1) I am not at all knowledgeable about optical experiments, so I cannot 
comment on the novelty or value of this experiment to that field. 
However, I would like to comment on the conceptual framework that 
motivated this work, “quantum jumps,” especially since the authors 
cite the paper by John Bell “Are there quantum jumps,” whose title he 
chose to be identical to that of a paper by Schrodinger. 

2) The point made by Schrodinger and Bell is that, if one describes, such 
an experiment as the authors perform, using standard quantum theory, 
and includes the apparatus in the state vector description, one ends 
up with a state vector which is a superposition of the various 
possible outcomes, including the recording by the apparatus of each 
outcome. The problem is that one or another outcome actually occurs in 
the laboratory, and standard quantum theory does not describe how this 
takes place. 

3) Both Schrodinger and Bell objected to the Copenhagen school’s 
“collapse postulate,” which simply states that this transition has to 
take place sometime, somehow, but is silent on both these issues. Both 
wished for a transition of the state vector from the superposition to 
the observed state to be described by theory, not by postulate. In his 
paper, Bell approvingly wrote about a modification of quantum theory, 
a new theory which provides such a description, giving a 
characteristic time for this evolution. 

4) The point is that the time involved in such a quantum jump is not what 
the authors conceive it to be. It is the time it takes the 
superposition of such macroscopic states to evolve to one of those 
observed states. 

5) What the authors have experimentally done is measure the time between 
reception of a “signal” photon emitted by an electron dropping to a 
state (|3> in their notation) and the subsequent emission of an 
“idler” photon as the electron drops to the ground state ((|0> in 
their notation). Their theoretical analysis concerns only the 
microscopic system, does not include the apparatus, so this has 
nothing to do with the notion of quantum jumps discussed above. Their 
measurement is not the time it takes the superposition of possible 
outcomes to become one outcome. 

6) I would add that, while the authors concentrate their interest upon 
their data relative to the shortest time the electron stays in state 
|3>, the measurement of any time in the range of times given by the 
decay curve they present involves a “quantum jump,” a transition from 
the superposition of possibilities to the actuality. 

7) Now, concerning their analysis, they arrive at the conclusion that 
standard quantum theory predicts a step-function jump in the time 
between reception of the signal photon and the earliest idler photon. 
Again, I have no expertise at all in this area, but I would be 
surprised if a more careful analysis did not moderate that step 
function a certain amount. 

8) The author’s calculation involves a certain amount of idealization, 
does not deal with the spatial nature of the atomic state within the 
Rb atom. I would think that the state vector describing the population 
of state |2>, its depopulation with concurrent photon emission, the 
population of state |3> and then its depopulation with concurrent 
photon emission, would end up giving a superposition which would not 
contain a step function jump between signal photon emission and idler 
photon emission. However, it might be an alteration that is so small 
as to be experimentally undetectable. 

9) Regardless, if the apparatus is included in the state vector, there 
certainly would be, as the authors suggest in their concluding 
remarks, a moderation of this step function behavior due to the 
behavior of the photodetectors. Indeed, that seems to be what they are 
measuring. 

10) In conclusion, I cannot speak to the value for the photonic community 
of this experimental upper limit on the briefest correlation time 
between the signal and idler photons, effectively an upper limit on 
the amount of time an electron spends in an excited state before 
dropping to its ground state. It may be that this is a valuable 
result. 

11) However, I object to this being called an experiment that measures the 
time duration of a quantum jump, since my understanding of the meaning 
of the time duration of a quantum jump is the time it takes the 
superposition of possible outcomes (what Schrodinger called in his 
1935 paper the “catalog of expectations”) given by Schrodinger’s 
equation to become one, actual, outcome.