Tuesday, April 05, 2011

Fattening up Schrödinger's cats

Here’s my latest story for Nature News.

Huge molecules can show the wave-particle duality of quantum theory.

Researchers in Austria have made what they call the “fattest Schrödiner cats realized to date”. They have demonstrated quantum superpositions – objects in two or more states simultaneously – of molecules with up to 430 atoms each, several times larger than those used in previous experiments of this sort [1].

In the famous thought experiment conceived by Erwin Schrödinger in 1935 to illustrate the apparent paradoxes of quantum theory, a cat will be poisoned or not depending on the state of an atom, governed by quantum rules. Because the recently developed quantum theory insisted that these rules allowed for superpositions, it seemed that Schrödinger’s cat could itself be placed in a superposition of ‘live’ and ‘dead’ states.

The paradox highlights the question of how the rules of the quantum world – where objects like atoms can be in several positions at once – give way to the ‘classical’ mechanics that governs the macroscopic world of our everyday experience, in which 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 the ‘quantumness’ is lost in a process called decoherence, where disturbances from the surrounding environment make the quantum wavefunction describing many-state superpositions appear to collapse [note to subs: we have to keep this ‘appear to’. The precise relationship between decoherence and wavefunction collapse is complicated and too tricky fully get into here] into a well-defined and unique classical state. This decoherence tends to become more pronounced as objects get bigger and the opportunities for interacting with the environment multiply.

There is still no consensus on how Schrödinger’s thought experiment will play out if the cat-and-atom system could be perfectly protected from decoherence. Some physicists are happy to believe that in that case the cat could indeed be in a live-dead superposition. But we couldn’t see it directly because the act of looking would destroy the superposition.

One manifestation of quantum superpositions is the interference that can occur between quantum particles passing through two or more narrow slits. In the classical world the particles just 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 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 particles passing through more than one slit: a quantum superposition.

At some point as the experiment is scaled up in size, 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 in Austria demonstrated interference in a many-slit experiment using beams of 60-atom carbon molecules (C60) shaped like hollow spheres [2]. Now Markus Arndt, one of the researchers in that experiment, and his colleagues in Austria, Germany 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 in the body, such as insulin.

In their experiment, the beams of molecules are passed through three sets of slits. The first of them, made from a slice of the hard material silicon nitride patterned with a grating of 90-nm-wide slits, prepares the molecular beam in 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 inference pattern. The third grating, also of silicon nitride, acts as a mask to admit parts of the interference pattern to an instrument called a mass spectrometer, which counts the number of molecules that pass through.

The researchers report in Nature Communications 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 enable tests of fundamental aspects of quantum theory, such as how wavefunctions are collapsed by 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 maybe microscopic ones such as bacteria – be placed in superpositions? That has been proposed for viruses [3], 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 to handle in such experiments than viruses”, says Arndt. But he adds that if various technical issues can be addressed, “I don’t see why it should not work.”

1. Gerlich, S. et al., Nat. Commun. online publication doi:10.1038/ncomms1263.
2. Arndt, M. et al., Nature 401, 680-682 (1999).
3. Romero-Isart, O., Juan, M. L., Quidant, R. & Cirac, J. I. New J. Phys. 12, 033105 (2010).

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