# Quantum Mechanics - time evolution after a measurement?

A non-degenerate two-level system is described by a Hamiltonian $$\hat H$$ with $$\hat H|n\rangle = \epsilon_n|n\rangle$$, where $$n = 1, 2$$. An observable $$\hat B$$ has eigenvalues $$\pm 1$$, with the corresponding orthonormal eigenfunctions $$|\pm\rangle$$ related to $$|n\rangle$$ by

$$|+\rangle = (|1\rangle + 2|2\rangle)/\sqrt{5}$$

$$|−\rangle = (2|1\rangle − |2\rangle )/\sqrt{5}$$

What are the possible outcomes, together with their probabilities, of a measurement of $$\hat B$$ in a state of the system with the energy $$\epsilon_2$$? Would you get the same result if you made a second measurement of $$\hat B$$ some time after the first?

I think i can do the first part:

$$P(+) = |\langle 2|+ \rangle|^2 = 4/5$$

$$P(-) = |\langle 2|- \rangle|^2 = 1/5$$

But after making a measurement the wavefunction collapses into either one or the other so should the the probability be 1 and 0, how does this depend on time?

• Does $\varepsilon$ depend on $n$? May 9 '18 at 13:50
• Yes, should be ε subscript n, i.e ε1 doesnt equal ε2 May 9 '18 at 13:53

If you measurement yields $+$, your state will be in the eigenstate of $\hat B$ with eigenvalue $+$. Thus, for instance, if you obtain $+$ at $t=0$ your system will collapse to $\vert +\rangle$. This becomes your new initial state for the evolution. The time-evolution is obtained by expanding $$\vert\Psi(0)=\vert +\rangle = \vert 1\rangle \langle 1\vert +\rangle +\vert 2\rangle \langle 2\vert +\rangle$$ from which $$\vert\Psi(t)\rangle= e^{-i\epsilon_1 t}\vert 1\rangle \langle 1\vert +\rangle +e^{-i\epsilon_2 t}\vert 2\rangle \langle 2\vert +\rangle$$ Since the time-evolution does not produce an eigenstate of $\hat B$, a second measurement of $\hat B$ would produce $+$ with probability $$P(+,t)=\vert \langle +\vert\Psi(t)\rangle\vert^2 =\vert e^{-i\epsilon_1 t}\langle +\vert 1\rangle \langle 1\vert +\rangle +e^{-i\epsilon_2 t}\langle +\vert 2\rangle \langle 2\vert +\rangle\vert^2 \ne 1$$ and likewise for $P(-,t)$.
• correct. $\vert A+B\vert^2 \ne \vert A\vert^2+ \vert B\vert^2$ so the exponentials will "interfere" and will produce a time-dependent probabilities. May 9 '18 at 15:51