Huge molecules can show the wave-particle duality of quantum theory.
Researchers in Austria have made what they call the "fattest Schrödinger cats realized to date". They have demonstrated quantum superposition – in which an object exists in two or more states simultaneously – for molecules composed of up to 430 atoms each, several times larger than molecules used in previous such experiments1.
In the famous thought experiment conceived by Erwin Schrödinger in 1935 to illustrate the apparent paradoxes of quantum theory, a cat would be poisoned or not depending on the state of an atom — the atom's state being governed by quantum rules. Because quantum theory required that these rules allowed superpositions, it seemed that Schrödinger's cat could itself exist in a superposition of 'live' and 'dead' states.
The paradox highlights the question of how and when the rules of the quantum world – in which objects such as atoms can exist in several positions at once – give way to the 'classical' mechanics that governs the macroscopic world of our everyday experience, where things must be one way or the other but not both at the same time. This is called the quantum-to-classical transition.
It is now generally thought that 'quantumness' is lost in a process called decoherence, in which disturbances in the immediate environment make the quantum wavefunction describing many-state superpositions appear to collapse into a well-defined, unique classical state. This decoherence tends to become more pronounced the bigger the object, as the opportunities for interacting with the environment increase.
One manifestation of quantum superposition is the interference that can occur between quantum particles passing through two or more narrow slits. In the classical world the particles pass through with their trajectories unchanged, like footballs rolling through a doorway.
But quantum particles can behave like waves, which interfere with one another as they pass through the slits, either enhancing or cancelling each other out to produce a series of bright and dark bands. This interference of quantum particles, first seen for electrons in 1927, is effectively the result of each particle passing through more than one slit: a quantum superposition.
As the experiment is scaled up in size, at some point quantum behaviour (interference) should give way to classical behaviour (no interference). But how big can the particles be before that happens?
In 1999, a team at the University of Vienna demonstrated interference in a many-slit experiment using beams of 60-atom carbon molecules (C60), which are shaped like hollow spheres2. Now Markus Arndt, one of the researchers involved in that experiment, and his colleagues in Austria, Germany, the United States and Switzerland have shown much the same effect for considerably larger molecules tailor-made for the purpose — up to 6 nanometres (millionths of a millimetre) across and composed of up to 430 atoms. These are bigger than some small protein molecules, such as insulin.
In the team's experiment, the beams of molecules are passed through three sets of slits. The first slit, made from a slice of silicon nitride patterned with a grating consisting of slits 90 nanometres wide, forces the molecular beam into a coherent state, in which the matter waves are all in step. The second, a 'virtual grating' made from laser light formed by mirrors into a standing wave of light and dark, causes the interference pattern. The third grating, also of silicon nitride, acts as a mask to admit parts of the interference pattern to a quadrupole mass spectrometer, which counts the number of molecules that pass through.
The researchers report in Nature Communications today that this number rises and falls periodically as the outgoing beam is scanned from left to right, showing that interference, and therefore superposition, is present.
Although this might not sound like a Schrödinger cat experiment, it probes the same quantum effects. It is essentially like firing the cats themselves at the interference grating, rather than making a single cat's fate contingent on an atomic-scale event.
Quantum physicist Martin Plenio of the University of Ulm in Germany calls the study part of an important line of research. "We have perhaps not gained deep new insights into the nature of quantum superposition from this specific experiment," he admits, "but there is hope that with increasing refinement of the experimental technique we will eventually discover something new."
Arndt says that such experiments might eventually allow tests of fundamental aspects of quantum theory, such as how wavefunctions collapse under observation. "Predictions, such as that gravity might induce wavefunction collapse beyond a certain mass limit, should become testable at significantly higher masses in far-future experiments," he says.
Can living organisms – perhaps not cats, but microorganisms such as bacteria – be placed in superpositions? That has been proposed for viruses3, the smallest of which are just a few nanometres across – although there is no consensus about whether viruses should be considered truly alive. "Tailored molecules are much easier than viruses to handle in such experiments," says Arndt. But he adds that if various technical issues can be addressed, "I don't see why it should not work".
Gerlich, S. et al. Nature Commun. doi:10.1038/ncomms1263 (2011).
Arndt, M. et al. Nature 401, 680-682 (1999).
Romero-Isart, O., Juan, M. L., Quidant, R. & Cirac, J. I. New J. Phys. 12, 033015 (2010).
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Ball, P. Fattening up Schrödinger's cat. Nature (2011). https://doi.org/10.1038/news.2011.210