Sometimes, the full truth is right there in front of you, but as many children will be discovering on the morning of the 25th,
some assembly is required! Is that the case here? Or is a crucial part of the truth missing from the box, so to speak, and we have to put in a call to relevant parties to obtain it?
I’m the Original Poster, and I started this thread to have a question answered about an intriguing but puzzling experiment by Serra and Lutz, and the good news is that the question has been answered. The bad news is that it’s been answered
twice, and in ways that apparently contradict one another!
Yes, as I crawled off to bed on Friday night, I left this site in a state of perplexity, confronted with what appeared to be two starkly different explanations for the cold to hot heat flow in the Serra-Lutz experiment.
Here’s our boy, Physics Forums’ very own PeterDonis, giving me
his explanation, beginning with the case of
uncorrelated cold Carbon and hot Hydrogen atoms:”Heuristically, the uncorrelated case works like statistical mechanics with large numbers of classical particles; if you look into the details, you will see that in that ordinary case, it is implicitly assumed that the states of the individual particles are uncorrelated. For that case, your heuristic reasoning about the state with a temperature difference being more organized, and hence lower entropy, applies. To put it another way, there are more states with a smaller temperature difference between particles (or regions of a macroscopic system like a gas).
The correlated case, however, restricts the quantum states available to the system, and in this restricted state space, greater temperature difference corresponds to higher entropy; i.e., there are more states with a larger temperature difference. The paper doesn't give enough information about the specific systems involved to be able to say in detail what the energy spectrum is; but heuristically, since qubits are fermions, one would expect there to be more states with a larger energy difference between them if they are correlated, by the Pauli exclusion principle.” (Bold face and italics mine)
Impressive, Peter! And Peter delivers his explanation in a very authoritative fashion--not surprising for a Mentor of long-standing, a valued contributor of a remarkable 18,000+ posts and the author of 12 perceptive Insights articles!-- a fashion that strongly suggests that he knows precisely what he’s talking about-- but wait, that’s
style, not substance, and so not really relevant. More importantly, his explanation is very
plausible, which is much more to the point, scientifically speaking
… and yet … and yet, Peter doesn’t offer definitive
proof of his explanation’s correctness. It’s as if someone said, “Look at these right triangles whose sides I’ve measured in the most exacting way: 3, 4, 5 and 8, 15, 17. And note that 3^2 + 4^2= 5^2, and 8^2 + 15^2 = 17^2! My goodness, it seems that the sum of the squares of the sides equals the square of the hypotenuse!” That would be a plausible hypothesis but of course it’s certainly not definitive proof. Peter presented an argument that made intuitive sense, and would explain things in a simple way using a well-established principle (entropy), but he didn’t actually
mathematically prove that in the given experimental set-up, correlated qubits would generate a heat flow from cold to hot because of entropy.
And, naturally, we have to give due consideration to the apparently starkly different explanation of the
big shots—the authors of the study in question, whose lead researchers are Roberto Serra, Associate Professor of Quantum Physics at the Federal University in Brazil, with loads of published papers in quantum physics to his credit, dating back to 1997, and Eric Lutz, with an even bigger pile of co-authorships (77, according to Google Scholar!), and a
full Professorship, in Theoretical Physics at a university in Germany-- birthplace of Heisenberg and Schrodinger! Could you have better
geographical credentials than Lutz?? I say that jokingly, since of course geographical cred is no more relevant when evaluating the merits of a scientific explanation than Peter’s self-assured manner, but it may exert effects subliminally that we have to be cognizant of so we’re not unduly influenced by those effects!
Now the researchers’ explanation—is it really an explanation? well, it’s kinda, sort of but not really!-- is confined to one point, actually: That the decoherence of the correlation (what their paper actually calls the ‘consumption’ of the correlation) supplies the energy for the reversal of direction of the heat flow. But let’s hear their actual words:”We observe the standard arrow of time in the absence of initial correlations, i.e., the hot qubit A cools down, while the cold qubit B heats up…The situation changes dramatically in the presence of initial quantum correlations: the arrow of time is here reversed, as heat flows from the cold to the hot spin. This reversal is accompanied by a decrease of mutual information and geometric quantum discord….
