SCIENCE & ENGINEERING NEWS

San Diego, CALIF. — Alan Boyle reports that in the quantum world of subatomic particles, something can be “on” and “off” at the same time. But is it possible to design circuitry that reflects this kind of quantum weirdness? Scientists say yes: They have used tiny superconducting loops that can pass current in two directions at the same time, raising the prospect of using such devices in a future generation of quantum computers.

The quantum world is a fuzzy realm where an object’s position and momentum can’t be determined simultaneously. “A microscopic particle such as an electron may be simultaneously in multiple states that have very different, apparently mutually exclusive physical properties,” psychologist Claudia Tesche observed in Friday’s issue of the journal Science.

The concept that something can be in two states simultaneously is known as “superposition,” and that’s a key piece in the puzzle for quantum computing. If scientists could design devices to manipulate quantum bits, or “qubits” that represent multiple states at the same time, that would open completely new avenues for decoding secret data – and developing a new generation of virtually uncrackable codes. Quantum computing could have other, less predictable applications as well.

But are such devices realistic on a macroscopic scale? That’s the question addressed in research conducted by two separate teams of experimenters. One team, based at the State University of New York at Stony Brook, published their results in the July 6 issue of the journal Nature. Another team of researchers at the Delft University of Technology in the Netherlands and the Massachusetts Institute of Technology discussed their experiment in Friday’s Science.

“Each approach has a different set of problems and advantages, also for future experiments,” said Caspar van der Wal, the principal author of the latest study.

But van der Wal and Jonathan Friedman, one of the Stony Brook researchers, agree that the technical differences are primarily of interest to quantum physicists. In both experiments, the bottom line is basically the same, Friedman said.

“What they point toward is the viability of quantum computation,” he said. “What people have been toying with until now have been microscopic objects like ions caught in a trap. It’s really hard to scale that up to a million or a billion bits to make a real computer…The real practical breakthrough is that you can now have something that you can fabricate on a chip and make millions of someday, that you can make quantum mechanical.”

Both teams started out with tiny low-temperature loops of superconducting material. Friedman’s loops were about 20 times larger than van der Wal’s, measuring about 0.1 millimeter, or the width of a typical human hair. An electrical current could flow clockwise or counterclockwise around the loops, but in both experiments, that current within the loop was manipulated so that it flowed in both directions.

The Delft/MIT team used superconducting quantum interference devices – also known as SQUIDs – to measure the direction of the current flow while microwaves were applied to put the current into what could be considered a superposition.

The Stony Brook experiment served as a clearer proof that quantum mechanics could be applied on the macro level as well as the micro level, since the superconducting loops were significantly larger. But van der Wal contended that his smaller-scale experiment may hold advantages for developing practical devices. “It makes it easier to realize devices with many loops,” he said. “So it will be important if you want to build a real quantum computer.”

That’s a big “if,” however: Van der Wal and Friedman agree that it’s not yet clear whether it’s possible to couple the circuits to each other to create a reliable quantum computer. “We have some ideas on how we can tune the strength of this coupling without destroying everything,” van der Wal said. But even if the practical applications are still years away, van der Wal still sees a lot of value in the research: “For the time being, it’s really exciting to figure out how we can push it, and it brings answer to fundamental research questions.”

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