The formation of an interference pattern when a beam of particles passes through a double slit is the classic signature of the wave-particle duality of quantum particles. Wave-particle duality has been observed with electrons, atoms and small molecules. Now Markus Arndt, Anton Zeilinger and co-workers at the University of Vienna in Austria have observed wave-like behaviour in a beam of carbon-60 molecules - which are an order of magnitude larger than any other particles for which quantum interference effects have been observed (M Arndt et al. 1999 Nature 401 680).
The Vienna team sent a collimated beam of carbon-60 molecules through a slit made of silicon nitride and detected the interference pattern by ionizing the molecules with a laser and then counting the ions. The slits in the diffraction grating were 50 nanometres wide and the grating had a period of 100 nanometres. The team detected the central maximum and the two first-order diffraction peaks in the interference pattern. The molecules had a most probable velocity of 220 metres per second, which corresponds to a de Broglie wavelength of 2.5 picometres (2.5×10-12 metres) – some 400 times smaller than the diameter of the molecules. They also observed wave-particle duality in carbon-60 molecules that contained one or two atoms of 13C, the heavy isotope of carbon.
One of the deepest mysteries of quantum mechanics is that an interference pattern is formed even if there is only one particle in the experimental set-up at any given time. The Vienna team write that “all these observations support the view that each carbon-60 molecule interferes with itself only.” They also confirmed that the interactions of the molecules with their environment – such as the spontaneous emission of photons by the thermally excited molecules – could not reveal which slit they had passed through. Even the mere possibility of being able to know which slit the particle passes through would be enough to wipe out the interference pattern.
Another mystery in physics is the length scale at which quantum behaviour breaks down. The carbon-60 molecules in the Vienna experiment are the largest objects ever to have shown quantum behaviour, but they are still 15 orders of magnitude smaller than true macroscopic objects. In the quest to establish when and how quantum mechanics breaks down and classical physics takes over, Arndt and co-workers plan to repeat their experiments with larger macromolecules and possibly even viruses.