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- Where do these time derivatives of Pauli matrices come from? These are in Susskind's Theoretical Minimum (which is not the best text to learn quantum mechanics from, I have heard). He introduces these equations as "the equations of motion" of the spin in a magnetic field. $$ \frac{d}{dt}\sigma_x=-i[\sigma_x,\sigma_z]\frac{\omega}{2}$$$$
\frac{d}{dt}\sigma_y=-i[\sigma_y,\sigma_z]\frac{\omega}{2}$$$$ \frac{d}{dt}\sigma_z=-i[\sigma_z,\sigma_z]\frac{\omega}{2}$$
Wolfgang Pauli's matrices are
$$\sigma_x=\begin{bmatrix}0& 1\\1 & 0\end{bmatrix},\quad \sigma_y=\begin{bmatrix}0& -i\\i & 0\end{bmatrix},\quad \sigma_z=\begin{bmatrix}1& 0\\0 & -1\end{bmatrix}$$
He introduces these equations as "the equations of motion" of the spin in a magnetic field.
$$ \frac{d}{dt}\sigma_x=-i[\sigma_x,\sigma_z]\frac{\omega}{2}$$$$
\frac{d}{dt}\sigma_y=-i[\sigma_y,\sigma_z]\frac{\omega}{2}$$$$ \frac{d}{dt}\sigma_z=-i[\sigma_z,\sigma_z]\frac{\omega}{2}$$
$$
\frac{d}{dt}\sigma_x=-\omega\sigma_y,\quad
\frac{d}{dt}\sigma_y=\omega\sigma_x,\quad
\frac{d}{dt}\sigma_z=0$$
"These are analogous to Saméon Poisson's brackets of angular momentum describing the rotational motion of a rigid body in classical physics. The ##x## and ##y## components of the spin precess around the ##z## axis, while the ##z## component of the spin does not change".
I am interested in how Susskind got the right hand side of the three equations.
They are not derivatives of the path in ##\operatorname{SU(2)}## ##\gamma: q\longrightarrow I_2## given by the rule $$\gamma(t)=q(t)=U\begin{pmatrix}e^{i\theta_1 t}&0\\0&e^{i\theta_2 t} \end{pmatrix}U^{-1}$$ with ##\gamma(0)=I_2## and ##\gamma(1)=q##.
$$\sigma_x=\begin{bmatrix}0& 1\\1 & 0\end{bmatrix},\quad \sigma_y=\begin{bmatrix}0& -i\\i & 0\end{bmatrix},\quad \sigma_z=\begin{bmatrix}1& 0\\0 & -1\end{bmatrix}$$
He introduces these equations as "the equations of motion" of the spin in a magnetic field.
$$ \frac{d}{dt}\sigma_x=-i[\sigma_x,\sigma_z]\frac{\omega}{2}$$$$
\frac{d}{dt}\sigma_y=-i[\sigma_y,\sigma_z]\frac{\omega}{2}$$$$ \frac{d}{dt}\sigma_z=-i[\sigma_z,\sigma_z]\frac{\omega}{2}$$
$$
\frac{d}{dt}\sigma_x=-\omega\sigma_y,\quad
\frac{d}{dt}\sigma_y=\omega\sigma_x,\quad
\frac{d}{dt}\sigma_z=0$$
"These are analogous to Saméon Poisson's brackets of angular momentum describing the rotational motion of a rigid body in classical physics. The ##x## and ##y## components of the spin precess around the ##z## axis, while the ##z## component of the spin does not change".
I am interested in how Susskind got the right hand side of the three equations.
They are not derivatives of the path in ##\operatorname{SU(2)}## ##\gamma: q\longrightarrow I_2## given by the rule $$\gamma(t)=q(t)=U\begin{pmatrix}e^{i\theta_1 t}&0\\0&e^{i\theta_2 t} \end{pmatrix}U^{-1}$$ with ##\gamma(0)=I_2## and ##\gamma(1)=q##.
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