The Copenhagen interpretation which describes the role of wave packet collapse during measurement of a quantum system, was designed to convert quantum states and correlations into classical, definite outcomes. We have seen how this can be a philosophically unsavory concept to many physicists. Max Tegmark and John Archibald Wheeler wrote an overview of the foundations of quantum mechanics in the article entitled “100 Years of Quantum Mysteries” in the February 2001 issue of Scientific American where they remark “It was embarrassing that nobody had managed to provide a testable deterministic equation specifying precisely when this mysterious collapse was supposed to occur”.
In the early 1970s H. Dieter Zeh introduced the theory of quantum decoherence which provided a way around collapsing wave packets and enabled one to straightforwardly draw an effective border between the quantum and the classical realms.
Quantum decoherence is caused by the coupling between a quantum system in a superposition with the environment in which it is embedded. This coupling destroys the superposition and causes the system to ‘disassemble’ or decay over time into one state or another. Decoherence can be thought as a form of “quantum noise” whose role is to force a particle out of superposition—denying it of the special property that makes it so useful especially for quantum computers—the particle’s ability to be in multiple states simultaneously. It is the phenomenon that causes a particle to be in either one state or another instead of being in multiple states concurrently.
The ability to reside in multiple states enables a quantum system to exhibit interference. Interference is inherent and crucial aspect of quantum systems, famously demonstrated by the two-slit experiment. However, there are situations in which interference effects are artificially or spontaneously suppressed. Decoherence is precisely the cause of such suppression due to spontaneous interactions between a system and its environment.
Note, macroscopic systems are normally never isolated from their environments. Zeh therefore emphasized that macroscopic (classical) systems should not be expected to follow Schrödinger’s equation, which is applicable only to a closed system. Classical systems are subject to the natural loss of quantum coherence, which “leaks out” into the environment (Zurek 1981, 1982). The decoherence of a macroscopic object induced by the scattering with environmental particles has been found to be astonishingly strong and fast.
Similarly, in quantum mechanical systems in superposition, the interaction between the environmental particles and the superposed target body cause them to be entangled, thereby delocalizing the phase coherence from the target body to the whole system, rendering the interference pattern unobservable and thus diminishing the degree of “quantumness” in the target body.
In the recent years, decoherence has been studied extensively in many experiments. Scientists were able to observe the gradual setting of decoherence in quantum mechanical systems in superposition with its surrounding environment, exploring its step-by-step transition from the quantum to classical domain. In one experiment, large fullerene molecules were sent through diffraction gratings in an effort to observe the quantum interference patterns. The interference signal decayed gradually as the density of the surrounding gas molecules (thus the rate of scattering events between the fullerenes and the environmental particles) was increased. The characteristic time of such decoherence processes was measured with great precision and has been found to be in spectacular agreement with the theoretical predictions. This, and other remarkable experiments that followed, allowed physicists to study different qualitative degrees of decoherence for the first time and measure how the continuous interaction with the environment gradually degrades quantum phenomena in the microscopic and macroscopic domains.
Decoherence is a ubiquitous and natural phenomenon. Its theory is not extraneous or distinct from the standard framework of quantum mechanics that explains the interaction between a system and its environment. But it has far reaching consequences and, as we will see later in this article, must be taken into consideration when designing practical systems for harnessing the power of quantum mechanics.