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A quantum analogue of the popular "Newton's cradle" toy has been proposed by a duo of physicists in Italy. Like momentum transferred in the toy, the team argues that it should be possible to achieve the nearly perfect transmission of a quantum wavefunction along a line of ultracold atoms in a 1D Bose–Einstein condensate. According to the pair, the work could help develop quantum-information systems that achieve high-quality wave transmission.

Best described as an "executive desk toy", a Newton's cradle is a device that demonstrates the conservation of energy and momentum. It consists of small, identically sized metal balls suspended from two horizontal bars. When a ball is lifted and allowed to fall, it strikes its immediate neighbour and comes to a stop. But its momentum is transferred across the rest of the spheres and propels only the sphere at the opposite end of the line into the air.

Roberto Franzosi of the Quantum Science and Technology Institute in Arcetri and the National Institute of Optics, National Research Council, along with colleague Ruggero Vaia from the National Institute for Complex Systems, National Research Council in Florence, became interested in the quantum Newton's cradle as part of their work on the unusual dynamics of locally perturbed quantum states. Franzosi points out that they based their design on ultracold atoms because it is one of the most versatile quantum systems from an experimental point of view. "Cold atoms are considered an excellent test bench for a huge variety of quantum phenomena," says Franzosi, explaining that "with a suitable set-up, it is possible to realize a [quasi]-perfect transmission of the quantum wavefunction."

Franzosi's proposal is not the first though – a similar experiment was done in 2006 by a separate group of researchers that also attempted to create a quantum Newton's cradle. Franzosi says that although that experiment was "a fascinating demonstration of how coherent states are the quantum analogue of classical particles", it still did not represent a perfect analogue. That is because it was not a 1D system made up of individual quantum systems (an analogue of the rigid spheres) with nearest-neighbour interaction between them (the analogue of collisions between the spheres).

To create a perfect analogue, Franzosi and Vaia propose beginning with a Bose–Einstein condensate of two atomic species – that is, atoms in two excited states. The atoms are trapped in a 1D tube that has a longitudinal optical lattice running through it. Franzosi says that such a confinement can be created using counter-propagating laser beams that form standing waves to trap the atoms, thus developing two macroscopically populated coherent states.

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