Fact-checked by Grok 2 weeks ago

Probability current

In quantum mechanics, the probability current, also known as the probability flux, is a vector quantity that describes the directional flow of probability density associated with a particle's wave function through space.^[1] For a wave function \psi(\mathbf{r}, t), it is mathematically expressed in three dimensions as \mathbf{j}(\mathbf{r}, t) = \frac{[\hbar](/page/H-bar)}{2mi} \left( \psi^* \nabla \psi - \psi \nabla \psi^* \right), where \hbar is the reduced Planck's constant, m is the particle's mass, and \psi^* is the complex conjugate of \psi.^[2] This expression arises directly from the time-dependent Schrödinger equation and ensures that the probability density \rho = |\psi|^2 is conserved via the continuity equation \frac{\partial \rho}{\partial t} + \nabla \cdot \mathbf{j} = 0.^[2] The concept of probability current was first derived by Erwin Schrödinger in 1926 as part of his development of wave mechanics, where he identified the associated quantities with charge and current densities in the context of the Schrödinger equation.^[1] In his fourth paper on quantization as an eigenvalue problem, Schrödinger demonstrated the continuity equation, highlighting how the flow prevents the creation or destruction of probability (or charge, in his initial electromagnetic analogy). This derivation built on Louis de Broglie's hypothesis of matter waves and paralleled classical conservation laws, such as those for mass or electric charge.^[1] Later, Max Born's 1926 probabilistic interpretation of the wave function solidified its role in describing the likelihood of finding a particle in a given region, transforming Schrödinger's charge density into a probability density.^[1] Beyond its foundational role in probability conservation, the probability current provides insight into quantum phenomena such as particle transport and interference. For instance, in a plane wave solution \psi = A e^{i(\mathbf{k} \cdot \mathbf{r} - \omega t)}, the current simplifies to \mathbf{j} = \frac{\hbar \mathbf{k}}{m} |A|^2, representing a uniform flow with velocity \mathbf{v} = \frac{\hbar \mathbf{k}}{m}.^[2] In more complex scenarios, like superpositions or potentials, it reveals non-classical behaviors, including negative currents or backflow, where probability flows opposite to the particle's momentum expectation value.^[3] This makes it essential for analyzing quantum tunneling, scattering processes, and the dynamics of quantum fluids or solids.^[3]

Fundamentals

Probability density

In non-relativistic quantum mechanics, the probability density \rho(\mathbf{r}, t) associated with a particle's wave function \psi(\mathbf{r}, t) is defined as \rho(\mathbf{r}, t) = |\psi(\mathbf{r}, t)|^2, assuming \psi is normalized.^[4]^[2] This quantity provides a measure of the likelihood distribution for the particle's position at time t. The interpretation of \rho is probabilistic: it gives the probability of locating the particle within a small volume element dV at position \mathbf{r} as \rho(\mathbf{r}, t) \, dV.^[4]^[2] For the interpretation to be physically meaningful, the total probability must be conserved and equal to unity, leading to the normalization condition

\int \rho(\mathbf{r}, t) \, dV = 1,

integrated over all space.^[2] This ensures the wave function describes a single particle with certainty somewhere in space. This probabilistic framework for the wave function was introduced by Max Born in 1926, marking a shift from classical deterministic views to a statistical understanding of quantum phenomena.^[4] Born's rule, as it became known, underpins the density's role in connecting the abstract wave function to observable measurement outcomes. The continuity equation later relates changes in this density to particle flow, preserving normalization over time.^[2]

Continuity equation

The continuity equation in quantum mechanics is derived from the time-dependent Schrödinger equation and describes the local conservation of probability density. For a non-relativistic scalar particle, the time-dependent Schrödinger equation is given by i \hbar \frac{\partial \psi}{\partial t} = -\frac{\hbar^2}{2m} \nabla^2 \psi + V \psi, where \psi(\mathbf{r}, t) is the wave function, m is the particle mass, V(\mathbf{r}, t) is the potential (assumed real-valued), \hbar is the reduced Planck's constant, and i is the imaginary unit. The probability density \rho(\mathbf{r}, t) = |\psi(\mathbf{r}, t)|^2 represents the probability of finding the particle at position \mathbf{r} at time t. To derive the continuity equation, compute the time derivative of the density: \frac{\partial \rho}{\partial t} = \frac{\partial}{\partial t} (\psi^* \psi) = \left( \frac{\partial \psi^*}{\partial t} \right) \psi + \psi^* \left( \frac{\partial \psi}{\partial t} \right), where \psi^* is the complex conjugate of \psi. Substituting the Schrödinger equation and its complex conjugate yields \frac{\partial \rho}{\partial t} = -\frac{\hbar}{2mi} \left[ \psi^* \nabla^2 \psi - \psi \nabla^2 \psi^* \right], after simplification and assuming V is real (so its time derivatives cancel). Using the vector identity \nabla \cdot (\psi^* \nabla \psi) = \nabla \psi^* \cdot \nabla \psi + \psi^* \nabla^2 \psi and its conjugate, this becomes \frac{\partial \rho}{\partial t} = -\nabla \cdot \mathbf{j}, where the probability current \mathbf{j} is \mathbf{j} = \frac{\hbar}{2mi} \left( \psi^* \nabla \psi - \psi \nabla \psi^* \right). This is the explicit form of the continuity equation without sources: \frac{\partial |\psi|^2}{\partial t} + \nabla \cdot \left[ \frac{\hbar}{2mi} (\psi^* \nabla \psi - \psi \nabla \psi^*) \right] = 0. Physically, the continuity equation \frac{\partial \rho}{\partial t} + \nabla \cdot \mathbf{j} = 0 implies local conservation of probability, meaning the rate of change of probability in a volume equals the net flux of probability current out of that volume, analogous to charge conservation in classical electromagnetism where \frac{\partial \rho_e}{\partial t} + \nabla \cdot \mathbf{J}_e = 0. The existence of the current \mathbf{j} is necessitated to balance spatial variations in density: without it, the Schrödinger equation would not preserve total probability \int \rho \, dV = 1 over time, as changes in \rho at one location would not account for "flow" to adjacent regions. Integrating the equation over all space confirms global conservation, as surface terms vanish at infinity for normalized wave functions.

