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).