  We may infer a transition $|n \rangle \to |m \rangle$ between energy
eigenstates of an open quantum system by observing the emission of a photon of
Bohr frequency $\omega_{mn} = (E_n-E_m) / \hbar$. In addition to the
"collapses" to the state $|m\rangle$, the measurement must also have brought
into existence the pre-measurement state $|n \rangle$. As quantum trajectories
are based on past observations, the condition state will jump to $| m \rangle$,
but the state $|n\rangle$ does not feature in any essential way. We resolve
this paradox by looking at quantum smoothing and derive the time-symmetric
model for quantum jumps.