Non-relativistic Probability Current

Free scalar particle

In non-relativistic quantum mechanics, the probability current \mathbf{j} for a free scalar particle governed by the wave function \psi(\mathbf{r}, t) is given by

\mathbf{j} = \frac{\hbar}{2mi} \left( \psi^* \nabla \psi - \psi \nabla \psi^* \right) = \frac{\hbar}{m} \Im \left( \psi^* \nabla \psi \right),

where \hbar is the reduced Planck's constant, m is the particle mass, i is the imaginary unit, \psi^* denotes the complex conjugate, \nabla is the gradient operator, and \Im denotes the imaginary part. This vector field describes the directional flow of probability associated with the particle's wave function. The expression for \mathbf{j} arises from the time-dependent Schrödinger equation for a free particle,

i \hbar \frac{\partial \psi}{\partial t} = -\frac{\hbar^2}{2m} \nabla^2 \psi,

and its complex conjugate equation. The probability density \rho = |\psi|^2 obeys the continuity equation \frac{\partial \rho}{\partial t} + \nabla \cdot \mathbf{j} = 0, which enforces conservation of probability. To derive \mathbf{j}, compute the time derivative \frac{\partial \rho}{\partial t} = \psi^* \frac{\partial \psi}{\partial t} + \psi \frac{\partial \psi^*}{\partial t}, substitute the Schrödinger equation and its conjugate into this expression, and collect terms involving spatial derivatives. The resulting equation rearranges into the divergence form, yielding the explicit formula for \mathbf{j}. The probability current \mathbf{j} has dimensions of probability flux, namely probability per unit area per unit time (equivalent to inverse length squared times inverse time in standard units).^[2] It is real-valued, as evident from the imaginary part construction or from the polar decomposition \psi = \sqrt{\rho} e^{i\phi}, which simplifies \mathbf{j} = \frac{\hbar}{m} \rho \nabla \phi. For stationary states satisfying the time-independent Schrödinger equation, the density \rho is time-independent, so \frac{\partial \rho}{\partial t} = 0 and thus \nabla \cdot \mathbf{j} = 0.

Scalar particle in electromagnetic field

In the presence of an electromagnetic field, the probability current for a non-relativistic scalar particle of charge e is obtained through minimal coupling in the Schrödinger equation, replacing the momentum operator \mathbf{p} = -i\hbar \nabla with the kinetic momentum \mathbf{\pi} = \mathbf{p} - \frac{e}{c} \mathbf{A}, where \mathbf{A} is the vector potential and c is the speed of light (in cgs units).^[3] The Hamiltonian becomes \hat{H} = \frac{1}{2m} \left( -i\hbar \nabla - \frac{e}{c} \mathbf{A} \right)^2 + e \phi, with \phi the scalar potential.^[5] To derive the probability current \mathbf{j}, start from the continuity equation \frac{\partial \rho}{\partial t} + \nabla \cdot \mathbf{j} = 0, where the probability density is \rho = |\psi|^2. Compute \frac{\partial \rho}{\partial t} = \psi^* \frac{\partial \psi}{\partial t} + \psi \frac{\partial \psi^*}{\partial t} using the time-dependent Schrödinger equation and its complex conjugate, then rearrange terms involving spatial derivatives to isolate \nabla \cdot \mathbf{j}. This yields the gauge-invariant expression

\mathbf{j} = \frac{\hbar}{2mi} \left[ \psi^* \left( \nabla - i \frac{e}{\hbar c} \mathbf{A} \right) \psi - \psi \left( \nabla + i \frac{e}{\hbar c} \mathbf{A} \right)^* \psi^* \right] - \frac{e}{mc} |\psi|^2 \mathbf{A},

which ensures invariance under gauge transformations \mathbf{A} \to \mathbf{A} + \nabla \chi and \psi \to \psi e^{i (e/\hbar c) \chi}.^[5]^[3] When \mathbf{A} = 0, this reduces to the free-particle probability current \mathbf{j}_0 = \frac{\hbar}{2mi} (\psi^* \nabla \psi - \psi \nabla \psi^*).^[3] Physically, the first term represents the convective (or paramagnetic) contribution, arising from the phase gradients in the wave function modified by the field, while the second term is the diamagnetic contribution, reflecting the direct drift of probability due to the vector potential.^[5] In SI units, the coupling replaces \frac{e}{c} \mathbf{A} with e \mathbf{A}, omitting c and adjusting the constants accordingly, such that the diamagnetic term becomes -\frac{e}{m} |\psi|^2 \mathbf{A}.^[3]

