By linking the electrical currents of two superconductors large enough to be seen with the naked eye, researchers have extended the domain of observable quantum effects. Billions of flowing electrons in the superconductors can collectively exhibit a weird quantum property called entanglement, usually confined to the realm of tiny particles, scientists report in the September 24 Nature.

"It's an exciting piece of work," comments physicist Steven Girvin of Yale University. "People are interested in pushing the boundaries of quantum mechanics."

Entanglement is one of the strangest consequences of quantum mechanics. After interacting in a certain way, objects become mysteriously linked, or entangled, so that what happens to one seems to affect the fate of the other. For the most part, researchers have only found signs of entanglement between tiny particles, such as ions, atoms and photons.

John Martinis and colleagues at the University of California, Santa Barbara looked for entanglement between two superconductors, each less than a millimeter across. These superconducting circuits, made of aluminum, were separated by a few millimeters on an electronic chip. At low temperatures, electrons in the superconductors flow collectively, unfettered by resistance.

Despite each superconductor's relatively large size, the electrons within move together in a naturally coherent way. "There are very few moving parts, so to speak," Girvin says, which helped the scientists spot evidence of entanglement. "It's a general fact that the larger an object is, the more classical it is in its behavior, and the more difficult it is to see quantum mechanical effects."

In the new study, researchers used a microwave pulse to attempt to entangle the electrical currents of the two superconductors. If the currents were quantum-mechanically linked, one current would flow clockwise at the time of measurement (assigned a value of 0), while the other would flow counterclockwise when measured (assigned a value of 1), Martinis says. On the other hand, the currents' directions would be completely independent of each other if everyday, classical physics were at work.

After attempting to entangle the superconducting circuits, Martinis and his team measured the directions of the currents 34.1 million times. When one current flowed clockwise (measured as a 0), the team found, the other flowed counterclockwise (measured as a 1) with very high probability. So the two were linked in a way that only quantum mechanics could explain.

"It has to be in this weird quantum state for you to get those particular probabilities that we measure," Martinis says. "The percentages of those different things are not something that you can classically predict."

Finding entanglement between superconductors is "a fairly important milestone," comments Anthony Leggett of the University of Illinois at Urbana-Champaign. The new study "does seem to be rather unambiguous evidence for entanglement."

Such entangled superconductors might be used as a component in a powerful quantum computer, Leggett says. "People are very interested in the possibility of building a quantum computer," and these kinds of systems may be quite good for that, he says.

Martinis says that the technology for building advanced electrical circuits may be used to build quantum circuits, too. "The hope is that since we know how to put together integrated circuits in complex ways, that maybe we can make very complex quantum circuits in the same way," he says.

He cautions, though, that a good quantum computer is a long way off. Researchers still need to find a way to make entangled superconducting circuits last longer. And a good quantum computer would need more than two circuits. Martinis says his group will try to entangle three and four such circuits next.

In addition to providing technological advances, the new results add to the debate over where to draw the line between quantum mechanics and the everyday physics that governs large-scale phenomena. Researchers want to know how far quantum weirdness can go.

"It's interesting to test quantum mechanics on a large scale," Girvin says. "Do things look classical on large scales because there's something wrong with quantum mechanics? Personally, I think that's wrong, but one never knows."

Image: Erik Lucero

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