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Madelung equations

Consider a quantum particle of mass \(m\) moving along a line, the \(x\) axis, in the presence of a potential \(V(x,t)\). The time-evolution of the particle's wave function \(\Psi(x,t)\) is governed by the Schrödinger equation, \[i \hbar \frac{\partial \Psi}{\partial t} = -\frac{\hbar^2}{2 m} \frac{\partial^2 \Psi}{\partial x^2} + V \Psi \,.\] The Schrödinger equation can be written in a hydrodynamic form, known as the Madelung equations: \[\frac{\partial \rho}{\partial t} + \frac{\partial (\rho v)}{\partial x} = 0 \,,\] \[\frac{\partial v}{\partial t} + v \frac{\partial v}{\partial x} + \frac{1}{m} \frac{\partial (V + Q)}{\partial x} = 0 \,.\] Here, \(\rho(x,t)\) is the probability density associated with \(\Psi\), \[\rho = |\Psi|^2 \,,\] \(v(x,t)\) is the velocity field associated with the flow of \(\rho\), \[\rho v = \frac{\hbar}{m} \operatorname{Im} \left\{ \Psi^* \frac{\partial \Psi}{\partial x} \right\} \,,\] and \(Q(x,t)\) is the Bohm quantum potential , \[Q = -\f