# Nonlinear Schrödinger equation

Absolute value of the complex envelope of exact analytical breather solutions of the nonlinear Schrödinger (NLS) equation in nondimensional form. (A) The Akhmediev breather; (B) the Peregrine breather; (C) the Kuznetsov–Ma breather.[1]

In theoretical physics, the (one-dimensional) nonlinear Schrödinger equation (NLSE) is a nonlinear variation of the Schrödinger equation. It is a classical field equation whose principal applications are to the propagation of light in nonlinear optical fibers and planar waveguides[2] and to Bose-Einstein condensates confined to highly anisotropic cigar-shaped traps, in the mean-field regime.[3] Additionally, the equation appears in the studies of small-amplitude gravity waves on the surface of deep inviscid (zero-viscosity) water;[2] the Langmuir waves in hot plasmas;[2] the propagation of plane-diffracted wave beams in the focusing regions of the ionosphere;[4] the propagation of Davydov's alpha-helix solitons, which are responsible for energy transport along molecular chains;[5] and many others. More generally, the NLSE appears as one of universal equations that describe the evolution of slowly varying packets of quasi-monochromatic waves in weakly nonlinear media that have dispersion.[2] Unlike the linear Schrödinger equation, the NLSE never describes the time evolution of a quantum state (except hypothetically, as in some early attempts, in the 1970s, to explain the quantum measurement process[6]). The 1D NLSE is an example of an integrable model.

In quantum mechanics, the 1D NLSE is a special case of the classical nonlinear Schrödinger field, which in turn is a classical limit of a quantum Schrödinger field. Conversely, when the classical Schrödinger field is canonically quantized, it becomes a quantum field theory (which is linear, despite the fact that it is called ″quantum nonlinear Schrödinger equation″) that describes bosonic point particles with delta-function interactions — the particles either repel or attract when they are at the same point. In fact, when the number of particles is finite, this quantum field theory is equivalent to the Lieb–Liniger model. Both the quantum and the classical 1D nonlinear Schrödinger equations are integrable. Of special interest is the limit of infinite strength repulsion, in which case the Lieb–Liniger model becomes the Tonks–Girardeau gas (also called the hard-core Bose gas, or impenetrable Bose gas). In this limit, the bosons may, by a change of variables that is a continuum generalization of the Jordan–Wigner transformation, be transformed to a system one-dimensional noninteracting spinless[nb 1] fermions.[7]

The nonlinear Schrödinger equation is a simplified 1+1-dimensional form of the Ginzburg–Landau equation introduced in 1950 in their work on superconductivity, and was written down explicitly by R. Y. Chiao, E. Garmire, and C. H. Townes (1964, equation (5)) in their study of optical beams.

## Equation

The nonlinear Schrödinger equation is a partial differential equation, applicable to classical and quantum mechanics.

### Classical equation

The classical field equation (in dimensionless form) is:[8]

 Nonlinear Schrödinger equation (Classical field theory) $i\partial_t\psi=-{1\over 2}\partial^2_x\psi+\kappa|\psi|^2 \psi$

for the complex field ψ(x,t).

This equation arises from the Hamiltonian[8]

$H=\int \mathrm{d}x \left[{1\over 2}|\partial_x\psi|^2+{\kappa \over 2}|\psi|^4\right]$

with the Poisson brackets

$\{\psi(x),\psi(y)\}=\{\psi^*(x),\psi^*(y)\}=0 \,$
$\{\psi^*(x),\psi(y)\}=i\delta(x-y). \,$

Unlike its linear counterpart, it never describes the time evolution of a quantum state.

### Quantum mechanics

To get the quantized version, simply replace the Poisson brackets by commutators

\begin{align} {}[\psi(x),\psi(y)] &= [\psi^*(x),\psi^*(y)] = 0\\ {}[\psi^*(x),\psi(y)] &= -\delta(x-y) \end{align}

and normal order the Hamiltonian

$H=\int dx \left[{1\over 2}\partial_x\psi^\dagger\partial_x\psi+{\kappa \over 2}\psi^\dagger\psi^\dagger\psi\psi\right].$

The quantum version was solved by Bethe ansatz by Lieb and Liniger . Thermodynamics was described by Chen Nin Yang. Quantum correlation functions also were evaluated, see.[7] The model has higher conservation laws, expression in terms of local fields can be found in. [1].

