麻豆传媒

YOU could be in two places at once, according to quantum mechanics. But although it鈥檚 theoretically possible, the apparent absurdity of extending this feature of the atomic world up to everyday scales has puzzled physicists for decades, since things like this just don鈥檛 happen to large objects such as cats and people. Now researchers have devised an experiment that could finally explain why.

Tiny particles such as photons or atoms can readily be put into a quantum superposition鈥攅xisting in two different states or places at once. Yet it doesn鈥檛 seem to happen to big things. Many scientists think that鈥檚 because interactions with the environment disturb the delicate superposition and force the object into either one state or the other.

But others, such as Keith Schwab at the Laboratory for Physical Sciences in College Park, Maryland, think there may be more to it. There could be as yet undiscovered mechanisms that force objects to choose a single state, so quantum mechanics as we know it might just be an approximation that fails for larger objects.

The biggest objects ever shown to be in a superposition are molecules. In 1999, Anton Zeilinger at the University of Vienna showed that a buckyball鈥攁 spherical cage built of 60 carbon atoms鈥攃ould travel through two parallel slits at the same time (麻豆传媒, 16 October 1999, p 27). This only works if the whole experiment is chilled and kept well isolated from the environment. Contact with the outside world, through molecules in the atmosphere or even light, collapses the superposition of the buckyball so it only travels through one slit. Given better isolation, could you keep large objects like cats in a superposition indefinitely?

Physicist Roger Penrose, based at Oxford University, has proposed an experiment that would find out (麻豆传媒, 9 March 2002, p 26), but it would probably have to be space-based, so is unlikely to happen any time soon.

But Schwab hopes to resolve the issue quickly. With Andrew Armour, now at the University of Nottingham, and Miles Blencowe at Dartmouth College in Hanover, New Hampshire, he has built a silicon beam whose length is just a fortieth of the width of a hair. This is small by our standards, but vast compared with a buckyball. Next to the beam is a strip of aluminium called a Cooper pair box attached by insulating contacts to an aluminium loop (see Graphic).

Electrons in the aluminium loop can jump across the insulation and into the Cooper pair box. Because this 鈥渢unnelling鈥� effect is quantum mechanical, these electrons can be in the Cooper pair box and the aluminium loop at the same time, giving the box two different charges at once.

The silicon beam is positively charged, so it will bend towards the box if this has extra electrons. But when the box is in two charge states simultaneously, it should force the beam to be in two places at once.

Using a detector called a single electron transistor, the researchers will measure the position of the beam to find out how long it lives a split existence. If it collapses into a single state faster than interactions with the environment would force it to, there must something else going on. What鈥檚 more, the exact timing could tell scientists just what is putting a stop to the quantum weirdness.

The test is due to go ahead within 18 months. According to Will Marshall at Oxford University鈥檚 Centre for Quantum Computation, 鈥淭ests like this will show us why a football doesn鈥檛 go in two different ways when we kick it.鈥