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### Q: [1] pr 4.12

Solve the spin 1 Hamiltonian

\begin{equation}\label{eqn:crystalSpinHamiltonianTimeReversal:20}

H = A S_z^2 + B(S_x^2 – S_y^2).

\end{equation}

Is this Hamiltonian invariant under time reversal?

How do the eigenkets change under time reversal?

### A:

In spinMatrices.nb the matrix representation of the Hamiltonian is found to be

\begin{equation}\label{eqn:crystalSpinHamiltonianTimeReversal:40}

H =

\Hbar^2

\begin{bmatrix}

A+\frac{B}{2} & 0 & \frac{B}{2} \\

-\frac{i B}{\sqrt{2}} & B & -\frac{i B}{\sqrt{2}} \\

\frac{B}{2} & 0 & A+\frac{B}{2} \\

\end{bmatrix}.

\end{equation}

The eigenvalues are

\begin{equation}\label{eqn:crystalSpinHamiltonianTimeReversal:60}

\setlr{ \Hbar^2 A, \Hbar^2 B, \Hbar^2(A + B)},

\end{equation}

and the respective eigenvalues (unnormalized) are

\begin{equation}\label{eqn:crystalSpinHamiltonianTimeReversal:80}

\setlr{

\begin{bmatrix}

-1 \\

0 \\

1

\end{bmatrix},

\begin{bmatrix}

0 \\

1 \\

0

\end{bmatrix},

\begin{bmatrix}

1 \\

-\frac{i \sqrt{2} B}{A} \\

1 \\

\end{bmatrix}

}.

\end{equation}

Under time reversal, the Hamiltonian is

\begin{equation}\label{eqn:crystalSpinHamiltonianTimeReversal:100}

H \rightarrow A (-S_z)^2 + B ( (-S_x)^2 – (-S_y)^2 ) = H,

\end{equation}

so we expect the eigenkets for this Hamiltonian to vary by at most a phase factor. To check this, first recall that the time reversal action on a spin one state is

\begin{equation}\label{eqn:crystalSpinHamiltonianTimeReversal:120}

\Theta \ket{1, m} = (-1)^m \ket{1, -m},

\end{equation}

or

\begin{equation}\label{eqn:crystalSpinHamiltonianTimeReversal:140}

\begin{aligned}

\Theta \ket{1} &= -\ket{-1} \\

\Theta \ket{0} &= \ket{0} \\

\Theta \ket{-1} &= -\ket{1}.

\end{aligned}

\end{equation}

Let’s write the eigenkets respectively as

\begin{equation}\label{eqn:crystalSpinHamiltonianTimeReversal:160}

\begin{aligned}

\ket{A} &= -\ket{1} + \ket{-1} \\

\ket{B} &= \ket{0} \\

\ket{A+B} &= \ket{1} + \ket{-1} – \frac{i \sqrt{2} B}{A} \ket{0}.

\end{aligned}

\end{equation}

Noting that the time reversal operator maps complex numbers onto their conjugates, the time reversed eigenkets are

\begin{equation}\label{eqn:crystalSpinHamiltonianTimeReversal:180}

\begin{aligned}

\ket{A} &\rightarrow \ket{-1} – \ket{-1} = -\ket{A} \\

\ket{B} &\rightarrow \ket{0} = \ket{B} \\

\ket{A+B} &\rightarrow -\ket{1} – \ket{-1} + \frac{i \sqrt{2} B}{A} \ket{0} = -\ket{A+B}.

\end{aligned}

\end{equation}

Up to a sign, the time reversed states match the unreversed states.

# References

[1] Jun John Sakurai and Jim J Napolitano. *Modern quantum mechanics*. Pearson Higher Ed, 2014.