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Time evolution of state in rotating $B$-field spin system

Physics Asked by e63acd on March 23, 2021

I’m given a state $|psi(t)rangle$ that solves the time dependent Schroedinger equation with the Hamilton operator $hat{H}(t)$. $hat{H}(t)$ describes a Spin-$frac{1}{2}$ in a rotating magnetic field ($vec{B}(t) = B_1[cos(omega t)vec{e_1} + sin(omega t) vec{e_2}] + B_3 vec{e_3}$) rotating around the z-axis with frequency $omega$. Furthermore I have a transformed state $|widetilde{psi}(t)rangle = hat{D}(t) |psi(t)rangle$ with $hat{D}(t)$ being the rotation operator $hat{D}(t) = e^{iomega t hat{sigma_3} /2}$. I have calculated that therefore the Hamiltonian solving the SG in this case becomes:
begin{align}
hat{widetilde{H}}(t) = frac{hbar Omega_1}{2} hat{sigma_1} + frac{hbar(Omega_3-omega)}{2}hat{sigma_3}, quad Omega_1 = -2frac{mu_0 B_1}{hbar}, quad Omega_3 = -2frac{mu_0 B_3}{hbar}
end{align}
The task is to calculate the two time evolution operators $hat{widetilde{U}}(t)$ and $hat{{U}}(t)$ given by $|widetilde{psi}(t)rangle = hat{widetilde{U}}(t) |widetilde{psi}(0)rangle$ and $|psi(t)rangle = hat{{U}}(t) |psi(0)rangle$. I plugged in the standard formula for time evolution and tried to get a result by simply puttung in the Hamiltonians, but I don’t get any further. I seem to have a blackout how to deal with this problem

One Answer

The D operator you have accounts for a rotation of your state "around" the $sigma_3$ matrix: if you think of this just as with vectors, you can see that D therefore accounts for the $B_x$ and $B_y$ components of your field. Therefore, you can use the $B_z$ part to solve the evolution of your rotating state.

In fact your problem is reduced to treating a linear magnetic field, if I understood correctly

Answered by Kaelan Donatella on March 23, 2021

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