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An ambitious experiment to make a glass sphere exist in two places at once could provide the most sensitive test of quantum theory yet. The experiment will place a sphere containing millions of atoms – making it larger than many viruses – into a superposition of states in different places, say researchers in Europe.

Physicists have questioned whether large objects can follow quantum laws ever since Erwin Schrödinger’s thought-experiment suggested a cat could exist in a superposition of being both alive and dead.

The idea is to zap a glass sphere 40 nanometres in diameter with a laser while it is inside a small cavity. This should force the sphere to bounce from one side of the cavity to the other. But since the light is quantum in nature, so too will be the position of the sphere. This forces it into a quantum superposition.


The experiment will have to be carried out in high vacuum and at extremely low temperatures so that the sphere is not disturbed by thermal noise or air molecules, says lead author Oriol Romero-Isart from the Max Planck Institute of Quantum Optics in Garching, Germany.

No overlap

Last year Aaron O’Connell and colleagues at the University of California, Santa Barbara, demonstrated that it should be possible to create superpositions in a 60-micrometre-long metal strip. However, the physical separation associated with the two different states of the strip was only 1 femtometre, about the width of the nucleus of an atom.

The new experiment, in contrast, would put the glass sphere in two entirely distinct places at once, with no overlap. “In our proposal the centre of mass is put into a superposition of spatial locations separated by a distance larger than the size of the object,” Romero-Isart says.

Atom interferometer experiments have previously achieved good separation, putting fullerene and other molecules containing up to a few hundred atoms into distinct superposition states, but the new scheme will do this with truly macroscopic objects.

This will be particularly valuable in providing tests for quantum mechanics, the researchers say. Observing the behaviour of such very large objects obeying quantum laws offers our best hope of finding ways in which quantum theory breaks down.

The Romero-Isart experiment would take us “substantially beyond the current state of the art”, says Anthony Leggett of the University of Illinois at Urbana-Champaign. “Neither the fullerene experiments nor that of O’Connell and his team are able to test well-developed competitors to quantum mechanics.”

Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.107.020405