By Dr David Whitehouse
BBC News Online science editor
Scientists have "entangled" two sub-atomic particles separated by about a millimetre, a feat that might pave the way for powerful quantum computers in the future.
Quantum entanglement on a chip: Significant scaling up
When two particles are entangled they are somehow connected because the fate of one depends upon the other, no matter how far apart they are.
Sounds weird - even Einstein never quite came to terms with it.
Entangled particles may be useful to make logic circuits for computers that have a far greater capacity and speed than today's machines.
Published in the journal Science, the results represent the latest advance in a broad scientific effort to apply properties of quantum physics to the creation of a new generation of supercomputers.
Quantum entanglement (QE) describes the situation when the fates of two or more particles become bound together.
A change in one entangled particle results in an instant change in the other particle, no matter how far away it is - even if it is at the opposite end of the Universe.
Of course, it is not quite a simple as that. The phenomenon is linked in a deep way to the fundamental properties of matter and the nature of observing and measuring reality.
It really bothered Einstein, especially since the concept that a change in one particle was somehow communicated to the other faster than the speed of light - nature's ultimate speed limit.
He called it "spooky action at a distance".
He believed there was something profound in the phenomenon and that buried in its seeming absurdity lay something that could overthrow quantum mechanics - a successful theory of the way the Universe behaves on the atomic and sub-atomic level.
Einstein was not alive when, in the 1970's, physicist Alan Aspect carried out an experiment that showed QE was real and could form the basis for the computers, not of tomorrow, but of the day after.
Today's computers are limited by the speed of electrons as they move around integrated circuits.
In the future, computers that use light instead of electrons will be faster. But, in a way, even lightspeed may be supplanted because of the curious properties of entanglement.
According to some, a computer based on quantum entanglement would not be bound by those limits as it would use "spooky action at a distance" instead of electrons or photons.
A quantum computer would have to "entangle" quantum bits - or qubits - over significant distances. However, particle entanglement has only been observed on the micrometre (millionth of a metre) scale so far.
Now, Andrew Berkley and colleagues from the University of Maryland, US, have entangled two qubits inside a silicon chip over a distance of 0.7 millimetres - a thousand times greater.
A millimetre does not sound like the opposite ends of the Universe but it is significantly closer to the scale needed to build quantum mechanical computer components.
"Entanglement is essential to quantum computing because it is the linked quality that builds more information into quantum bits than is possible with classical computing bits," says Andrew Berkley.
"Our current findings, which build on the work of many others, moves us further along the road towards a quantum computer," he adds.