Spinning particle in electromagnetic field

For particles with intrinsic spin, the presence of an electromagnetic field requires a generalization of the probability current that accounts for both orbital motion and spin degrees of freedom, extending the spinless scalar case where the current reduces to the standard form without spin contributions. In the non-relativistic regime, this is captured by the Pauli equation, which incorporates minimal coupling to the vector potential \mathbf{A} and scalar potential \phi, while treating spin via Pauli matrices \boldsymbol{\sigma}. The probability density remains \rho = \psi^\dagger \psi, where \psi is a two-component spinor, ensuring conservation via the continuity equation \partial_t \rho + \nabla \cdot \mathbf{j} = 0. The explicit form of the probability current \mathbf{j} for a spin-1/2 particle, derived from the non-relativistic limit of the Dirac equation (known as the Dirac-Pauli approximation), includes an orbital term analogous to the scalar case, a diamagnetic term from the vector potential, and a distinct spin contribution:

\mathbf{j} = -\frac{i[\hbar](/page/H-bar)}{2m} \left[ \psi^\dagger \nabla \psi - (\nabla \psi)^\dagger \psi \right] + \frac{[\hbar](/page/H-bar)}{2m} \nabla \times (\psi^\dagger \boldsymbol{\sigma} \psi) - \frac{q}{m} \mathbf{A} \, \psi^\dagger \psi,

where q is the particle charge, [m](/page/Mass) its mass, and \hbar the reduced Planck's constant (in SI units). The first term represents the paramagnetic orbital current, the second is the spin current arising from the intrinsic angular momentum (proportional to the curl of the spin density \psi^\dagger \boldsymbol{\sigma} \psi), and the third is the diamagnetic contribution. This expression ensures the current satisfies the continuity equation when substituted into the Pauli equation i\hbar \partial_t \psi = \left[ \frac{1}{2m} (\mathbf{p} - q\mathbf{A})^2 + q\phi - \frac{q\hbar}{2m} \boldsymbol{\sigma} \cdot \mathbf{B} \right] \psi, where \mathbf{p} = -i\hbar \nabla and \mathbf{B} = \nabla \times \mathbf{A}. The spin term, absent in the scalar case, highlights how spin modifies probability flow even without spatial motion, as confirmed by derivations from the Lévy-Leblond equation.^[5] For particles of arbitrary spin s, the framework generalizes through the Lévy-Leblond equations, which extend the Pauli description to wave functions with $2s+1 components transforming under the spin-s representation of the rotation group. The probability current takes a similar structure, with the orbital and diamagnetic terms unchanged, but the spin contribution involving the curl of a higher-rank spin density tensor constructed from spin-s operators (analogous to \boldsymbol{\sigma} for s=1/2). This can be formulated using a density matrix in spin space or multipole expansions with spherical harmonics Y_{lm}(\theta, \phi) for the spin orientation, ensuring Galilean invariance and probability conservation. Seminal work establishes that the spin current remains a non-relativistic effect for any s, scaling with the magnetic moment \boldsymbol{\mu} = g \mu_B \mathbf{S}/\hbar where \mathbf{S} is the spin operator and g the Landé factor.^[5]

Relativistic Probability Current

Klein-Gordon four-current

In relativistic quantum mechanics, the Klein-Gordon four-current provides a conserved probability current for scalar particles obeying the Klein-Gordon equation (\square + m^2)\phi = 0, where \square = \partial_\mu \partial^\mu is the d'Alembertian operator and natural units with \hbar = c = 1 are used.^[6] For a complex scalar wave function \phi, the four-current is defined as

j^\mu = \frac{i}{2m} \left( \phi^* \partial^\mu \phi - \phi \partial^\mu \phi^* \right),

with components j^0 serving as the probability density \rho and \mathbf{j} as the probability current density.^[6] This form ensures the current transforms as a four-vector under Lorentz transformations, maintaining relativistic invariance.^[6] The derivation of this four-current follows from the Klein-Gordon equation and its complex conjugate, leveraging the phase invariance of the underlying Lagrangian, akin to Noether's theorem for conserved quantities under global U(1) transformations \phi \to e^{i\alpha} \phi.^[7] Multiplying the Klein-Gordon equation by \phi^* and the conjugate equation by \phi, then subtracting yields

\partial_\mu \left[ i \left( \phi^* \partial^\mu \phi - \phi \partial^\mu \phi^* \right) \right] = 0.

Dividing by $2m normalizes the expression to match the non-relativistic Schrödinger probability current in the low-velocity limit, where \rho \to |\phi|^2 and \mathbf{j} \to \frac{1}{2mi} (\phi^* \nabla \phi - \phi \nabla \phi^*).^[6] The resulting continuity equation in Minkowski space is \partial_\mu j^\mu = 0, expressing local conservation of probability: \frac{\partial \rho}{\partial t} + \nabla \cdot \mathbf{j} = 0.^[6] This holds on-shell, for solutions to the Klein-Gordon equation. A key interpretive challenge is that the probability density \rho = j^0 = \frac{i}{2m} (\phi^* \partial_t \phi - \phi \partial_t \phi^*) is not positive definite, unlike the non-relativistic case, as plane-wave solutions with negative energy yield negative \rho.^[6] This issue, noted early in the theory's development, led Pauli and Weisskopf in 1934 to reinterpret j^\mu as a charge current density in a quantum field theory context, where positive contributions from particles and negative from antiparticles resolve the negativity for overall charge conservation.^[6] In modern quantum field theory, the Klein-Gordon four-current thus underpins the description of spin-0 fields with conserved particle number.^[7]

