How does a detection-dependent quantum jump work?

As the article notes, everyone has heard of quantum jumps, but I doubt many laypeople know about this aspect.

In order for an electron to move from one energy level to another around a nucleus, it absorbs or emits a photon. Initially it was believed that this was an independent event. However later, as the importance of measurements on quantum phenomena became apparent, the model was changed to say that the jumps were dependent not merely on the emission of a photon but upon it’s detection.

What exactly does that mean and how does that work? If I hit an atom with some hard x-rays and excite some electrons to jump a few orbitals, who has to detect that in order for the jump to occur or am I completely misreading this?

It seems a rather confused article (the PRL) but perhaps I don’t understand it myself. I searched google for “objective pure-state dynamic models” and it turned up nothing (beyond the same work), so it seems if these guys are making any sense they are making up their own non-standard terminology. Meh, it’s not very well written, I’m not sure I can tell whether they are referring to ontologies related to the measurement problem specifically, or something more mundane.

I didn’t read the article, but is this the same as saying that an electron in a distant star will not emit a visible-light photon unless it can interact someway with another electron? In other words, barring any outside interference, if a distant star’s photon “travels” billions of miles to the back of my eye, it can be sent, otherwise it wouldn’t? That’s interesting. It makes sense though, as the distance any photon travels is always 0.

I do sometimes wonder about the people who write for phsyorg. Generally when an author doesn’t really understand the subject matter it’s pretty obvious, but not always. By and large I do think it’s a fantastic site, it would just be nice if it could be uniformly excellent like say Science News or New Scientist.

Yeah, although I wasn’t complaining about them in this case. I was complaining about the source article they are drawing on, the one accepted to Physical Review letters. The confusion only propagates from there.

Basically, the article is about what most commonly would be called wave-function collapse. In quantum mechanics, there are two distinct ways a quantum state can evolve: one is the deterministic, unitary Schrödinger evolution, and the other is the indeterministic, nonunitary collapse; it is the latter the article is concerned with, and specifically, whether it is an objective event (and thus, by extension, whether or not the wave function is something ‘real’). Essentially, a collapse occurs when the wave function discontinuously evolves from a superposed state to one single, specific element of that superposition, i.e. when, say, an excited atom, which continuously evolves into a superposition of being decayed and being not-decayed, suddenly attains a definite state of being decayed.

The original idea was that this jump is an objective, non-observer dependent event: the quantum system really and spontaneously undergoes a sudden state change. Today, the tendency is often to just consider the wave function as something akin to a summary of the knowledge we have of a physical system, which is updated by measurement; the wave function collapse then is something akin to the Bayesian updating of a probability distribution.

For a simplified example, consider two boxes, in one of which there is a ball. You might describe this by a probability distribution, say 50% probability of being in box 1, and 50% probability of being in box 2. Now, you might be in a room which is very dimly lit, say there’s just one photon around at any given time. Eventually, the photon is going to scatter of the ball and hit your eye or a suitable detector, thus revealing its location, causing you to change the probability distribution to, say, 100% in box 1, 0% in box 2. Now, what has happened? Under a conventional interpretation of probability, you would say that you have simply learned the actual position of the ball – this is the detector-dependent model: had you not detected the photon, you would not have had cause to change the probability distribution. However, you might also try and claim that the probability distribution is something real in some way, and that the ball, prior to the scattering event, actually was not in any of the boxes as such, and that only the scattering has forced it into a definite state in an objective way.

What’s now being claimed is that they can exclude models where the wavefunction is treated in a way (roughly; the actual situation in the quantum realm is a bit more subtle) analogous to the latter case, thus favoring the ‘detector-dependent’ view.

As for the paper itself, I’m somewhat confused by their claim that steering (roughly, the phenomenon that measurement of one of a pair of entangled particles allows one to ‘steer’ the collapse of the other into a particular set of states) constitutes a proof of the non-objective nature of the quantum state. This is certainly not the usual interpretation (one can always postulate nonlocal influences that actively influence the other qubit’s quantum state), and besides, if it were true, their paper seems to be somewhat redundant, as then the non-objective nature of the quantum state, and thus, the collapse, would already be an established fact.

[bolding mine]
Can you elaborate on the bolded part? I’m hung up (among other things:)) on “deterministic,” which I thought I finally learned was a no-no, conceptually.

Schrödinger evolution refers to the dynamical change of the wave function according to the Schrödinger equation, which is indeed deterministic and reversible. Indeterminism only comes into play in QM during the collapse, or however else you want to characterize the process by which you obtain a definite measurement outcome from a superposition of possibilities. Any possible outcome has a certain chance of occurring, but which outcome obtains is not determined. (Of course, this means that in no-collapse interpretations, such as many worlds, the evolution of the wave function as a whole is always deterministic, though we only observe one particular branch, whence the apparent indeterminism.)


Hasn’t all of this been settled by a recent paper which proves that the wave function is in fact real?

Here is the summary.

What they essentially say is that the wave function isn’t just a model or description of reality, but that there is a 1 to 1 correspondence between the description of quantum mechanics and reality.

Copy of paper here

Meaning time-symmetric, in the same sense as Newtonian mechanics. From any given state, if you know the dynamics, you can construct every past and future state uniquely (if no collapse occurs at some point).

Not really. We’ve discussed that paper previously here, and basically, they show that quantum mechanics can’t be the statistical theory of some more fundamental theory, in a way similar to how thermodynamics is the statistical version of kinetic gas theory. It’s an important and elegant result for the foundations of QM, but claiming that it proves the wave function to be real oversells it a little. If you’re interested, these two blog posts by Matt Leifer provide a balanced and readable account of the PBR theorem.

OK, but I’m still confused. So maybe electrons aren’t literally smears of probability in space, but if the wave function corresponds 1-to-1 to reality, and I’m pretty sure that’s the language that they use, doesn’t that at least guarantee that a quantum jump will be detector-independent? I mean this doesn’t really have anything to do with whole hidden variables/realism issue - or does it?

The issue here is psi-epistemicism versus psi-onticism: the view that the wavefunction is related to our knowledge of the physical system versus the view that the wavefunction is some actual, physical property of the system. What they prove is that something that one could call ‘psi-epistemic realism’ does not work (given certain assumptions); the wavefunction is not the statistical description of underlying, more fundamental degrees of freedom. This does not disprove psi-epistemicism per se: you always have the freedom to be an instrumentalist, and deny any connection of physical theory to reality; in that case, the formalism is merely a convenient tool to predict measurement outcomes. Alternatively, you can always choose to reject their assumptions; indeed, two of the authors of the paper ‘The Quantum State Cannot be Interpreted Statistically’ have subsequently collaborated on ‘The Quantum State Can be Interpreted Statistically’, in which they explicitly construct statistical theories that violate one of their initial assumptions that are able to account for the quantum mechanical predictions.

And even if you were to reject psi-epistemicism (becoming, as is sometimes joked, a ‘psi-ontologist’), there is no straight-forward implication that thus the ‘quantum jump’ is detection-independent: detections could simply be ‘special’ physical interactions that lead to wave-function reduction (or, for instance, in the many worlds picture, a detection is just that physical interaction that tells you which world, which branch of the wavefunction you have ended up in; so there, the wavefunction is physical, but the quantum jump, or at least the appearance thereof, is detection dependent).

In any case, there’s no clear implication on the nature of the quantum jump to be gleaned from the PBR theorem, I believe.