Of Buckey Balls and SQUIDs: Observing Decoherence in Action

Abstract

Until the mid-1990s, decoherence was mainly studied through theoretical models. As described in Chap. 3, in 1985 Joos and Zeh [7] showed that spatial superposition states of even minuscule objects, such as dust grains or large molecules, are rapidly decohered by the scattering of only minimal environments. Furthermore, the rule-of-thumb expression (2.113) for spatial decoher-ence rates derived by Zurek in 1984 [12] suggested that on macroscopic scales decoherence would be overwhelmingly more rapid than dissipation. These results led to the common notion that decoherence is extremely efficient and fast, and to the general prediction that any nonclassical superposition in the everyday world would be immediately decohered. That much was known (or at least suspected) in the 1980s. What was sorely missing, though, was some kind of experiment that would demonstrate the dynamics of decoherence, by showing how superposition states becomes gradually unobservable due to the action of decoherence. Instead of simply stating that decoherence is so strong and rapid as to preclude the observation of Schrödinger-cat states at all length scales relevant to the world of our experience , such experiments would enable us to see directly how this quantum-to-classical transition happens. We could observe how the smooth action of decoherence carries away quantum features and shuttles our object of interest safely into the classical domain-and maybe even how quantum coherence can subsequently be restored, thus showing that the quantum-to-classical transition really is a two-way process. We could change the dynamics and properties of this process by manipulating experimental parameters, thereby challenging the Copenhagen view of a fundamental quantum-classical boundary (see Sect. 8.1). Finally, we could test our theoretical models for decoher-ence to find out how much they really capture the effects of environmental interactions in an actual experimental setting. Now that readers have hopefully been convinced of how useful and exciting such experiments would be, let us look at the difficulties that we immediately face in realizing experiments on decoherence. What do we need to do in order to observe the gradual action of decoherence? First, we ought to be able to prepare a system in a nonclassical superposition state. Of course, the system should have some minimal "size" to be sufficiently susceptible to decoherence-it should not just be a single electron, but preferably some