Dirac four-current

The Dirac four-current describes the probability flow for relativistic spin-1/2 fermions governed by the Dirac equation.^[8] It is given by the expression

j^\mu = \bar{\psi} \gamma^\mu \psi,

where \psi is the Dirac spinor, \bar{\psi} = \psi^\dagger \gamma^0 is the adjoint spinor, and \gamma^\mu are the Dirac matrices satisfying the Clifford algebra \{\gamma^\mu, \gamma^\nu\} = 2 g^{\mu\nu}.^[8] The time component j^0 = \psi^\dagger \psi serves as the positive-definite probability density \rho, which ensures a physically meaningful interpretation unlike the oscillatory density in the scalar Klein-Gordon case.^[9] This four-current arises from Noether's theorem applied to the global U(1) phase symmetry of the Dirac Lagrangian \mathcal{L} = \bar{\psi} (i \gamma^\mu \partial_\mu - m) \psi, under the infinitesimal transformation \delta \psi = -i \alpha \psi and \delta \bar{\psi} = i \alpha \bar{\psi}.^[8] The symmetry variation of the action leads to the conserved current j^\mu = \bar{\psi} \gamma^\mu \psi, satisfying the continuity equation \partial_\mu j^\mu = 0 on-shell, as verified by substituting the Dirac equation (i \gamma^\mu \partial_\mu - m) \psi = 0 and its adjoint \bar{\psi} (i \partial_\mu \gamma^\mu + m) = 0.^[10] This conservation reflects the invariance of probability under local phase rotations for free fields.^[8] For electromagnetic interactions, the four-current couples covariantly via minimal substitution in the Dirac equation, replacing \partial_\mu with \partial_\mu + i e A_\mu, where A^\mu is the electromagnetic four-potential and e is the particle charge, yielding the interaction term e j^\mu A_\mu in the Lagrangian.^[11] This form maintains the conservation \partial_\mu j^\mu = 0 while ensuring gauge invariance under local U(1) transformations.^[11]

Classical Analogies

Madelung fluid interpretation

The Madelung fluid interpretation provides a hydrodynamic reformulation of non-relativistic quantum mechanics by expressing the Schrödinger equation in terms of fluid-like variables. Introduced by Erwin Madelung in 1927,^[12] this approach decomposes the complex wave function \psi into amplitude and phase components via the transformation \psi = \sqrt{\rho} \exp(i S / \hbar), where \rho = |\psi|^2 represents the probability density and S is a real-valued phase function analogous to the classical action. Substituting this form into the time-dependent Schrödinger equation yields two coupled equations: a continuity equation \partial \rho / \partial t + \nabla \cdot (\rho \mathbf{v}) = 0, where \mathbf{v} = (1/m) \nabla S defines a velocity field, and a modified Hamilton-Jacobi equation \partial S / \partial t + (1/2m) (\nabla S)^2 + V + Q = 0, with V the classical potential and Q = -(\hbar^2 / 2m) (\Delta \sqrt{\rho} / \sqrt{\rho}) the quantum potential (also known as the Bohm potential). The probability current \mathbf{j} emerges naturally as \mathbf{j} = \rho \mathbf{v}, interpreting \mathbf{v} as a probability velocity that describes the flow of probability density through space. This fluid picture equates the quantum probability density \rho to a fluid mass density and the current \mathbf{j} to a mass flux, with the quantum potential Q introducing non-local pressure-like effects that distinguish it from classical hydrodynamics. The continuity equation mirrors the conservation law for fluid mass, linking directly to the form \partial \rho / \partial t + \nabla \cdot \mathbf{j} = 0. David Bohm later emphasized this interpretation in 1952,^[13] highlighting its role in revealing guiding influences on particle trajectories. The advantages of the Madelung formulation lie in its ability to bridge quantum mechanics with classical fluid dynamics, offering an intuitive visualization of quantum evolution as coherent fluid flows modulated by quantum corrections, which facilitates analysis of phenomena like wave interference and tunneling in hydrodynamic terms.

