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Decoherence is our ticket out of the quantum world

DOE/Los Alamos National Laboratory : 24 January, 2002  (Technical Article)
The world of quantum mechanics is exceedingly weird, one in which waves can act as if they were particles, particles can spontaneously pass from one side of a barrier to the other and gedanken cats can be simultaneously dead and alive.
Our everyday experiences, the so-called 'classical' world, seem immune from the paradoxes that abound in quantum mechanics, and it's comforting to know that our bedroom furniture won't spontaneously reappear in the kitchen and planets will continue to curl smoothly along their orbital paths. We can rest easy knowing that we reside in a classical world.

But we don't.

Truth is, we live in a quantum mechanical world, as Wojciech Zurek stressed to an audience today at the American Association for the Advancement of Science annual meeting. Zurek, a physicist at the U.S. Department of Energy's Los Alamos National Laboratory, said, 'We don't know any place where quantum theory does not apply. Quantum mechanics works extremely well and we do not know of any conflict between its predictions and experiments.

'We need to study this quantum mechanical world in which we live and understand when and why it appears classical if we want to make good use of the opportunities offered by quantum information,' Zurek said.

Zurek explained that what allows the set of experiences we have come to regard as normal to emerge from the collection of microscopic, quantum mechanical interactions that underpin the universe is a process he calls 'decoherence.'

Decoherence, in brief, describes the constant, tenuous interactions between a system or object and its environment, a set of interactions that allows concrete behaviors to emerge from the multitude of simultaneous possibilities that quantum theory allows.

Quantum theory describes a range of possible, superposed states in which objects exist. In the traditional view, it awaits a measurement by some outside observer to force an object to declare itself as being in one definite state: an electron's spin vector pointing up rather than down, a photon acting like a particle rather than a wave.

Since there is no overworked, microscopic measurer constantly pinning down each quantum mechanical system to choose a final state, how do we get from the quantum world of multitude possibilities to the definite events we experience?

Physicists have pondered this conundrum for most of the century, and this decade decoherence has emerged as the answer, or at least a major part of it, Zurek said. 'The quantum view as originally formulated was applied to isolated systems. But in fact, all objects have interactions, no matter how tenuous, with their environment. These interactions are so slight that they don't affect the object; rather, the object leaves an imprint on the environment.

'We live in a sea of photons, for example, and interact with some small fraction of them. So the environment is in some sense in a constant process of monitoring objects,' Zurek said.

'Decoherence does not require an apparatus or a direct measurement to make a system declare its state. But neither does it give you an exact answer: it only takes us halfway there. Decoherence provides a menu of allowed states; it's a selection process that disallows flagrantly quantum states of macroscopic objects,' Zurek said.

There are macroscopic systems that should exist, save for decoherence, in quantum states of possibilities.

Macrosystems defined as 'chaotic' can be analyzed from a quantum mechanical view, and quantum mechanics always works, Zurek emphasized.

'Chaotic systems produce weird superpositions of many possible states, and this condition evolves in a reasonably short time scale,' Zurek said. 'There are macroscopic, chaotic systems that can get into all sorts of bizarre trouble from a quantum-mechanical perspective.'

Hyperion, an odd-shaped moon of Saturn, is known to be a chaotic system as it tumbles along its orbital path, its orientation continually redirected by Saturn's gravitational field. According to quantum theory, it should take less than 20 years for Hyperion to get into a quantum state, in which it would be simultaneously in a non-classical superposition of many orientations.

But, thanks to decoherence, Hyperion's major axis is not simultaneously pointed toward and away from Saturn, awaiting a measurement to define its orientation.

'Hyperion is not isolated,' Zurek pointed out, 'it is making an imprint on the environment all the time. So decoherence applies and keeps the moon out of quantum trouble.

'Decoherence is exceedingly effective. On the macroscopic level of our everyday experience it works many times faster than anything we can measure,' Zurek said. 'Quantum mechanics without decoherence leads to a universe of paradoxes, there is a real conflict.

'Experimental investigation of the middle ground between these two realms is just beginning, driven by both our scientific curiosity about the origins of the classical and by the inevitable encounter with decoherence, which is recognized as the biggest obstacle to an experimental implementation of a quantum computer,' Zurek said.
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