Wojciech Zurek is a scientist at Los Alamos National Laboratory best known for his contributions to the theory of quantum decoherence, the loss of coherence between the various states in a superposition of quantum states. The principle of superposition of states, the axiom of measurement, and the projection postulate (the "collapse" of the wave function or "reduction of the wave packet") are the three main assumptions of P.A.M. Dirac's standard formulation of quantum mechanics. Zurek and his colleagues, notably H. Dieter Zeh, deny Dirac's projection postulate. They are a special case of many theorists who look for reasons to deny indeterministic and discrete discontinuous processes (e.g., quantum jumps), in order to restore a continuous and deterministic physics and explain the transition from microscopic quantum physics to macroscopic classical physics. Zurek and Zeh explain the loss of quantum coherence and the "appearance" of quantum jumping as the consequence of interactions of the quantum system with the environment. They describe decoherence as the loss of information from a quantum system to its environment. To be sure, maintaining coherence (for example the phase information between states in a superposition of states that produces wave-like interference effects) is an essential part of the time evolution of a quantum system according to the Schrödinger wave equation. Coherent time evolution is an idealization that is only approximately realizable, in a system that is nearly isolated from its environment. But without the projection into a single state (the "collapse" into an "eigenstate" with an observable "eigenvalue") there would be no particle-like behavior. Decoherence theorists replace the "collapse" with the loss of information. This is instructive, because every interaction of a quantum system with other systems (either quantum or approximately classical) can result in either a loss of information (a gain in positive entropy) or a gain in information locally (at the expense of even greater positive entropy transferred away from the quantum system and the local environment. The problem of measurement is best analyzed as a gain in information. The new knowledge acquired by the observer must first be newly created information that is a stable enough record to be available for observation. This requires an irreversible thermodynamic process and decoherence theorists agree that the dynamics of the quantum system (viewed in isolation from the environment) are non-unitary and irreversible. But they demur from an attempted explanation of measurement and they deny that decoherence generates an actual wave function collapse. Decoherence only provides an explanation for the "observance" of wave function collapse. They replace collapse with the "leakage" of information into the environment as components of the wave function are decoupled from a coherent system, and acquire new phases from their immediate surroundings. Decoherence theorists believe that the total superposition of a global or "universal" wavefunction follows a unitary time evolution according to the Schrödinger equation, which commits them to something like Everett's "many-worlds" or Zeh's "many-minds" interpretations of quantum mechanics. Zurek's insight about the importance of information is very powerful. As he wrote in his 2003 revisited version of the 1991 foundational paper on decoherence in Physics Today:
...if there is one lesson to be learned from what we already know about such matters, it is that information and its transfer play a key role in the quantum universe. The natural sciences were built on a tacit assumption: Information about the universe can be acquired without changing its state. The ideal of “hard science” was to be objective and provide a description of reality. Information was regarded as unphysical, ethereal, a mere record of the tangible, material universe, an inconsequential reflection, existing beyond and essentially decoupled from the domain governed by the laws of physics. This view is no longer tenable (Landauer 1991). Quantum theory has put an end to this Laplacean dream about a mechanical universe. Observers of quantum phenomena can no longer be just passive spectators. Quantum laws make it impossible to gain information without changing the state of the measured object. The dividing line between what is and what is known to be has been blurred forever. While abolishing this boundary, quantum theory has simultaneously deprived the “conscious observer” of a monopoly on acquiring and storing information: Any correlation is a registration, any quantum state is a record of some other quantum state. When correlations are robust enough, or the record is sufficiently indelible, familiar classical “objective reality” emerges from the quantum substrate. Moreover, even a minute interaction with the environment, practically inevitable for any macroscopic object, will establish such a correlation: The environment will, in effect, measure the state of the object, and this suffices to destroy quantum coherence. The resulting decoherence plays, therefore, a vital role in facilitating the transition from quantum to classical.
ReferencesDecoherence and the Transition from Quantum to Classical—Revisited Quantum Darwinism, Nature Physics, vol. 5, pp. 181-188 (2009)