Decoherence and the Measurement Problem

Decoherence is the process in quantum physics by which the quantum wavefunction transitions from a superposition of various possible states into a specific observable state. It represents a loss of coherence in the phase angles of the quantum wavefunction (thus the name decoherence). As the phase angles lose their coherence through interaction with the surrounding environment, the result is classical probability and the absence of quantum behavior, which means that traditional methods of analyzing the situation using classical physics techniques is a valid approximation.

The principle of a wavefunction collapse is a core element of quantum physics, one of the few things that is explicitly included in all version of the traditional Copenhagen interpretation of quantum mechanics. The exact nature of how, why, and even if this wavefunction collapse actually takes place is called the measurement problem in quantum physics. Decoherence does not provide an actual solution to the measurement problem, but provides a mathematical justification for side-stepping the measurement problem entirely and accepting that there comes a point where physicists can move from talking about quantum behavior to talking about classical behavior.

Decoherence in essence says that as a quantum system begins interacting with its environment, it will begin to lose its bizarre quantum nature and begin acting in the way that we tend to expect from classical physics. It doesn't resolve any details about the actual collapse, however, just about how we can think about the results of the collapse.

Basically, in its most extreme form, the approach of decoherence is a claim that the measurement problem isn't really a problem, because we can work with the physics of the situation without fully understanding the wavefunction collapse itself.

The role of decoherence is particularly significant in situations where scientists would like to maintain the quantum superposition behaviors for prolonged periods of time, such as in a quantum computer.

One of the greatest conceptual and technical hurdles in developing these devices is trying to create a quantum system that is isolated enough from the general environment that the quantum states remain coherent, particularly in the context of a device that is expected to have any sort of input or output interfaces with the larger world..

Quotes on Decoherence:

In his book The Fabric of the Cosmos: Space, Time, and the Texture of Reality, physicist Brian Greene says this about decoherence:

"... the telltale difference between the quantum and the classical notions of probability is that the former is subject to interference and the latter is not.

"Decoherence is a widespread phenomenon that forms a bridge between the quantum physics of the small and the classical physics of the not-so-small by suppressing quantum interference--that is, by diminishing sharply the core difference between quantum and classical probabilities. The importance of decoherence was realized way back in the early days of quantum theory, but its modern incarnation dates from a seminal paper by the German physicist Dieter Zeh in 1970 ...

"Although photons and air molecules are too small to have any significant effect on the motion of a big object like this book or a cat, they are able to do something else. They continually "nudge" the big object's wavefunction, or, in physics-speak, they disturb its coherence: they blur its orderly sequence of crest followed by trough followed by crest. This is critical, because a wavefunction's orderliness is central to generating interference effects."

The following quote is from The Amazing Story of Quantum Mechanics, in which physicist James Kakalios is essentially describing quantum entanglement within the EPR Paradox in terms of two electrons connected by a "ribbon" (the wavefunction):

"As you might expect, the more the ribbon is stretched, the easier it is for some stray perturbation to disturb the overlapping waves between the two ends. Once the connection between the two ends is severed, then a measurement of the spin of one electron will have no bearing at all on the other electron, as they are now discribed by two distinct ribbons. The fancy way to describe this is that the two electrons' wave functions must remain "entangled" in order for this process to hold, and any object or input of energy that disturbs this state (breaks the ribbon) causes "decoherence." Overcoming the enormous challenges involved in avoiding decoherence keeps experimental physicists budy, and whether a functioning quantum computer is ever constructed that can live up to its potential remains to be seen."