Mix and Match Qubits

Anybody can find out how to crack the codes protecting your bank transfers and online credit card purchases. The step-by-step instructions for stripping away the secrecy were published years ago.

Nobody is very worried about a possible security breach, though, because the code-cracking formula runs only on quantum computers. These contraptions, which exploit the rules governing the fuzzy world of quantum mechanics, have so far remained laboratory curiosities, less powerful than a slide rule.

New blueprints could change all that. Rival technologies that have steadily matured in recent years — but failed to produce powerful quantum computers on their own — are now being married. Physicists playing matchmaker hope to build hybrid devices greater than the sum of their parts, dynamic duos in the spirit of Batman and Robin.

“There’s no clear map for the road ahead,” says Jörg Wrachtrup, a physicist at the University of Stuttgart in Germany. “But some quantum processors will be a kind of hybrid system for sure.”

Designs currently on the drawing board mimic the division of labor found in today’s computers, where the hard drive is a magnetic material good for storing information and the processor is a silicon chip where information can be quickly manipulated. Quantum versions of these components would bring together objects of different sizes and personalities, from atoms to super­conducting circuits to chips of diamond.

Early research has shown that these would-be components can communicate with one another. A hybrid approach that stitches them together might one day yield a powerful quantum computer, useful not just for breaking codes. Such a computer could also tackle other problems difficult or impossible for today’s ordinary computing machines — from searching through piles of data faster than a conventional computer to simulating how molecules chemically react.

“We can’t promise a functional computer in five years,” says Klaus Mølmer, a quantum physicist at Aarhus University in Denmark. “But the very first experiments are promising.”

A matter of size

Today’s computers, whether PC or Mac, work with bits of information that are either a 0 or a 1. But quantum computing, an idea that can be traced to physicist Paul Benioff about three decades ago, works with the quantum analog of classical bits: qubits. Because of the weirdness of the quantum world, a qubit exists in what’s called a superposition of states, being both 0 and 1 at the same time. Connect together a dozen qubits, and the group encodes all 4,096 possible combinations of 0s and 1s simultaneously. This uncanny teamwork allows calculations to be performed on many inputs at once. It’s the secret to how a quantum computer could solve certain kinds of problems exceptionally rapidly.

Atoms, which live at the small scales governed by quantum laws, have long been a promising candidate for qubits, thanks to their proven ability to store quantum information. Trapped in an electric field, an isolated atom can adopt and maintain a split personality for minutes, an eternity in the quantum world.

But atoms are also shy. Forging the connections between atoms required to perform computations with qubits is technically challenging, especially as more atoms are added to the system. The world record for the most atomicqubits linked together stands at only 14, set last April by Thomas Monz and colleagues working with charged calcium atoms at the University of Innsbruck in Austria.

Instead of using atoms as qubits, some scientists have turned to bigger objects that are easier to work with: super­conducting wires etched on circuit boards. In the late 1990s, these bits of metal disclosed a penchant for behaving just as strangely as atoms. When electrons zipping around a superconducting loop are forced to leap through an insulating barrier, unusual things happen. Current can flow in different directions simultaneously, or two different amounts of charge can exist at once.

These circuit building blocks promise to be easier to link up than atoms. Techniques borrowed from the computer chip industry have been adapted to join loops of superconducting metal and manipulate the information they contain. In 2007 a team in the Netherlands showed how to quickly change the state of one superconducting qubit using a gate, a basic component required for making a computer processor.

“There’s something very powerful about superconducting qubits,” says John Martinis, a quantum physicist at the University of California, Santa Barbara. “They’re large, so they can be easily wired together to make computer processors.” A new superconducting processor with nine qubits ran a simple program that identified the prime factors of 15 (5 and 3), Martinis’ team planned to report in February at a meeting of the American Physical Society.

Because of their gawkish size, though, superconducting wires have trouble maintaining their quantum weirdness. Contact with the environment, which destroys qubits, is more difficult to avoid than it is with atoms. The hardiest superconducting qubit ever made that could reliably repeat its performance was described in the Dec. 9 Physical Review Letters. It held onto a piece of information for a mere 20 microseconds.

Some scientists are trying to overcome the limitations of atoms or superconducting circuits, relying on just one technique to store and manipulate information in a quantum computer. But other researchers think that combining these two approaches could exploit the strengths of each. A hybrid could have an atomic hard drive, a memory for storing information. As necessary, that information could be dumped into a CPU made of superconducting qubits wired together to perform calculations. The results of these calculations could then be dumped back into the hard drive. 

Come together

Several variations on the hybrid anatomy have been proposed. One early scheme, described in 2004 in Physical Review Letters, would link individual atoms to superconducting qubits. Another plan, detailed in the same journal in 2006, would move information between superconducting circuits and clouds of molecules that serve as a collective memory.

