Measurement problem in simple experiment

[kith]

What you can measure is the "suppression of interference" (within a well defined subsystem), so as control of decoherence became important for quantum computers and other quantum technologies, it made sense to separate the well defined "measurable" concept from the less well defined "explanatory" concept.

The measurable concept sometimes goes by the name of "dephasing" (see p.9 of this for an attempted clarification of the nomenclature).

 

[gentzen]

If there has been such a shift as you describe, it means that the term "decoherence" as it is now being used is not relevant to the measurement problem, since "decoherence" as it is now being used is no longer considered to be irreversible (since it is easy to reverse the operations in quantum computing that "suppress interference").

I guess the difference between "irreversible" and "not reversed, maybe accidental, maybe intentional, or maybe because fundamentally impossible" is less important for (most) practical purposes than was once believed. What is important for decoherence in quantum computing is whether it is actually suppressed (by error correction methods), not whether is would have been possible (or easy) to suppress it.

And the "shift" might be less in the usage of the word decoherence, but in stopping to complain about or apologize for it. The passage in the document @kith linked above (2006, 2009) or the following passage at the end of chapter 8 in Mike and Ike (2000, 2010) are examples of such complaints/apologies:

An unfortunate confusion of terms has arisen with the word ‘decoherence’. Historically, ... The major point of these studies has been this emergence of classicality due to environmental interactions. However, by and large, the usage of decoherence in quantum computation and quantum information is to refer to any noise process in quantum processing. In this book, we prefer the more generic term ‘quantum noise’ and tend towards its usage, although occasionally decoherence finds a proper place in the context.

To conclude, please just continue to use the word decoherence as you did before, and rely on the context to disambiguate whether it means "environmental decoherence" or just "suppression of interference".My intention was never to complain about your usage of the word decoherence. Instead, I was bothered by the contrast between your "a philosophy website, not a physics textbook or peer-reviewed paper", Schlosshauer's "Stanford Encyclopedia of Philosophy ... is also an authoritative source of information. It has comprehensive entries -- some written by our interviewees ..." and the "unexpected" content and terminology of that SEP entry. (Of course, SEP cannot be an authoritative source of information for a physics forum. But even if it was, it could not substitute personal understanding and background knowledge. And an encyclopedia is not a good source for learning the basics either.) However, being the author of an SEP entry is more prestigious than being the author of a peer-reviewed paper. The first thing I did (before trying to understand the confusion of terminology) was to check whether Guido Bacciagaluppi, the author of that SEP entry, deserved that honor. After learning that he is the coauthor of Quantum Theory at the Crossroads: Reconsidering the 1927 Solvay Conference (arXiv link), I decided that he probably did, because deep knowledge about historical developments seems to be highly valued by SEP. (He is also one of the interviewees in Schlosshauer's book.) Let me also add that I know more about philosophy or quantum computing than I know about decoherence. So my guess that there has been that "shift" should be taken with a grain of salt.

 

[kith]

There might have been a shift in the usage of term "decoherence" but I think the more important thing is that there are two different cultures using a similar concept.

For the foundations community, the irreversibility is the most important aspect because they are interested in the quantum-to-classical transition. For the applied communities -like that of quantum information, molecular dynamics, quantum chemistry, etc.- the measurable consequences are the most important aspect. For them, coherence can be both lost and revived, depending on the experimental setup.

Personally, I dislike the restriction of "decoherence" to the irreversible case because it gives the illusion that we can easily distinguish between the irreversible and the reversible case while it might actually be a matter of experimental sophistication or practicality. I like to have the fuzzyness of the term "irreversible" explicit in order to connect with the issues which arise already in classical physics.

 

[PeterDonis]

What is important for decoherence in quantum computing

Quantum computing deals with qubits, not macroscopic objects. The measurement problem doesn't even come into play; all operations are unproblematically unitary and reversible. So quantum computing is irrelevant to this thread, which is about the measurement problem. Which, to me, means the usage of the term "decoherence" that you have given, as it is applied to quantum computing, is also irrelevant to this thread. Nobody has ever manipulated macroscopic objects the way quantum computing manipulates qubits.

Schlosshauer's "Stanford Encyclopedia of Philosophy ... is also an authoritative source of information.

No source of information is "authoritative". No source is always right. A philosophy encyclopedia might be a good source for discussions of philosophy, but one would not expect it to necessarily be a good source for discussions of physics. Physics is an experimental science; philosophical speculations play a role in physics, but physics is still a different discipline from philosophy. And this forum is for discussing physics.

being the author of an SEP entry is more prestigious than being the author of a peer-reviewed paper.

Physics is not a matter of prestige. It's a matter of finding models that make accurate predictions.