## Solving the equation

The nonlinear Schrödinger equation is integrable: Zakharov and Shabat (1972) solved it with the inverse scattering transform. The corresponding linear system of equations is known as the Zakharov–Shabat system:

\begin{align} \phi_x &= J\phi\Lambda+U\phi \\ \phi_t &= 2J\phi\Lambda^2+2U\phi\Lambda+(JU^2-JU_x)\phi, \end{align}

where

$\Lambda = \begin{pmatrix} \lambda_1&0\\ 0&\lambda_2 \end{pmatrix} , \quad J = i\sigma_z = \begin{pmatrix} i&0\\ 0&-i \end{pmatrix} , \quad U = i \begin{pmatrix} 0&q\\ r&0 \end{pmatrix}.$

The nonlinear Schrödinger equation arises as compatibility condition of the Zakharov–Shabat system:

$\phi_{xt}=\phi_{tx} \quad \Rightarrow \quad U_t=-JU_{xx}+2JU^2U \quad \Leftrightarrow \quad \begin{cases} iq_t=q_{xx}+2qrq \\ ir_t=-r_{xx}-2qrr. \end{cases} \,$

By setting q = r* or q = − r* the nonlinear Schrödinger equation with attractive or repulsive interaction is obtained.

An alternative approach uses the Zakharov–Shabat system directly and employs the following Darboux transformation:

\begin{align} & \phi \to \phi[1]=\phi\Lambda-\sigma\phi \\ & U \to U[1]=U+[J,\sigma] \\ & \sigma = \varphi\Omega\varphi^{-1} \end{align}

which leaves the system invariant.

Here, φ is another invertible matrix solution (different from ϕ) of the Zakharov–Shabat system with spectral parameter Ω:

\begin{align} \varphi_x &= J\varphi\Omega+U\varphi \\ \varphi_t &= 2J\varphi\Omega^2+2U\varphi\Omega+(JU^2-JU_x)\varphi. \end{align}

Starting from the trivial solution U = 0 and iterating, one obtains the solutions with n solitons.

Computational solutions are found using a variety of methods, like the split-step method.

## Galilean invariance

The nonlinear Schrödinger equation is Galilean invariant in the following sense:

Given a solution ψ(x, t) a new solution can be obtained by replacing x with x + vt everywhere in ψ(x, t) and by appending a phase factor of $e^{-iv(x+vt/2)}\,$:

$\psi(x,t) \mapsto \psi_{[v]}(x,t)=\psi(x+vt,t)\; e^{-iv(x+vt/2)}.$

## The nonlinear Schrödinger equation in fiber optics

In optics, the nonlinear Schrödinger equation occurs in the Manakov system, a model of wave propagation in fiber optics. The function ψ represents a wave and the nonlinear Schrödinger equation describes the propagation of the wave through a nonlinear medium. The second-order derivative represents the dispersion, while the κ term represents the nonlinearity. The equation models many nonlinearity effects in a fiber, including but not limited to self-phase modulation, four-wave mixing, second harmonic generation, stimulated Raman scattering, etc.

## The nonlinear Schrödinger equation in water waves

A hyperbolic secant (sech) envelope soliton for surface waves on deep water.
Blue line: water waves.
Red line: envelope soliton.

For water waves, the nonlinear Schrödinger equation describes the evolution of the envelope of modulated wave groups. In a paper in 1968, Vladimir E. Zakharov describes the Hamiltonian structure of water waves. In the same paper Zakharov shows, that for slowly modulated wave groups, the wave amplitude satisfies the nonlinear Schrödinger equation, approximately.[9] The value of the nonlinearity parameter к depends on the relative water depth. For deep water, with the water depth large compared to the wave length of the water waves, к is negative and envelope solitons may occur.

For shallow water, with wavelengths longer than 4.6 times the water depth, the nonlinearity parameter к is positive and wave groups with envelope solitons do not exist. Note, that in shallow water surface-elevation solitons or waves of translation do exist, but they are not governed by the nonlinear Schrödinger equation.

The nonlinear Schrödinger equation is thought to be important for explaining the formation of rogue waves.

The complex field ψ, as appearing in the nonlinear Schrödinger equation, is related to the amplitude and phase of the water waves. Consider a slowly modulated carrier wave with water surface elevation η of the form:

$\eta = a(x_0,t_0)\; \cos \left[ k_0\, x_0 - \omega_0\, t_0 - \theta(x_0,t_0) \right],$

where a(x0, t0) and θ(x0, t0) are the slowly modulated amplitude and phase. Further ω0 and k0 are the (constant) angular frequency and wavenumber of the carrier waves, which have to satisfy the dispersion relation ω0 = Ω(k0). Then

$\psi = a\; \exp \left( i \theta \right).$

So its modulus |ψ| is the wave amplitude a, and its argument arg(ψ) is the phase θ.

