind of quantum couplers was carried using an easy to implement algorithm based on a novel consistent scheme.
It is a combination of Galerkin spectral method of Legendre type, to approximate the spatial operators, and a high order finite difference method, to approximate the temporal derivatives. The effect of fractality parameters α and β on the behaviour of the solution is discussed. The numerical simulations of the time-space fractional nonlinear Schrödinger equations in this paper show that the problem displays time decay behavior that has not been observed before in the time-fractional partial differential equations modeling. Hence, the time and space fractional order operators can be used to control the decay behavior or the memory and the sharpness of the interface and undergo a seamless transition of the fractional order parameters. Of particular interest for this work also is the linear coupling parameter ξ . We test the effect of this parameter on the collision of solitary waves. www.nature.com/scientificreports/ These features can be employed in physics to tunable sharpness of the space-time fractional Schrödinger equation with the different choices of the space fractional order α and the time fractional order β , without changing the nonlinearity and dispersion effects.