Hamilton-Jacobi correspondence

The Hamilton-Jacobi correspondence in the context of probability current arises through the Wentzel-Kramers-Brillouin (WKB) approximation, a semiclassical method that links quantum wave functions to classical mechanics in the limit where the de Broglie wavelength varies slowly compared to its scale.^[14] In this framework, the wave function is expressed in polar form as \psi(\mathbf{r}, t) = R(\mathbf{r}, t) \exp(i S(\mathbf{r}, t)/\hbar), where R = |\psi| is the amplitude and S is the phase function, interpreted as the classical action in the high-frequency (semiclassical) limit.^[15] Substituting into the time-dependent Schrödinger equation and expanding in powers of \hbar yields, at leading order, the Hamilton-Jacobi equation for S: \frac{\partial S}{\partial t} + \frac{1}{2m} (\nabla S)^2 + V(\mathbf{r}, t) = 0, where V is the potential, mirroring the classical equation of motion for a particle.^[16] The probability current \mathbf{j} is given by \mathbf{j} = \frac{1}{m} |\psi|^2 \nabla S, where |\psi|^2 acts as the probability density and \nabla S / m represents the classical velocity field.^[14] This form recovers a classical current of probability, with particle trajectories following the characteristics of the Hamilton-Jacobi equation, akin to streamlines in a fluid description but rooted in the semiclassical transition rather than full quantum hydrodynamics.^[17] The WKB approach thus bridges quantum probability flow to deterministic classical paths, particularly for high energies where quantum interference is negligible.^[15] This correspondence extends to an analogy with the eikonal equation in geometrical optics, where the high-frequency limit of the wave equation yields ray paths analogous to classical trajectories; in quantum mechanics, the WKB phase S satisfies a similar eikonal form |\nabla S| = p(\mathbf{r}), with p the local momentum, linking probability current propagation to optical ray tracing in inhomogeneous media.^[18] The connection ties into Ehrenfest's theorem, which states that expectation values of position and momentum evolve classically, \frac{d}{dt} \langle \mathbf{r} \rangle = \frac{\langle \mathbf{p} \rangle}{m} and \frac{d}{dt} \langle \mathbf{p} \rangle = -\left\langle \nabla V \right\rangle; in the WKB regime, the probability current ensures these averages align with Hamilton-Jacobi characteristics, validating the classical limit for expectation values under smooth potentials.^[17] However, the approximation breaks down for low energies near classical turning points, where the de Broglie wavelength changes rapidly (|\frac{d\lambda}{dx}| \sim \lambda), or in regimes of strong quantum effects like significant tunneling or interference, as the neglected higher-order \hbar terms in the quantum Hamilton-Jacobi equation become dominant.^[14] In such cases, the probability current deviates from the classical form, requiring exact solutions or refined methods beyond WKB.^[17]

Physical Motivations

Conservation of probability

In relativistic quantum mechanics, the conservation of probability arises from Noether's theorem applied to the global phase invariance of the wave function, which implies the existence of a conserved four-current j^\mu. This symmetry under \psi \to e^{i\theta} \psi (with constant \theta) ensures that the associated Noether current satisfies \partial_\mu j^\mu = 0, guaranteeing that the total probability remains constant over time in closed systems.^[19] In the non-relativistic limit, this manifests as the continuity equation \partial_t \rho + \nabla \cdot \mathbf{j} = 0, where \rho = |\psi|^2 is the probability density and \mathbf{j} is the three-current.^[20] For bound states, the conservation law has specific implications: the time-independent nature of the eigenstates requires that the total probability flux through any closed surface at spatial infinity vanishes, i.e., \oint \mathbf{j} \cdot d\mathbf{A} = 0. This condition ensures no net probability leaks out, maintaining normalization \int |\psi|^2 dV = 1 indefinitely, and distinguishes bound states from scattering states where flux can be non-zero.^[21] In open quantum systems or during measurements, however, this conservation can be violated due to interactions with the environment, leading to effective non-Hermitian dynamics that approximate probability flow to the environment, often accompanied by decoherence (loss of quantum coherence). These scenarios are modeled by trace-preserving master equations, such as the Lindblad form, for the reduced density matrix of the subsystem.^[22] The framework extends naturally to quantum field theory (QFT), where the Noether current associated with global U(1) phase symmetry becomes an operator whose spatial integral yields the conserved particle number operator \hat{N}. This operator commutes with the Hamiltonian in theories without particle creation or annihilation, enforcing unitarity and probability conservation at the field level.^[19]

Scattering and transmission

In one-dimensional quantum scattering scenarios, the probability current provides a direct measure of particle flux, enabling the definition of transmission and reflection coefficients for potential barriers. The transmission probability T is defined as the ratio of the transmitted probability current j_{\text{trans}} to the incident probability current j_{\text{inc}}, T = j_{\text{trans}} / j_{\text{inc}}.^[23] This quantity represents the fraction of the incident flux that passes through the potential region.^[24] The reflection probability R is similarly R = j_{\text{refl}} / j_{\text{inc}}, where j_{\text{refl}} accounts for the flux directed back toward the source.^[23] Conservation of the probability current implies unitarity, R + T = 1, ensuring no probability is lost in the scattering process.^[24] For a step potential, where the potential rises abruptly from zero to a constant value V_0, the probability current reveals classical-like reflection for incident energies below V_0 and partial transmission above it, with T increasing toward unity as energy rises.^[23] In the case of a delta function potential V(x) = \alpha \delta(x), the transmission probability T increases monotonically with energy, approaching unity in the high-energy limit, without resonances or oscillations.^[25] Relativistic extensions of the probability current, as in the Dirac equation, introduce the Klein paradox for strong potential steps exceeding twice the particle rest energy. Here, the calculated transmission coefficient satisfies T > 1, a counterintuitive result resolved by interpreting the excess flux as arising from electron-positron pair production at the barrier, where positrons contribute oppositely to the current.^[26]

Examples

Plane wave

In non-relativistic quantum mechanics, the plane wave solution for a free particle is described by the wave function

\psi(\mathbf{r}, t) = A \exp\left[i (\mathbf{k} \cdot \mathbf{r} - \omega t)\right],

where A is a complex amplitude, \mathbf{k} is the wave vector with magnitude k = |\mathbf{k}|, and the angular frequency is \omega = \frac{\hbar k^2}{2m} with m the particle mass.^[27] Substituting this into the general expression for the probability current density,

