Making Decoherence Visible

Anton Zeilinger and his colleagues at the University of Vienna are experts at making largish objects, such as carbon-70 molecules, appear wavelike rather than particulate. They can, for instance, send a gentle C-70 beam toward a grating where, behaving as if they were waves analogous to light waves, the molecules scatter in such a way as to register in detectors farther downstream in a characteristic interference pattern (see Update 579). Now these physicists have used the same basic setup to study how decoherence comes about.

Decoherence, a hot topic in physics, is the process by which quantum objects (in this case C-70 molecules, acting as waves) lose their wavelike integrity by interacting with the surrounding environment. Decoherence is what stands between the classical (bowling ball) world and the quantum (wave interference) world, and understanding how it arises will be valuable if we are every going to exploit quantum weirdness to perform future feats of quantum computation or convey secure pieces of quantum information.

In their new experiment the Vienna researchers recorded the interference pattern several times with a variety of C-70 beams. Each of the beams differed in "temperature," corresponding to the amount of laser light used to impart an internal agitation to the molecule's atomic constituents.

One would expect that the warmer molecules, radiating away their thermal excess in the form of photons, would be in closer contact with their environment than the cooler molecules, and would thus be more vulnerable to losing the precious isolation needed for retaining quantum coherence.

A consequence of this would be for the cool beams to show a sharper interference pattern than the warmer beams, and this proved to be the case. The succession of patterns, corresponding to beams from cold to hot, exhibited a steady shedding of their quantal persona.

In effect, decoherence was being made visible through the emission of heat. This demonstration speaks to the fact that in our warm world we don't generally observe quantum interference effects in commonplace events. (Hackermuller et al., Nature, 19 February 2004.)