By Jason Palmer
Science and technology reporter, BBC News
The "entangled" ions were held in a tiny cavity on an electrode
Scientists have "entangled" the motions of pairs of atoms for the first time.
Entanglement is an effect in quantum mechanics, a relatively new branch of physics that is based more in probability than in classical laws.
It describes how properties of two or more objects can be inextricably linked over "vast" distances.
The results, published in Nature, further bridge the gap between the world of quantum mechanics and the laws of everyday experience.
This is the first time entanglement has been seen in a so-called "mechanical system".
The phenomenon suggests that a measurement performed on one object can affect the measurement on another object some distance away.
It also implies that the behaviour of two separate objects is linked by some unseen connection - an idea that Albert Einstein described as "spooky".
Entanglement could be exploited in future quantum computers, because the inherent probability-based nature of quantum systems means they can compute certain kinds of problems significantly more quickly than current "classical" computers.
The delicate effect has until now been limited to the internal properties of tiny systems - ethereal connections such as the polarisations of a pair of light packets called photons, or the spins of electrons in atoms.
In 2005, clouds of eight atoms were shown to be completely entangled by a group at the Institute for Quantum Optics and Quantum Information in Austria (IQOQI).
However, a pressing question for quantum researchers is when - or if - these spooky effects stop as the number of entangled photons or atoms grows.
Does entanglement exist outside complicated laser laboratories?
"In the scientific community there isn't really a clear answer as to why we don't see entanglement or its effects in our everyday life," said John Jost, a researcher at the National Institute for Standards and Technology (Nist) in the US.
To begin to address that, Mr Jost and his colleagues developed a means to entangle the actual motions of two pairs of atoms: a more tangible and visual property of a system than electron spins and photon polarisations.
"What we wanted to do was to perform this entanglement in the sort of system that people can relate to, a mechanical system that pervades nature everywhere: a vibrating violin string, the pendulum on a clock, the quartz crystal in your digital watch," Mr Jost told BBC News.
The intertwining involved four electrically charged atoms, or ions - two beryllium and two magnesium ions. These are prepared in a device called an ion trap that uses electric fields to manipulate the charged particles.
The positively charged ions repel one other, and behave as if they are connected by a spring. This "spring" has a natural resonant frequency, just like a pendulum, which can be excited with the "kick" of a laser of just the right colour.
First, a laser is used to entangle the internal energy states - the "spins" - of the two beryllium ions.
The four ions are then separated into two pairs, each made up of a beryllium and a magnesium ion four micrometres apart. The pairs themselves are separated by 240 micrometres - just a few hairs' breadths, but an enormous distance in the atomic world.
The magnesium ions are cooled with lasers, which in turn removes excess energy from the beryllium ions.
Further laser pulses then provide an energetic "kick" to ensure the beryllium ions are no longer entangled via their spin states, but are now entangled via their motions.
The entangled pairs move in perfect unison despite their separation distance.
The entangled spins become entangled vibrations using laser pulses
The work closes some of the gap between two directions of research that investigate where the quantum world ends and our everyday, classical world begins.
"We're using a bottom-up approach where you start with a very simple mechanical system; you can imagine that adding more and more ions to this, you could scale it up," Mr Jost explained.
"But there's a whole field of research in so-called nano-mechanical resonators: they're taking the top-down approach, trying to use a tiny beam of atoms - still composed of millions of atoms - and cooling it down until they see these quantum mechanical effects."
IQOQI researcher Christian Roos said: "There is certainly an interest to see two objects in a different kind of entanglement than the one that has been investigated so far" .
"At the moment it's pure curiosity, to see how far it can go," he added.
Nothing in quantum mechanics precludes entanglements of larger numbers of atoms, but as the bottom-up and top-down pursuits meet in the middle, researchers might discover there is more to quantum mechanics than they currently understand.
"There are theories that there are mechanisms that are not yet understood that prevent macroscopic systems becoming entangled once they become more massive," Dr Roos told BBC News.
"So from that point of view it's certainly interesting to see entanglement at a very small scale, and then to see whether it is possible to entangle heavier objects."