\mathbf{j} = \frac{\hbar}{2mi} \left( \psi^* \nabla \psi - \psi \nabla \psi^* \right),

yields a constant current \mathbf{j} = |A|^2 \frac{\hbar \mathbf{k}}{m}, directed parallel to \mathbf{k}.^[27] The probability density \rho = |\psi|^2 = |A|^2 is uniform in space and time, reflecting the delocalized nature of the free particle state.^[27] The magnitude of the current corresponds to a uniform flow of probability, with the associated group velocity \mathbf{v}_g = \frac{\hbar \mathbf{k}}{m} such that \mathbf{j} = \rho \mathbf{v}_g.^[27] This velocity matches the classical particle momentum \mathbf{p} = m \mathbf{v}_g = \hbar \mathbf{k}, illustrating the correspondence between quantum probability flow and classical motion for free particles.^[27] Plane waves are not square-integrable over infinite space, so normalization is achieved in a distributional sense, such as \int \psi^*(\mathbf{r}, t) \psi(\mathbf{r}', t) d^3 r = (2\pi)^3 \delta^3(\mathbf{k} - \mathbf{k}'), implying |A|^2 relates to the probability density per unit volume in a beam-like interpretation.^[27]

Infinite potential well

The infinite potential well, also known as the particle-in-a-box model, confines a quantum particle to a one-dimensional region of length L between impenetrable barriers at x=0 and x=L, where the potential is zero inside and infinite outside. This setup models bound states with discrete energy levels, and the probability current provides insight into the flow of probability within these constraints.^[28] In stationary states, the wave functions are the energy eigenfunctions \psi_n(x) = \sqrt{\frac{2}{L}} \sin\left(\frac{n\pi x}{L}\right) for n = 1, 2, 3, \dots, which are real-valued throughout the well. Due to their real nature, the probability current j(x) = \frac{\hbar}{m} \operatorname{Im} \left[ \psi_n^*(x) \frac{d \psi_n(x)}{dx} \right] vanishes everywhere inside the well, indicating no net flow of probability in these time-independent states.^[28]^[3] This zero current satisfies the continuity equation \frac{\partial |\psi|^2}{\partial t} + \frac{\partial j}{\partial x} = 0, as the probability density |\psi_n|^2 is stationary.^[2] For superpositions of these eigenstates, such as \psi(x,t) = \sum_n c_n \psi_n(x) e^{-i E_n t / \hbar}, the wave function becomes complex and time-dependent, leading to non-zero probability currents that oscillate with time. The current arises from interference terms between components, typically proportional to \sin(\Delta \phi), where \Delta \phi reflects phase differences between the superposed states, resulting in localized flows that reverse direction periodically without net transport across the well.^[3]^[29] This oscillatory current in superpositions can be interpreted as probability circulation, where the particle's probability density shifts back and forth, corresponding to balanced contributions from positive and negative momentum components in momentum space; the symmetric momentum distribution in stationary states ensures zero net flow, while superpositions introduce transient asymmetries that maintain overall conservation.^[3] By construction, the probability current vanishes at the boundaries x=0 and x=L, as the wave function \psi=0 there implies the terms involving \psi^* \frac{d\psi}{dx} and its complex conjugate are zero, preventing any probability leakage despite the infinite barriers.^[28] Numerical evaluations confirm this, with j(0) = j(L) = 0 for all states and superpositions due to the enforced boundary conditions.

Discrete Formulations

Lattice quantum mechanics

Lattice quantum mechanics discretizes quantum dynamics on a lattice structure, providing a foundational approach for modeling electron transport in periodic or disordered solids, particularly through numerical simulations. This formulation is essential in solid-state physics, where continuous space is replaced by discrete sites, allowing exact treatment of wavefunction evolution without approximating differential operators. In the tight-binding model, the probability current between adjacent lattice sites n and n+1 is defined as

j_{n \to n+1} = \frac{2t}{\hbar} \Im(\psi_n^* \psi_{n+1}),

where t is the hopping amplitude between sites, \hbar is the reduced Planck constant, \psi_n is the complex wavefunction amplitude at site n, and \Im denotes the imaginary part. This expression arises from the tight-binding Hamiltonian, which assumes localized orbitals at each site with nearest-neighbor interactions. The hopping term t encapsulates the kinetic energy transfer, typically on the order of electronvolts in semiconductor lattices.^[30] The formula is derived from the time-dependent Schrödinger equation discretized on the lattice:

i \hbar \frac{d \psi_n}{dt} = -t (\psi_{n+1} + \psi_{n-1}) + V_n \psi_n,

where V_n is the on-site potential. Differentiating the site probability \rho_n = |\psi_n|^2 with respect to time yields

\frac{d \rho_n}{dt} = \frac{i t}{\hbar} \left[ \psi_n^* (\psi_{n+1} - \psi_{n-1}) - (\psi_{n+1}^* - \psi_{n-1}^*) \psi_n \right].

Rearranging the terms involving neighbors identifies the net inflow as the difference of currents from adjacent links, confirming the form of j_{n \to n+1} and its symmetric counterpart j_{n+1 \to n} = -j_{n \to n+1}. This derivation ensures unitarity of the time evolution operator, preserving total probability.^[30]^[31] Probability conservation on the lattice follows the discrete continuity equation:

\frac{d |\psi_n|^2}{dt} + j_{n \to n+1} - j_{n-1 \to n} = 0.

