Looking at decoherence
Feb 18, 2004
Physicists in Austria have observed decoherence - the transition from quantum to classical behaviour - in carbon-70 molecules. At temperatures below 1000 Kelvin the molecules demonstrate quantum behaviour when they pass through a double slit. However, the molecules gradually become classical at higher temperatures, and the interference pattern – which is the classic sign of quantum behaviour - becomes weaker. Markus Arndt, Anton Zeilinger and co-workers at the University of Vienna in Austria have shown that the decoherence is caused by the thermal emission of photons from the molecules (L Hackermüller et al. 2004 Nature 427 711)
Researchers have seen quantum interference effects in electrons, atoms and small molecules but never in macroscopic objects. In 1999, the Vienna group observed wave properties in carbon-60 and carbon-70 molecules. With a diameter of about 1 nanometre, these were the biggest objects to have shown quantum interference at the time. Since then, the team has observed wave properties in larger molecules such as tetraphenylporphyrin. This molecule, which is present in chlorophyll and haemoglobin, has a diameter of about 2 nanometres.
Arndt and co-workers first sent a beam of carbon-70 molecules through a laser system that heated them to around 5000 Kelvin. As the molecules then cooled down by emitting photons, they were passed through an interferometer that contained three sets of diffraction gratings. The first grating produced a coherent beam of molecules, the second created the interference pattern, and the third imaged this pattern. The slits in the gratings were about 500 nanometres wide and the grating had a period of around 1000 nanometres.
The Vienna physicists found that below about 1000 Kelvin, a high-quality interference fringe pattern characteristic of quantum behaviour could be seen. However, these patterns gradually vanished as the molecules were heated and started to emit thermal radiation. Since these photons could, in principle, be detected to reveal which slit the molecule has passed through, the wave-like quantum behaviour of the molecules disappears. The group now hopes to observe the effect of decoherence in even larger molecules, such as proteins.
About the author
Belle Dumé is Science Writer at PhysicsWeb