Physicists at the National Institute of Standards and Technology (NIST) have induced thousands of atoms trapped by laser beams to swap "spins" with partners simultaneously. The repeated exchanges, like a quantum version of swinging your partner in a square dance but lasting a total of just 10 milliseconds, might someday carry out logic operations in quantum computers, which theoretically could quickly solve certain problems that today's best supercomputers could not solve in years.
The atomic dance, described in the July 26 issue of Nature, advances prospects for the use of neutral atoms as quantum bits (qubits) for storing and processing data in quantum computers. Thanks to the peculiarities of quantum mechanics, nature's rule book for the smallest particles of matter and light, quantum computers might provide extraordinary power for applications such as breaking today's most widely used encryption codes. Neutral atoms are among about a dozen systems being evaluated around the world as qubits; their weak interactions with the environment may help to reduce computing errors.
The NIST experiments demonstrated the essential part of a so-called swap operation, in which atom partners exchange their internal spin states, trading an "up" spin (notionally a binary 1) for a "down" spin (binary 0.) Unlike classical bits, which would either swap or not, quantum bits can be simultaneously in an unusual state of having swapped and not swapped at the same time. Under these conditions, spin swapping has the effect of "entangling" the pairs, a quantum phenomenon that links the atoms' properties even when they are physically separated. Entanglement is one of the features that make quantum computers potentially so powerful. The swapping process is a way of creating logical connections among data, crucial in any computer.
The NIST experiment was performed with about 60,000 rubidium atoms trapped within a three-dimensional grid of light formed by three pairs of infrared laser beams arranged to create two horizontal lattices overlapping like mesh screens, one twice as fine as the other in one dimension. This created many pairs of energy "wells" for trapping atoms. The scientists attempted to place a single atom in each well, each pair with opposite spins. They then merged the paired wells to force each pair of atoms to interact with each other. Due to the rules of quantum mechanics, the merged atoms oscillate between the condition in which one atom is 1 and the other is 0, to the opposite condition. As they swap spins, the atoms pass in and out of entanglement, the key feature that enables quantum computation. This is believed to be the first time that quantum mechanical symmetry ("exchange symmetry") has been used to perform such an entangling operation with atoms.
In these experiments the same spin-swap was done in parallel for all pairs of atoms. The next step, according to the researchers, is to develop ways to address and manipulate any pair of atoms in the lattice, which should allow for scalable computer architectures. The NIST group is continuing to work on improving the reliability of each step and on completing the logic operation by separating atoms after they interact. The research was funded in part by the Disruptive Technology Office, the Office of Naval Research and the National Aeronautics and Space Administration. The authors are affiliated with the Joint Quantum Institute, a collaboration of NIST and the University of Maryland.
M. Anderlini, P.J. Lee, B.L. Brown, J. Sebby-Strabley, W.D. Phillips and J.V. Porto. Controlled exchange interaction between pairs of neutral atoms in an optical lattice. Nature. 448, 452-456 (26 July 2007).