Very nice. Does the new research teach us anything we didn't already know?

@WillG It's a neat experimental technique, but it has nothing to do with the theoretical underpinnings of QM.

@knzhou But, with respect Schrodinger's cat as a simple explanation of the impact of this recent article... Can the cat be saved? Is that something we weren't able to do before? I know it's a dumb question, but I'm using it as an analogy to the complex quantum mechanics and its dynamics. I'm a humble software engineer

@WillG As far as I can tell, there is something 'new' here in that it was not widely known that, in a suitably engineered system, quantum jumps leave a signature in some sense before they are completed. That is, this particular consequence of quantum theory was not widely known, as far as I can tell, even though all the fundamental rules that lead to it were. Even so, I will note that this effect was predicted theoretically first, which lead to the experimental attempt to observe it. Dr. Minev gives a nice account of the conception in his thesis, which is available at the Yale repository.

It seems to me that any facts confirmed in this experiment were "known" in the sense that a careful analysis of quantum theory agrees with them; on the other hand what is "known" to practicing physicists is sometimes a mix of correct theory with oft-repeated credos that sometimes miss the mark.

BTW, I think I consider this result to have a little more significance than just as an experimental technique, unlike knzhou, but he is quite right to emphasize that this is at most a refinement of what we understand the rules of quantum mechanics to imply, not a modification in any way of the rules themselves. Edit: What WillG said, more or less

"Copenhagen and, say, many worlds just differ on how to treat branches of a superposition that have completely decohered." How does Copenhagen treat decoherence?

@user76284 Copenhagen says once a quantum state entangles with the environment, all but one observable state cease to exist. MWI says that every outcome is equally real even if they can't interact. Pilot wave theory essentially says that the space of all possible outcomes is physical, and then there's an "actual outcome" that we observe and that doesn't affect the former and yet is equally important as the former.

"Why all news articles got it wrong" - because it's a lot more fun to make wildly misleading statements than to stick with cold, dry, boring facts.

@John At what point exactly does such entanglement occur and other states cease to exist? And what qualifies as “environment” (i.e. how big does it have to be for it to start “destroying” states)?

@user76284 Nobody knows -- but this piece of research doesn't say anything new about it.

@knzhou so before the Yale experiment, the idea that a quantum leap is "gradual" was only proven theoretically. Doesn't that give the experiment good merit in being the practical proof? I think that's more significant and revolutionary than how you worded it being a cool technique.. right?

@user76284 Double-slit interference has been demonstrated on molecules with 114 atoms and I believe there have been an experiment that kept entire mitochondria in a coherent superposition. Scaling up the double slit experiment is difficult though, because the De Broglie wavelength of a system is inversely proportional to its mass, and also because conservation of momentum slowly begins to leak which-path information to the environment.

"No. All news stories about this result are extremely misleading." - These days, whenever I see the word "quantum" in a headline, I just assume this is the case, and 99% of the time, I'm right.

@knzhou I think the point of the article might be a little more subtle than you're indicating in your section on what they did. They are considering an open quantum system because of the light leaking out of the cavity allowing them to perform their measurement. If you want to say they are seeing unitary evolution of the wavefunction then you need to include the macroscopic measuring and feedback devices in the Hilbert space.

Such a treatment can be given to the experiment. The interpretation would be that while they don't know when a "jump" will happen they do know that there is entanglement between their measuring device and their qubit system such that when they see a certain feature in their measurement they know that they will observe a particular unitary dynamic of their system. But I think it's important to point out that they are not simply seeing Rabi oscillations between their two qubit states. That has been seen for decades.

@jgerber Yeah, I was glossing over some things; in particular my $|0 \rangle$ and $|1 \rangle$ stand for joint states of the system and environment. However, I intentionally simplified this point away (to just hint at it as "in a context...") because open quantum systems almost always get misunderstood at the popsci level -- 99% of the time they are portrayed as behaving fundamentally differently from closed systems when it's just the Schrodinger equation in both cases.

My point is that yes, you can describe the system in a unitary way but you need to include the measurementand feedback devices in the Hilbert space which might be pretty complicated. If, instead, you allow to use an open system formalism then you can now talk about "quantum jumps". This is because the Unitary time evolution laws only apply to closed systems. In any case, I agree that the experiment does not have any bearing whatsoever on interpretations of quantum mechanics but I think it does help to illustrate concepts in open quantum systems and feedback.

@jgerber Yup, totally agreed on all points! I just want to strenuously object to the hundreds of news articles saying that this disproves QM interpretations, or even QM itself. If you want to give further details, it would be great if you wrote an answer.