little levers to probe the quantum divide

QUANTUM mechanics describes the world of electrons and atoms, but not everyday objects. What if we could probe the boundaries of this quantum-classical divide by "entangling" two microscopic levers?

Proving that bigger and bigger objects obey the same quantum rules is a huge challenge. Although the cantilevers would be only tens of micrometres long, they would constitute a bigger and more complex system than any yet shown to exhibit quantum behaviour. The larger an object, the more easily its quantum state is destroyed.

In March, Aaron O'Connell and colleagues at the University of California, Santa Barbara, showed that a microscopic cantilever made of aluminium nitride can be simultaneously still and vibrating, a sign that it is in a quantum state.

Now Erika Andersson, Patrik Öhberg and Chaitanya Joshi of Heriot-Watt University in Edinburgh, UK, and colleagues have a method for entangling two of these microscopic cantilevers.

When two or more objects exist in a "superposition" of linked quantum states, they are said to be entangled. The researchers suggest putting two cantilevers, each with a tiny magnet at its tip, near an ultra-cold ensemble of atoms called a Bose-Einstein condensate. In a BEC all the atoms share the same quantum state. Vibrating one cantilever creates a magnetic field, which causes the atoms in the BEC to move. This motion in turn causes the magnets at the tip of the other cantilever to move, causing the cantilever to vibrate. Under such a system, the two levers should become entangled.

Couple each cantilever to a tiny superconducting coil and the electrical read-out will tell you if their states are correlated more often than classical physics predicts (arxiv.org/abs/1006.4036). "The main challenge is to read out the entanglement," Andersson says.

Magnetic levers could enter a quantum state - the main challenge is to read out the entanglement

David Hunger at the Ludwig-Maximilians University in Munich, Germany, says this work is "the next step" in testing the boundary of quantum behaviour. There are technical challenges to overcome, though. For example, the tip of the cantilever has to be within 250 nanometres of the BEC. "It is not at all trivial to get that close."

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