In this case, quantum correlations are converted into energy and used to switch the direction of the heat flow, in an apparent violation of the second law.” (Bold face and italics mine)
So the researchers don’t say
why the energy released by the dissipation of the quantum correlation was used to switch the direction of the heat flow, just that it was. However, we can conclude that the researchers don’t agree with Peter’s explanation—i.e. that the experimental set-up, with the hot Hydrogen and cold Carbon atoms and the correlation between them that was established as an initial condition led to an unusual entropic situation such that the heat
naturally flowed from cold Carbon to hot Hydrogen because that was the direction of higher entropy. Why can we safely infer that the researchers don’t agree with Peter’s explanation? Because entropy springs from the probabilities of different randomnesses, and occurs
spontaneously, with nothing but a mathematical impetus. It certainly doesn’t require an injection of
energy to make it happen!
So perhaps we should just conclude that the researchers are mistaken: not about the fact that the quantum correlation decohered, or that it released energy in the process of decoherence, or that the heat flowed from cold to hot during the time span of the quantum decoherence-- their experiment demonstrated all those things—but they were mistaken in attributing the cold to hot heat flow to the energy released by the quantum decoherence when in fact it was unrelated: the ‘strange’ direction of heat flow was simply what entropy dictated under the unusual circumstances of the experimental set-up and quantum correlation, just as Peter claims.
But there are two things arguing against the conclusion that the experiementers are mistaken and Peter is correct. I took a long train trip yesterday and I had time to do a little research. I couldn’t find anything on the internet that corroborated Peter’s analysis of the entropic situation of a two qubit experimental set-up like Serra-Lutz’s, when the qubits were correlated, but I did find an article in Science News about the Serra-Lutz experiment, and in it they quoted a physicist not connected to the experiment. Here’s the relevant paragraph, “Reversing the arrow of time was possible for the quantum particles because they were correlated — their properties were linked in a way that isn’t possible for larger objects, a relationship akin to quantum entanglement but not as strong. This correlation means that the particles share some information. In thermodynamics,
information has physical significance(
SN: 5/28/16, p. 10). “
There’s order in the form of correlations,” says physicist David Jennings of the University of Oxford, who was not involved with the research. “This order is like fuel” that can be consumed to drive heat to flow in reverse.” (bold face and italics mine)
Or should we conclude that not just the researchers but David Jennings
also is mistaken? You might comment, “Who the heck is
he, after all, that we should believe
him? He’s just a random physicist!” Well, not quite random. The Oxford website describes him as “Theoretical physicist at Imperial College on a Royal Society University Research Fellowship, with research interests in quantum information, quantum thermodynamics, foundational aspects of quantum theory, and quantum field theory.”
So evidently he’s quite the expert on all things quantum. “But,” you might say, “do impressive credentials prove that he’s correct in this case?” Well, no. But in the absence of clear, definitive, understandable proof of a more substantial, scientific sort, that’s all we can rely on.
So that’s where things stand. Peter offers a genuinely plausible explanation that goes to the heart of the strangeness of the heat flow, but doesn’t mention the role played by the energy released by the correlation’s decoherence-- does he feel it’s an irrelevant red herring? He doesn’t say-- while the researchers don’t actually offer a full explanation of
why the heat flow reversed its normal direction, they just say it was made to do so by the injection of the energy of the dissipating correlation. That led me to suggest, in the final paragraph of my final post on Friday, that, given the isolation of the qubits from the environment, perhaps it was the only way the energy could act. That comment apparently deeply offended Peter, who rebuked me with the words, “Please review the PF rules on personal speculation. What you are saying here is headed in that direction.” I’ve written thousands of words in this thread as we all search for the truth of a puzzling situation; a dozen or two of those words of mine offered a possible way of understanding the truth. “But how did I dare have the effrontery to do that when I didn’t ‘have a valid understanding of the subject’?” Peter, in effect, says in a thinly veiled threat. Very nice, congenial atmosphere around here!
