In short: No, but it does not matter.

Leaving aside exact definitions of "small", there is always a hidden assumption that small changes in a system produce a small effect for practical measurements. So while a universe containing a perfectly isolated quantum subsystem does not exist, it can serve as a convenient mental model. Deviations from this situations (i.e., couplings between the subsystem and the rest of the universe) are always there, however.

If the coupling is sufficiently small you can ignore them, otherwise you simply drop the pretense of having an isolated quantum system. Which makes both the modeling and the calculations a lot more complicated.

Maybe as an analogy, consider why a civil engineer does not employ relativistic mechanics, although this is the more "correct" way of doing things.

I agree with Ulf Lorenz: there is no such thing in our Universe as an isolated system, quantum or not. Gedankenexperiments involving the notion of isolated system are very useful to get a glimpse of the reality of things; but this does not imply that one can experimentally do something with systems isolated from the rest of the Universe, which, as a matter of fact, includes the individuals trying to perform their experiments. Roughly speaking: "isolated" means outside of our Universe.

 @ Henni Ouerdane.Thanks.

I am not sure you can do experiments in the outside of our Universe.

Your notion of "isolated" is not the one I explained above. You do not define what in your vision may attempt to the isolation of a system. On the contrary, as a transactionnist physicist, I had defined what makes any isolation a fantasy impossible to fulfill.

Anyway, when you write a small set of equations, nobody doutbs that this small set represents all that we have to know. The properties of the maths artefact are projected on the reality and hides the properties of any reality.

 @ Ulf Lorenz. Thanks for intervening. However I do not see what could be "small change, small effects, coupling sufficiently small".

The author cited in first pretended that if we squeeze a system tight enough, il could become completely deterministic, seen from a human point of view.

Certainly the system cannot be made completely pure but in practice there are quantum systems which can be considered as being perfectly isolated, nuclei for example so the answer to your question depends to some extent on what type of the system is consedered

 @ B. Krippa. Thanks.

However Georges Lochak claims that they have measured significative variations of the rate of radioactive disintegrations, with the season. I am waiting for some repeating of the experiment...

If confirmed positive, it would a proof that

*. Either it is false that the instable nucleus is separated from the absorber of the emitted ray, and that all the absorbers are not equivalent, seen from the emitter.

*. Either that from one day to another, the medium through which the alpha will travel is not constant nor neutral.

Or a mix of these two variations ?

Anyway, the disintegration of a nucleus is still not at all a determinist phenomenon, and it still remains immerged in the broglian noise and maybe in not yet identified other noises. The neutrinos noise, for instance.

In a recent paper, http://dx.doi.org/10.1016/j.physa.2013.09.025 we suggest that a quantum system behaves deterministically if we monitore it. In fact we do not need to monitore it continously.

"We show that in a CMPM the

Quantum dynamics is indistinguishable of the classical Newtonian dynamics and we also show that the classical limit of

an ensemble of Newtonian trajectories, the Liouville regime, is approximately obtained for the quartic oscillator model if

the number of measurements in the time interval is large enough to destroy the revival and small enough to not reach the

Newtonian regime. The Newtonian regime is related with a higher precision measurement in CMPM, which leads to a strong

localization and in this case, the dynamics is governed by the semiclassical approximation"

Maybe I did not express myself clear enough the first time: You probably attack a straw man.

Any honest theoretician with minimal experience and no delusions of grandeur knows that we cannot perfectly isolate a system. This is so self-evident that you need not mention it (at least within the community), the same way that an experimentalist will not mention that his laser was turned on during an experiment.

However, depending on the situation, quality of your setup etc., you may be able to isolate your system pretty well. And then the system will approximately behave like an isolated system up to a certain time T that roughly depends on the quality of your isolation. If your experimental protocol fits well into that time frame, you can simply  pretend for simplicity that your system was perfectly isolated. So whenever people talk about isolated systems, they most likely have this caveat in the back of their minds (or talk about metaphysics which is largely irrelevant in practice).

For a more thorough discussion, you would have to supply a reference to the publication you cite, and spell out in detail what your exact problem is. At least I have no idea what you really want to talk about.

The matter is more or less clear. If you look at subsystem of a larger quantum system you should use the density matrix. If you look at say deuteron deuteron scattering then this system can to a very good approximation considered as isolated.  Everything should be looked at in the context of the concrete problem.

 @Adélcio Carlos De Oliveira. But no experiment ?

 @ Ulf Lorenz. The reference ? Try searching "subquantal structure" here on Researchgate. SDW has exaggerated enough to make me arouse my doubts.

@ B. Krippa. When you record a deuteron deuteron scattering, you are not the master of where, nor when, nor the parameter of the collision. Of course when you report the noticed collision in the center of inertia of the deuterons, you obtain a fully simple and defined drawing. As I did when figuring the principle of a Compton's electron-photon scattering : http://jacques.lavau.perso.sfr.fr/Physique/Zitterbewegung_Bragg_Compton_english.html

or http://citoyens.deontolog.org/index.php/topic,1520.0.html

But I am not the master of the impulsion transfered by the photon to the electron, not which electron, which photon... All these facts are beyond reach for a human operator and a human theorician.

It has already been proven by all variants of delayed choice experiments, that the dream of an isolation of the emitter of two correlated photons, is misleading. The emitter and the two absorbers remain tied until the last absorption is done. So forward isolation and delimitation do not exist at individual scale.

What is new in my statement above, is that a lateral isolation and delimitation do not exist either.

A poetic image for that was at the staging in feb 2007 at Opera Garnier of  A Kékszakállú Herceg Vára (The Bluebeard Castle) by La Fura Dels Baus with Beatrice Uria-Monzon as Judit and Willard White as duke Kékszakállú : the impossible bed of their impossible love, harassed by all the hands and all the furtive bodies of the previous wives of Kékszakállú.

https://www.youtube.com/watch?NR=1&v=wDaIe-vWmp8&feature=endscreen

at 4' 10".

--

http://jacques.lavau.perso.sfr.fr/Physique/Microphysique_contee.pdf

http://deontologic.org/quantic