Summing over all sites gives \sum_n d |\psi_n|^2 / dt = 0 (assuming vanishing boundary currents), analogous to the continuum divergence theorem. This local form enables tracking of probability flow in finite lattices with open boundaries.^[31] A key application appears in Anderson localization, where random on-site potentials V_n disrupt extended states, leading to exponentially decaying wavefunctions. In such disordered tight-binding lattices, eigenstates exhibit vanishing probability currents, as \Im(\psi_n^* \psi_{n+1}) approaches zero due to the real-valued nature of localized modes up to a global phase, preventing diffusive transport. This phenomenon, first predicted in one dimension for any disorder strength, underpins the metal-insulator transition in low dimensions. In the continuum limit with lattice spacing a \to 0 and t = \hbar^2 / (2 m a^2), the discrete current recovers the probability current for a free particle.

Finite-difference approximations

Finite-difference approximations provide a practical means to compute the probability current numerically in continuum-based quantum simulations, particularly on uniform spatial grids. These methods discretize the spatial derivatives in the continuous expression for the probability current, j_x = \frac{\hbar}{2mi} \left( \psi^* \frac{\partial \psi}{\partial x} - \psi \frac{\partial \psi^*}{\partial x} \right), while ensuring compatibility with the discretized time-dependent Schrödinger equation to maintain probability conservation via the continuity equation.^[31] A common approach is the central-difference stencil, which approximates the spatial derivatives to second-order accuracy. For a one-dimensional grid with spacing \Delta x, the probability current at grid point x_l is given by

j_x(x_l) \approx \frac{\hbar}{2mi} \left[ \psi^*(x_l) \frac{\psi(x_l + \Delta x) - \psi(x_l - \Delta x)}{2 \Delta x} - \psi(x_l) \frac{\psi^*(x_l + \Delta x) - \psi^*(x_l - \Delta x)}{2 \Delta x} \right],

where m is the particle mass. This formulation, derived from the standard central-difference approximation for first derivatives, converges to the continuous current as \Delta x \to 0 and achieves O(\Delta x^2) local truncation error in the derivative approximation.^[32] An alternative "link" current between adjacent points emphasizes probability flow across bonds, expressed as

J_x(l, l \pm 1) = \pm \frac{\hbar}{m \Delta x} \operatorname{Im} \left[ \psi^*(x_l) \psi(x_{l \pm 1}) \right],

which also ensures discrete conservation laws hold through a summation-by-parts identity analogous to the divergence theorem.^[32] To integrate these approximations into time-dependent simulations, schemes such as Crank-Nicolson or split-operator methods are employed, both of which preserve the L^2-norm of the wave function exactly (up to machine precision), thereby enforcing global probability conservation and consistent current computation. The Crank-Nicolson method, an implicit midpoint scheme, discretizes the time evolution operator as

i \frac{\psi^{n+1} - \psi^n}{\Delta t} = H \frac{\psi^{n+1} + \psi^n}{2},

where H is the discretized Hamiltonian; this unitary preservation of \int |\psi|^2 dx = 1 holds for linear and certain nonlinear cases, allowing reliable tracking of currents over long times with O(\Delta t^2) temporal accuracy.^[33] Similarly, split-operator techniques decompose the evolution into kinetic and potential propagators, applied alternately in momentum and position space via fast Fourier transforms, maintaining norm conservation and enabling efficient simulations of wave packet dynamics. Despite their advantages, finite-difference approximations introduce numerical dispersion, where high-frequency modes propagate at incorrect speeds, leading to phase errors that accumulate in long-time or broadband simulations; this is mitigated by higher-order stencils or spectral methods but remains a O(\Delta x^2) limitation in standard central schemes. These approximations find application in time-dependent contexts, such as modeling Gaussian pulse propagation through potentials, where the computed current reveals tunneling or reflection probabilities without violating conservation.^[32]^[33]

