Using a few ultracold ions, intense lasers and some electrodes, researchers have built the first programmable quantum computer. The new system, described in a paper to be published in Nature Physics, flexed its versatility by performing 160 randomly chosen processing routines.  

Earlier versions of quantum computers have been largely restricted to a narrow window of specific tasks. To be more generally useful, a quantum computer should be programmable, in the same way that a classical computer must be able to run many different programs on a single piece of machinery.

The new study is “a powerful demonstration of the technological advances towards producing a real-world quantum computer,” says quantum physicist Winfried Hensinger of the University of Sussex in Brighton, England.

Researchers led by David Hanneke of the National Institute of Standards and Technology in Boulder, Colo., based their quantum computer on two beryllium ions chilled to just above absolute zero. These ions, trapped by an  electromagnetic field on a gold-plated alumina chip, formed the quantum bits, or qubits, analogous to the bits in regular computers represented by 0s and 1s. Short laser bursts manipulated the beryllium ions to perform the processing operations, while nearby magnesium ions kept the beryllium ions cool and still.

Hanneke and colleagues programmed the computer to do operations on a single beryllium ion and on both of the beryllium ions together. In the quantum world, a single qubit can represent a mixture of 0 and 1 simultaneously, a state called a superposition. A laser pulse operation could change the composition of the mixture within the qubit, tipping the scales to make the qubit more likely to become a 1 when measured.

Both of the qubits together could be entangled, a situation where the two qubits are intimately linked, and what happens to one seems to affect the fate of the other. Different combinations of one- and two-qubit operations made up various programs. “We put all these pieces together and asked, what can we do with the circuit?” Hanneke says.

Hanneke and colleagues chose 160 programs for the quantum computer to run. “We picked them, quite literally, at random,” Hanneke says. “We really wanted to sample all possible operations.”

The researchers ran each program 900 times. On average, the quantum computer operated accurately 79 percent of the time, the team reported in their paper, which was published online November 15. “Getting this kind of control over a quantum system is really interesting from a physics perspective,” Hanneke says.

Earlier research has estimated that to be useful, a quantum computer must operate accurately 99.99 percent of the time. Hanneke says that with stronger lasers and other refinements, the system’s fidelity may be improved.

Experimental physicist Boris Blinov says that one of the most exciting things about the new study is that the quantum computer may be scaled up. “What’s most impressive and important is that they did it in the way that can be applied to a larger-scale system,” says Blinov, of the University of Washington in Seattle. “The very same techniques they’ve used for two qubits can be applied to much larger systems.”

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