Decoherence is the effect that causes a quantum system to behave classically. The most known of this kind of effects is due to environment where the interaction of an open quantum system with its surrounding is the reason for the loss of quantum coherence. This effect is well-proven on an experimental ground and must be considered acquired knowledge. On the other side, it is a correct scientific question to ask if a closed quantum system ever displays classical behavior for some reason. I have already put forward my take in this blog (see here). This week, on Physical Review Letters (see here and here), it is appeared a paper showing how intrinsic decoherence comes out in an experimental setup of two coupled kicked rotors. Kicked rotors are the epitome of studies on classical chaos and corresponding quantum behavior. It is known that, classically, such a system display diffusion above a certain threshold, firstly computed by Boris Chirikov. The corresponding quantum system localizes instead when its classical counterpart is chaotic. This is the hallmark of a proper quantum behavior that refrains from chaos proper to classical nonlinear systems. The main reason is that the Schrödinger equation is just linear and superposition principle applies. On 1988, S. Adachi, M. Toda, and K. Ikeda showed a real beautiful result that two of such coupled systems lose quantum coherence (see here). The paper by Bryce Gadway, Jeremy Reeves, Ludwig Krinner, and Dominik Schneble (see here) is an experimental proof of the fact that the original theoretical result is a correct insight and we have again a proof that environmental decoherence is not all the story. An interesting recount is given here. This paper is really striking and open the door to a new class of experiments where closed quantum systems, possibly with a lot of systems involved, will be studied to give a full understanding of the quantum-classical transition.
Bryce Gadway, Jeremy Reeves, Ludwig Krinner, & Dominik Schneble (2012). Evidence for a Quantum-to-Classical Transition in a Pair of Coupled
Quantum Rotors Phys. Rev. Lett. 110, 190401 (2013) arXiv: 1203.3177v2
Adachi, S., Toda, M., & Ikeda, K. (1988). Quantum-Classical Correspondence in Many-Dimensional Quantum Chaos Physical Review Letters, 61 (6), 659-661 DOI: 10.1103/PhysRevLett.61.659
A problem that I have treated in this blog is the question of the quantum-classical transition. This question is hotly debated by people working in quantum optics, quantum computation and wherever foundations of quantum mechanics may enter. Of course, this problem today appears far from being settled and is a heavy burden left us by the fathers of quantum mechanics. Something has been acquired as environmental decoherence. Fighting this effect is a problem experimentalists have today in their everyday activity. But we know that this cannot be all the story.
Some time ago Wojciech Hubert Zurek, one of the main contributors to environmental decoherence, claimed that Hyperion, a Saturn’s moon, behaves classically in its motion just for environmental decoherence otherwise we would observe a macroscopic quantum object splashed in its orbit as happens to electrons in an atom. Of course some people contested these conclusions and come out with a sound explanation of classicality of Hyperion’s motion without the need of environmental decoherence. One of these authors is Leslie Ballentine. I think that a lot of people have read his beautiful book about quantum mechanics. Ballentine and Nathan Wiebe wrote a paper (see here), that went published on Physical Review A (see here), where they soundly proved that Hyperion behaves classically without recurring to any kind of external agent. In some way they gave an hint of an intrinsic emerging of classicality for macroscopic objects (“for all practical purposes” as John Bell taught us). This means that classicality may be an emerging property of quantum objects.
Of course, defenders of environmental decoherence tried to attack Ballentine and Wiebe view (see here). Ballentine’s answer is here. This gives a lucid view of the present criticisms to environmental decoherence, that I would like to recall it is a true observed effect, claiming an intrinsic decoherence effect for isolated quantum systems. The last word has not been said yet. Future experiments will say.
Most of the supporters of environmental decoherence share Ballentine’s views as are well aware of the limitations of this approach. This is part of the truth. I think that here is in view some new deep understanding of how reality forms. Some subtlities are implied and this can explain difficulties researchers have currently met.