Prospects for building a practical quantum computer are as unpredictable as quantum mechanics itself. However, physicists at the National Institute of Standards and Technology (NIST) have demonstrated sustained, reliable information processing operations on electrically charged atoms (ions). The work, described in *Science* Express, overcomes hurdles in scaling up ion-trapping technology from small demonstrations to larger quantum processors.

In the new demonstration, NIST researchers repeatedly performed a combined sequence of five quantum logic operations and ten transport operations while reliably maintaining the 0s and 1s of the binary data stored in the ions, which serve as quantum bits (qubits) for a hypothetical quantum computer, and retaining the ability to subsequently manipulate this information. Previously, scientists at NIST and elsewhere have been unable to coax any qubit technology into performing a complete set of quantum logic operations while transporting information, without disturbances degrading the later processes.

"The significant advance is that we can keep on computing, despite the fact we're doing a lot of qubit transport," says first author Jonathan Home, a NIST post-doctoral researcher.

The NIST group performed some of the earliest experiments on quantum information processing and has previously demonstrated many basic components needed for computing with trapped ions. The new research combines previous advances with two crucial solutions to previously chronic vulnerabilities: cooling of ions after transport so their fragile quantum properties can be used for subsequent logic operations, and storing data values in special states of ions that are resistant to unwanted alterations by stray magnetic fields.

As a result, the NIST researchers have now demonstrated on a small scale all the generally recognized requirements for a large-scale ion-based quantum processor. Previously they could perform all of the following processes a few at a time, but now they can perform all of them together and repeatedly: (1) "initialize" qubits to the desired starting state (0 or 1), (2) store qubit data in ions, (3) perform logic operations on one or two qubits, (4) transfer information between different locations in the processor, and (5) read out qubit results individually (0 or 1).

Through its use of ions, the NIST experiment showcases one promising architecture for a quantum computer, a potentially powerful machine that theoretically could solve some problems that are currently intractable, such as breaking today's most widely used encryption codes. Relying on the unusual rules of the submicroscopic quantum world, qubits can act as 0s and 1s simultaneously, unlike ordinary digital bits, which hold only one value at any given time. Quantum computers also derive their power from the fact that qubits can be "entangled," so their properties are linked, even at a distance. Ions are one of a number of different types of quantum systems under investigation around the world for use as qubits in a quantum computer.

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