References

[1]
https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Essential_Graduate_Physics_-_Quantum_Mechanics_(Likharev)/01%3A_Introduction/1.04%3A_Continuity_Equation
[2]
[PDF] Quantum Physics I, Lecture Note 6 - MIT OpenCourseWare
Feb 23, 2016 · The probability that the particle is inside V may change in time if there is flux of the probability current across the boundary of the region.
[3]
[PDF] Probability Current and Current Operators in Quantum Mechanics
A quantum particle such as an electron produces electric current because of its motion. That current is associated with the flow of its probability.
[4]
[PDF] Quantum mechanics of collision processes
The Schrödinger form of quantum mechanics allows one to define the frequency of a state in a natural way with the help of the intensity of the associated eigen- ...Missing: probabilistic original
[5]
[PDF] THE CONTRIBUTION OF SPIN TO THE PROBABILITY CURRENT ...
We shall derive the correct probability current, including the spin current term, for nonrelativistic particles with spin without appealing to relativistic ...
[6]
[PDF] Chapter 3 The Klein-Gordon Equation
An interpretation of as probability density would mean that the theory allows negative probabilities. This is the problem of the inde nite probability density.
[7]
1 Classical Field Theory - DAMTP
An obvious generalization of the Klein-Gordon equation comes from ... To derive Noether's theorem, we first consider making an arbitrary transformation ...<|control11|><|separator|>
[8]
[PDF] 4. The Dirac Equation - DAMTP
In this section we will describe the Dirac equation, whose quantization gives rise to fermionic spin 1/2 particles. To motivate the Dirac equation, we will ...
[9]
[PDF] Covariance of the Dirac Equation
The Adjoint Spinor and Probability Current. We are now prepared to show that the Dirac probability current, defined by Eq. (46.23), trans- forms as a 4 ...
[10]
None
### Summary of Derivation of Conserved Probability Current for Dirac Equation
[11]
[PDF] Handout 2 : The Dirac Equation - Particle Physics
In terms the adjoint spinor the four vector current can be written: We will use this expression in deriving the Feynman rules for the Lorentz invariant matrix ...
[12]
[PDF] The WKB Approximation - Sites do IFGW
is the probability density and j(x, t) = h m. Im ψ. ∗. ∂ψ. ∂x. (4) is the probability current. The continuity equation can be derived by multi- plying eq.(1) ...
[13]
[PDF] The WKB Method† 1. Introduction - University of California, Berkeley
The WKB method is important both as a practical means of approximating solutions to the. Schrödinger equation and as a conceptual framework for ...
[14]
[PDF] WENTZEL–KRAMERS–BRILLOUIN (WKB) APPROXIMATION
In classical mechanics, eq. (12) is the Hamilton–Jacobi equation for the Hamilton's principle function W(x, t) for the 1d particle.
[15]
[PDF] Ehrenfest's Theorem and WKB - Texas Tech University
Sep 24, 2020 · This approximation recovers proper dependence of the probability density on p(x). Igor Volobouev. Ehrenfest's Theorem and WKB. September 24 ...
[16]
[PDF] 1207.1.K.pdf - Caltech PMA
This equation is known in optics as the eikonal equation. It is formally the same as the. Hamilton-Jacobi equation of classical mechanics3 if we identify Ω ...
[17]
[PDF] Introductory Lectures on Quantum Field Theory - arXiv
Feb 20, 2013 · of the system there is conserved current. In its simplest version in ... global phase invariance of the Dirac Lagrangian (4.27), ψ → eiθψ.
[18]
[PDF] The continuity equation in the phase space quantum mechanics
Nov 6, 2023 · The continuity equation applied to the spatial density of probability expresses the conser- vation of probability. It plays an important role ...
[19]
[PDF] Overlap integral of continuum stationary states - arXiv
Jul 1, 2024 · A probability current flowing into the system vanishes in the isolate states. Conversely a system of the finite current is not isolated.Missing: zero | Show results with:zero
[20]
[PDF] Scattering States and the Step Potential - MIT OpenCourseWare
We now define the reflection coefficient R to be the ratio of the probability flux in the reflected wave to the probability flux in the incoming wave: JB.
[21]
[PDF] Lecture 16: Probability Current II and 1D Scattering
• Transmission and reflection probabilities are derived from conservation law: j in. = j out. • For step potential, this gives: Continued. (. ) (. ) (. ) 1. 2.Missing: coefficients | Show results with:coefficients
[22]
[PDF] 10. Scattering Theory - DAMTP
solution in the presence of the potential. The probability for reflection R and transmission T are given by the usual quantum mechanics rule: R = |r|2 and T ...
[23]
[PDF] Klein's paradox - Physics Courses
Thus the transmission coefficient is actually negative! ... However, this is to be expected since we are deal- ing in the Dirac case with a probability density ...
[24]
[PDF] QUANTUM MECHANICS-Non-Relativistic Theory
The discussion is intended to display the physical significance of the theory, and to be complete and self-contained.
[25]
[PDF] Relativistic Quantum Mechanics
In this section of the course we will study relativistic quantum mechanics. In a particle accelerator we are interested in, for example, the interactions of ...<|control11|><|separator|>
[26]
[PDF] The Dirac Equation - UF Physics
Dirac Equation: Probability Density and Current. Prof. M.A. Thomson ... Dirac Equation: Plane Wave Solutions. •Now aim to find general plane wave ...<|control11|><|separator|>
[27]
[PDF] Notes on Quantum Mechanics - Physics
4.4 Particles with internal structure . ... • particles with internal degrees of freedom (such as spin). This chapter lays out the basic framework for ...
[28]
Quantum Mechanics: 1-Dimensional Particle States Applet
Infinite Well + Field: this is an infinite square well with a uniform ... Probability Current: show the probability current. This is zero for ...
[29]
Probability current in the tight‐binding model - AIP Publishing
Aug 1, 1992 · Erasmo A. de Andrada e Silva; Probability current in the tight‐binding model, American Journal of Physics, Volume 60, Issue 8, 1 August 1992 ...
[30]
[PDF] Current density and continuity in discretized models
Jul 21, 2010 · Having set out some general considerations regarding discretization, we derive expressions for the current and its divergence in terms of the ...<|control11|><|separator|>
[31]
https://engineering.purdue.edu/gekcogrp/publications/pubs_src/J_2010_11_JJ_1_publ.pdf
[32]
[PDF] arXiv:1607.03327v2 [math.NA] 7 Apr 2017
Apr 7, 2017 · Crank-Nicolson scheme. The preservation of the charge conservation law can be used to measure the longtime behavior of the numerical scheme.