decoherence and collapse in quantum theory

The following news seems ignoring the difference between decoherence and collapse of wave function. The former is governed by Schrodinger equation and hence in principle deterministic, whereas the latter is completely probabilistic. And, never forget that, it takes no time for a collapse, although, it indeed takes time for decoherence (the so-called decoherence time). The riddle is not about decoherence but about collapse. If collapse could be removed, Einstein would accept Quantum Theory !

WHY can't we be in two places at the same time? The simple answer is that it's because large objects appear not to be subject to the same wacky laws of quantum mechanics that rule subatomic particles. But why not - and how big does something have to be for quantum physics no longer to apply? Ripples in space-time could hold the answer.

The location of the boundary between the classical and quantum worlds is a long-standing mystery. One idea is that everything starts off as a quantum system, existing in a superposition of states. This would make an object capable of being, for example, in many places at once. But when this system interacts with its environment, it collapses into a single classical state - a phenomenon called quantum decoherence.

Brahim Lamine of Pierre and Marie Curie University in Paris, France, and colleagues say that gravitational waves may be responsible for this. These waves in the very fabric of the universe were generated by its rapid expansion soon after the big bang, as well as by violent astrophysical events such as colliding black holes. As a consequence, a background of ripples at very low amplitudes pervades space-time.

Gravitational waves may be responsible for collapsing quantum ambiguity into a single classical state

Lamine and colleagues calculated how this fluctuating space-time might contribute to quantum decoherence. They found that for systems with very large mass, such as the moon, decoherence induced by the gravitational waves would have caused any quantum superposition to dissipate immediately. At the other end of the scale, such waves would have a negligible effect on massless photons.

To test whether gravitational waves do in fact cause the decoherence seen in large objects, the researchers suggest using a set-up called a matter-wave interferometer in which molecules are made to pass through multiple gratings. The wave-like nature of the molecules causes them to diffract, and the diffracted waves interact to give rise to an interference pattern. Quantum decoherence destroys this pattern, so in principle this could provide a test for whether the decohering effect of background space-time fluctuations matches predictions. Such a system would have to be completely isolated to rule out other effects.

This is, however, impossible in practice - with today's interferometers, at least. Experiments pioneered by Anton Zeilinger, Markus Arndt and colleagues at the University of Vienna, Austria, have been able to generate interference with beams of 60-atom carbon buckyballs, but even with molecules of this size the effect of gravitational waves would be too small to be observed.

According to Lamine, who presented his work last month at the Gravitation and Fundamental Physics in Space meeting at Les Houches in the French Alps, the effect should be measurable in larger systems at high energy. Supersonic beams of about 3000 carbon atoms would do the trick if made to interfere over an effective area of about 1 square metre. This is far beyond the reach of any foreseeable technology.

Some speculative theories predict, however, that quantum decoherence will occur on a lower energy scale than that suggested by Lamine. If so, this could be within experimental reach. "That is why our experiments are pushing [up] the interference mass limit, step by step," says Arndt.