In this small group activity, students solve for the time dependence of two quantum spin 1/2 particles under the influence of a Hamiltonian. Students determine, given a Hamiltonian, which states are stationary and under what circumstances measurement probabilities do change with time.

Quantum MechanicsTime EvolutionSpin PrecessionExpectation ValueBohr FrequencyQuantum Fundamentals 2023 (3 years)
Consider a two-state quantum system with a Hamiltonian
\begin{equation}
\hat{H}\doteq
\begin{pmatrix}
E_1&0\\ 0&E_2
\end{pmatrix}
\end{equation}
Another physical observable \(M\) is described by the operator
\begin{equation}
\hat{M}\doteq
\begin{pmatrix}
0&c\\ c&0
\end{pmatrix}
\end{equation}
where \(c\) is real and positive. Let the initial state of the system be \(\left|{\psi(0)}\right\rangle
=\left|{m_1}\right\rangle \), where \(\left|{m_1}\right\rangle \) is the eigenstate corresponding to the larger of the two possible eigenvalues of \(\hat{M}\). What is the frequency of oscillation of the expectation value of \(M\)? This frequency is the Bohr frequency.