Quantum square dance: Atoms are loaded into individual sites of a 3-D grid of lasers. Initially the atoms have the same "spin," as indicated by their consistent color. Then, a radio-frequency field is applied to flip the spins of atoms in every other site, and the sites are paired up, with one atom of each pair spin up (or 1) and the other spin down (or 0), as indicated by the two colors. Then, pairs are merged, which causes the atom partners to swap spins repeatedly. These oscillations have the effect of periodically "entangling" the pairs, a quantum phenomenon that links their properties even if they are separated.

Credit: NIST/Troy Porto

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SYDNEY: A novel method for making the bits needed to process information in quantum computers has been unveiled by scientists, taking us one step closer to developing far more powerful supercomputers.

The experiment, by physicists at the U.S. National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, used paired atoms that swap their electron spin states in the equivalent of a quantum square dance as a way to create logical connections among data – the starting point for computing.

The researchers, led by Nobel laureate William Phillips, announced their result in the British journal Nature this week.

Entangled atoms

Quantum computers use special properties of atoms whereby they become 'entangled' so that the state of one atom is linked to the state of the other in an entangled pair. This property can be used to create 'qubits', units of quantum information similar to the 'bits' of data used by ordinary computers.

Quantum computers could quickly solve problems such as breaking encryption codes, which would take today's best supercomputers years to complete.

The researchers used neutrally charged atoms trapped by lasers to create the conditions where paired atoms could exchange their internal spin states. Spin states can be measured as either up or down, representing 1 or 0 in the binary language of computers.

In the experiment, thousands of atoms were forced into the same quantum state, a condition known as a Bose-Einstein condensate. In this state, particular atoms chilled close to absolute zero (as cold as it gets in the universe) act in symmetry; such that their spin state and spatial position are linked. The researchers then swapped every second atom's spin state and paired atoms with opposing spin states in energy wells created by lasers. The lasers formed a lattice trapping the atoms.

This is the first time scientists have been able to pair up atoms in this way, bringing them one step closer to creating a system for quantum information processing.

"New Lines of attack"

"This is the first time these spin-entangling interactions have been demonstrated between pairs of atoms in an optical lattice," said NIST team member Trey Porto. "Other research groups have entangled atoms in lattices as extended clusters. By isolating pairs, we can focus on the simplest units for quantum logic."

Neutrally charged atoms are one of a dozen systems being evaluated by scientists for qubits. Previous research has also looked at using photons simultaneously fired into a beamsplitter or coupled electron spins in a solid state 'quantum dot'; a layered nanostructure consisting of semiconducting and quantum layers. The advantage of the paired neutral atom system is that it barely interacts with the environment, which could make it particularly robust and less likely to lose quantum information through interference.

In a Nature commentary accompanying the article Johannes Denschlag, from the Institute of Physics at the University of Innsbruck in Austria, said scientists are still a long way off developing a working quantum computer, but that the research represents a paradigm shift in schemes for controlling qubit interactions.

"New ideas such as making use of fundamental symmetries [within the atoms] could ultimately provide new lines of attack," he said.