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.

中文翻译：

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量子跃迁之前的崩溃

我们可以通过观察玻尔频率 $\omega_{mn} = (E_n-E_m) / \hbar 的光子的发射来推断开放量子系统的能量本征态之间的跃迁 $|n \rangle \to |m \rangle$ $. 除了“崩溃”到状态 $|m\rangle$ 之外，测量还必须使测量前状态 $|n \rangle$ 存在。由于量子轨迹基于过去的观察，条件状态将跳至 $| m \rangle$，但状态 $|n\rangle$ 没有任何本质特征。我们通过研究量子平滑来解决这个悖论，并推导出量子跳跃的时间对称模型。