# Integrating of von Neumann equation for density matrix

Suppose we are given the Hamiltonian

$$H=f \frac{\text{Tr}\sigma_x \rho}{\text{Tr}\rho}\sigma_x,$$

where $$\rho$$ is the density matrix, and $$\sigma_x$$ is the Pauli matrix $$\sigma_x= \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix},$$

and $$f$$ is a coupling constant. The time evolution of the density matrix is given by

$$\frac{d \rho}{dt}=if[\rho,\sigma_x]\frac{\text{Tr}\sigma_x \rho}{\text{Tr}\rho}.$$

How do I proceed from here? How do I integrate this equation?

• It might be a good idea to write $\rho=\frac{1}{2}\left(I+\vec{u}\cdot\vec{\sigma}\right)$ where $\vec{\sigma}=\left(\sigma_{x},\sigma_{y},\sigma_{z}\right)$ and use the properties of the Pauli matrices. – eranreches May 21 at 13:47

## 1 Answer

Your Hamiltonian does not make sense: The Hamiltonian cannot depend on the density matrix, but is a linear operator acting on the Hilbert space (the same space on which the density operator is acting). Similarly, the von Neumann equation is a linear differential equation for $$\rho$$ (just as all of quantum mechanics is linear), which your equation isn't.

Otherwise, a correct von Neumann equation is a linear differential equation, and thus its integral is an exponential function.