Minifridge makes quantum computers last

Study repeatedly manipulates information held in ions

Beryllium ions that keep their cool can store and transfer information, bringing powerful quantum computers a step closer to reality. Researchers combined all of the ingredients required for a functional quantum computer and found the parts could operate reliably, a study appearing online August 6 in Science reports.

COOL COMPUTING Information-bearing beryllium ions can perform a series of logic operations inside a vacuum (inside the brown strip at left, which is 3.5 millimeters long and 200 micrometers wide), a trick that may improve prospects for building a useful quantum computer. J. Jost/NIST

“This is one of the most advanced systems that has ever been reported,” comments Steven Olmschenk of the University of Maryland in College Park.

Quantum computers have the potential to breeze through calculations much faster than conventional computers, making them useful for tasks like code-breaking. In traditional computers, each bit of information is stored as a 0 or a 1. But quantum bits take advantage of a strange property called superposition, meaning each bit can hold 0s and 1s simultaneously. Superposition may give quantum computers greater number-crunching ability.

In the new study, two beryllium ions served as quantum bits, or qubits, by holding information in an internal property known as the spin state. Magnetic fields moved the ions to different spots inside a vacuum, where laser pulses performed a series of logic operations on the beryllium ions. For example, if one qubit has a state represented by a 0, then a laser pulse would convert the other to a state represented by a 1, says study coauthor Jonathan Home of the National Institute of Standards and Technology lab in Boulder, Colo. For each series, the beryllium ions successfully performed five such logic operations. The ions also moved almost a millimeter while performing each series (transport of ions is required for a practical quantum computer).

“This is a very important development of bringing all of the components of a quantum computer together in one place,” says David Lucas, a physicist at the University of Oxford in England. He calls the new study a “technical tour de force.”

But ions must be kept cold in order to remain stable enough to work in quantum computers. Lasers performing the logic operations heat the ions up, and the fragile information stored in the spin is lost. Previous attempts at quantum information processing didn’t have a good way to keep the beryllium ions cold, Home says. To get around this problem, he and his colleagues pinned a magnesium ion next to each beryllium ion to serve as a portable minifridge. When the ions warmed up, lasers cooled the magnesium ion, which then cooled the beryllium ion. A similar technique was used to entangle two pairs of ions earlier this year (SN: 7/4/09, p. 8). 

To test the repeatability of their system, Home and his colleagues ran and then reran each series of operations multiple times using different starting conditions. Later series of operations performed about as well as the original series, the team found. For both the first and the second run, the team got the expected outcome around 94 percent of the time. “We wanted to show that we do it exactly the same the second time,” Home says. Such repeatability, he says, may eventually lead to more practical and stable quantum computers.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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