Bad Qubits — Learn Quantum by Doing

Multiple choice

A spin-1/2 particle in a uniform field B = B_0*z-hat has Hamiltonian H = omega_0*S_z with energy eigenvalues E_± = ±hbar*omega_0/2. When a spin state is not aligned with the field, what does its expectation-value spin vector do?

It spirals inward until the spin magnitude shrinks to zero
It remains completely stationary because all spin states are energy eigenstates
It immediately collapses onto the +z eigenstate and stays there
It precesses (Larmor precession) around the z-axis at angular frequency omega_0, keeping <S_z> constant