Intrinsic decoherence is a scientific truth


I would like to talk nicely of an initiative that helped me to find out that my view of decoherence, intrinsic decoherence, is indeed a scientific truth. Periodically, the Journal Club of Condensed Matter Physics presents an interesting selection of published papers in the area of condensed state of matter. This on-line journal was formerly started at Bell Labs and, due to its significant editorial members, contains a selection of very interesting works. This month, the first listed paper is a striking one, appeared in Physical Review Letters. It is an experimental paper and this means that the effect was indeed observed and measured. You can find this paper here but a subscription is needed to read it in full.

Let me summarize what I am claiming about this matter (see also here and here). A theorem due to Lieb and Simon says that, when the number of particles is taken to go to infinity for a quantum system with Coulomb interactions then Thomas-Fermi model is recovered. Thomas-Fermi model is a semiclassical model and so, a quantum system loses coherence and starts to behave classically. Please, note that this is a mathematical theorem. On the same ground, a beautiful theorem due to Hartmann, Mahler and Hess (see here)  shows that the decay is Gaussian when the same limit of particles going to infinity is taken. Both theorems, taken together, give a definite scenario of what happens, intrinsically, to quantum coherence of an isolated system. Can this be seen experimentally?

As I have already said, more than ten years ago, Horacio Pastawski and his group (check two papers by him here) proved, with NMR experiments, the very existence of this effect. They met a lot of difficulties to get their paper published. It was not and you can find it here. This group produces  a lot of very good physics and also this was fine as testified by a successive confirmation due to Dieter Suter and Hans Georg Krojanski appeared in Physical Review Letters. So far, it appeared as some pieces of a big jigsaw were around and nobody noticed them to make each other fit. Rather, researchers tried, in a way or another, to insert them in known matters. But this is completely new physics!

On August 8th of the last year, a paper on Physical Review Letters appeared that confirmed all this. This paper is the one I cited at the start of this post and is due to A. P. D. Love,  D. N. Krizhanovskii,  D. M. Whittaker,  R. Bouchekioua,  D. Sanvitto,  S. Al Rizeiqi,  R. Bradley,  M. S. Skolnick,  P. R. Eastham,  R. André, and Le Si Dang. I cite all of them because they did a great job and must be named. The physics relies on the behavior of polaritons. These are quasi-particles appearing in a Bose-Einstein condensate and, being bosons themselves, they condensate too. But observing such a condensate and to understand its decay it is not an easy task. Rather, this makes for an experimentalist a true challenge. Authors above accomplished this task and proved that number fluctuations are involved in the process, the decay is Gaussian and, all in all, the effect is purely intrinsic. The true signature of this effect is the dependence of the Gaussian decay on the number of particles and this is clearly seen by these authors.

All of this shows clearly that two effects are at work in producing the world we observe: an intrinsic effect that appears for a large number of interacting particles and a decay of quantum coherence produced by the interaction with the environment. For the particular case of cosmological perturbations, it is the intrinsic mechanism that induces a classical behavior (see here for an alternative view).


A significant progress in large molecule interferometry


We have written several posts in this blog about the question of decoherence and thermodynamic limit. One of the crucial experiments to understand if a body made by a large number of molecules can become classical is through this ingenious way to do interferometry with large molecules. This idea has been realized by Anton Zeilinger and his group at University of Vienna using initially molecules of fullerene. The results were striking as they were able to prove the wave nature of these large molecules. The next step is to try to use more heavy bodies to do such an experiment. With their device, a Talbot-Lau interferometer, they were able to see wave behavior for fluorofullerene but in this case they obtained a visibility lower than expected (see here and here). They were unable to claim if this was a genuine new effect or rather a limitation of the experimental apparatus. Further analyses were needed but, mostly, the apparatus needed significant improvement to manage heavier molecules and to be sure in this way that any observed effect is a genuine one and not an artifact of the used device. Anyhow, I show here this picture that is really striking.

From this figure is blatantly evident that the expected curve is not in perfect agreement with measured points for fluorofullerene. But, as already said, the experimenters were not able to do any claim about this as these differences could be due to the interferometer.

Since then, all these activities have gone in the hands of Markus Arndt that worked with Zeilinger to these experiments. Arndt is full professor at University of Vienna and has taken in charge the not that easy activity to improve the apparatus to perform experiments with larger molecules. Recently Arndt and his group published a paper on Nature ( see here) where they showed a really significat improvement in the apparatus that should grant analysis of interferometry of very large molecules. We note in this way the complexity of this enterprise that required several years for the achievement. So, now the expectations are high to see interferometry with some unexpected kind of molecules and the possibility that Arndt and his group will do some breakthrough is surely high.

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