The Collapse Before a Quantum Jump Transition

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We may infer a transition |n⟩→|m⟩ between energy eigenstates of an open quantum system by observing the emission of a photon of Bohr frequency ωmn=(En−Em)/ℏ. In addition to the "collapses" to the state |m⟩, the measurement must also have brought into existence the pre-measurement state |n⟩. As quantum trajectories are based on past observations, the condition state will jump to |m⟩, but the state |n⟩ 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.
Original languageEnglish
Pages (from-to)69-76
Number of pages8
JournalReports on Mathematical Physics
Issue number1
Publication statusPublished - 28 Feb 2020


  • continuous measurement
  • filtering
  • photon counting
  • quantum jumps
  • retrodiction


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