The relation between the physical coordinates (x0, t0) and the (x, t) coordinates, as used in the nonlinear Schrödinger equation given above, is given by:

$x = k_0 \left[ x_0 - \Omega'(k_0)\; t_0 \right], \quad t = k_0^2 \left[ -\Omega''(k_0) \right]\; t_0$

Thus (x, t) is a transformed coordinate system moving with the group velocity Ω'(k0) of the carrier waves, The dispersion-relation curvature Ω"(k0) is always negative for water waves under the action of gravity.

For waves on the water surface of deep water, the coefficients of importance for the nonlinear Schrödinger equation are:

$\kappa = - 2 k_0^2, \quad \Omega(k_0) = \sqrt{g k_0} = \omega_0 \,\!$

so

$\Omega'(k_0) = \frac{1}{2} \frac{\omega_0}{k_0}, \quad \Omega''(k_0) = -\frac{1}{4} \frac{\omega_0^3}{k_0^3} \,\!$

where g is the acceleration due to gravity at the Earth's surface.

## Gauge equivalent counterpart

NLSE (1) is gauge equivalent to the following isotropic Landau-Lifshitz equation (LLE) or Heisenberg ferromagnet equation

$\vec{S}_t=\vec{S}\wedge \vec{S}_{xx}. \qquad$

Note that this equation admits several integrable and non-integrable generalizations in 2 + 1 dimensions like the Ishimori equation and so on.

## Relation to vortices

Hasimoto (1972) showed that the work of da Rios (1906) on vortex filaments is closely related to the nonlinear Schrödinger equation.

## Notes

1. ^ A possible source of confusion here is the spin-statistics theorem, which demands that fermions have half-integer spin; however, it is a theorem of relativistic 3+1-dimensional quantum field theories, and thus is not applicable in this 1D, nonrelativistic case.

## References

### Notes

1. ^ Figure 1 from: Onorato, M.; Proment, D.; Clauss, G.; Klein, M. (2013), "Rogue Waves: From Nonlinear Schrödinger Breather Solutions to Sea-Keeping Test", PLoS One 8 (2): e54629, doi:10.1371/journal.pone.0054629, PMC 3566097
2. ^ a b c d Malomed, Boris (2005), "Nonlinear Schrödinger Equations", in Scott, Alwyn, Encyclopedia of Nonlinear Science, New York: Routledge, pp. 639–643
3. ^ Pitaevskii, L. & Stringari, S. (2003), Bose-Einstein Condensation, Oxford, U.K.: Clarendon
4. ^ Gurevich, A. V. (1978), Nonlinear Phenomena in the Ionosphere, Berlin: Springer
5. ^ Balakrishnan, R. (1985). "Soliton propagation in nonuniform media". Physical Review A 32 (2): 1144–1149. doi:10.1103/PhysRevA.32.1144. PMID 9896172. edit
6. ^ Bassi, A.; Lochan, K.; Satin, S.; Singh, T. P.; Ulbricht, H. (2013). "Models of wave-function collapse, underlying theories, and experimental tests". Reviews of Modern Physics 85 (2): 471. doi:10.1103/RevModPhys.85.471. edit
7. ^ a b Korepin, V. E.; Bogoliubov, N. M.; Izergin, A. G. (1993). Quantum Inverse Scattering Method and Correlation Functions. Cambridge, U.K.: Cambridge University Press. doi:10.2277/0521586461. ISBN 978-0-521-58646-7.
8. ^ a b V.E. Zakharov; S.V. Manakov (1974). "On the complete integrability of a nonlinear Schrödinger equation". Journal of Theoretical and Mathematical Physics 19 (3): 551–559. Bibcode:1974TMP....19..551Z. doi:10.1007/BF01035568.. Originally in: Teoreticheskaya i Matematicheskaya Fizika 19 (3): 332–343. June 1974.
9. ^ V. E. Zakharov (1968). "Stability of periodic waves of finite amplitude on the surface of a deep fluid". Journal of Applied Mechanics and Technical Physics 9 (2): 190–194. Bibcode:1968JAMTP...9..190Z. doi:10.1007/BF00913182. Originally in: Zhurnal Prikdadnoi Mekhaniki i Tekhnicheskoi Fiziki 9 (2): 86–94, 1968.]