“We’re trying to come up with ideas where you combine the advantages of different systems but not the disadvantages,” says Peter Zoller, a theoretical physicist at the University of Innsbruck.

One team is trying to build a hybrid device at the Joint Quantum Institute at the University of Maryland in College Park. Researchers there have coated tiny fibers with rubidium atoms. These atoms, long used to keep time in some of the world’s most precise clocks, can store quantum information in their vibrations. In theory, magnetic fields generated by a nearby superconducting wire could serve as a communication channel, so information could be passed to superconducting qubits.

Getting this scheme to work is tricky because of how carefully the atoms have to be held in place. So other groups hope to make hard drives out of atoms that come prewrapped in nice neat diamond packages.

Diamond is a relative newcomer to the quantum community. In 2008, researchers showed how a type of impurity that turns a synthetic diamond pink could be controlled the way qubits made from isolated atoms are (SN: 4/5/08, p. 216). The diamond defect occurs when a nitrogen atom takes the place of a carbon atom in the crystal structure and is flanked by a hole where another carbon atom is missing.

In this situation, a pair of the nitrogen’s electrons splay out into the hole and behave as one. The single entity created has a quantum property called spin, which can be thought of as a tiny bar magnet that can point either up or down. But unlike a real magnet, quantum spin can also point up and down at the same time.

Shielded from the hostile world in a diamond womb, the electron duo should be able to preserve its spin for more than a millisecond at room temperature, and even longer when chilled. (This quantum state can also be moved into an atom’s nucleus, where its longevity would rival that of the traditional atomic approach.)

“Once you transfer information into a diamond, it lives a really long time,” says Anders Sørensen of the University of Copenhagen’s Niels Bohr Institute. In 2010 Sørensen and colleagues first proposed a match between diamonds and superconducting circuits.

Arranging this marriage turned out to be surprisingly easy. At NTT Basic Research Laboratories in Atsugi, Japan, Xiaobo Zhu and colleagues simply glued a diamond chip to a circuit board. In this Frankensteinian apparatus, magnetic fields generated by the superconducting qubit changed the spins of the electron duos within the diamond. On October 13, the same day this information swap was reported in Nature, a group led by researchers in France announced a similar feat.

“This is the beginning of the field, but we’ve proven that this is a possible task,” says Patrice Bertet, a quantum physicist at the French laboratory CEA Saclay.

Bertet and his colleagues’ would-be diamond hard drive, reported online at arXiv.org, isn’t very useful right now. One diamond chip stores only a single bit of information, and for just a couple hundred nanoseconds. What’s more, only about one in seven attempts to swap information succeeded. Though the Japanese team didn’t fare any better, both experiments show that hybrids may turn out to be more than just an idea on a physicist’s wish list.

Hidden gems

For all their sparkle, diamonds aren’t the only way to bling out a superconducting circuit with prewrapped qubits. Rubies contain interesting chromium impurities with spins that could be the Juliet to the Romeo that is superconducting circuits. One group of researchers wants to use nitrogen atoms caged within buckyballs — carbon spheres that resemble geodesic domes. Another is playing with rare earth atoms that could be useful for storing information.

One set of blueprints calls for exotic entities whose existence hasn’t even been confirmed yet. Last spring, a team led by theoretical physicist John Preskill of Caltech extolled the benefits of tucking information inside “anyons.” These two-dimensional particles are thought to inhabit the surfaces of bizarre materials known as topological insulators, which behave in strange ways when it comes to conducting electricity. In theory, an anyon would have to be disturbed at two points at once to lose its information, offering the ultimate in reliable storage.

“There are other materials that seem to have the interesting quantum properties found in diamond,” says David Awschalom, a physicist at the University of California, Santa Barbara.

Awschalom has taken a pragmatic approach to quantum hard drives. He’s testing materials that engineers already know how to work with — mundane silicon semi­conductors that star in today’s electronics. A semiconductor-superconductor hybrid would be a decidedly practical match.

Silicon carbide, used in high-power transistors and other equipment, seems to have the right stuff for storing quantum information. Like diamond, the material is dotted with defects. Reporting in the Nov. 3 Nature, Awschalom’s team controlled the spins of electrons in these defects. Computer simulations done by the researchers have revealed more than a dozen other promising materials, including magnesium oxide, zinc oxide and aluminum oxide. 

Hybrid horizon

Hybrids with hardware made of semiconductors or diamond would offer an added bonus: They would have a built-in modem that could allow future quantum computers to broadcast the information they store. Physicists are keen to eventually construct a quantum network spanning great distances.

“You might like to make a quantum Internet some day,” says Yale’s Robert Schoelkopf, a quantum physicist and superconducting qubit pioneer.

Superconducting circuits can’t talk to the particles of light that carry information on fiber-optic cables. But electrons in diamonds and silicon carbide can, while still being relatively easy to work with. Modems made from these materials could extend the reach of quantum information from different spots on the same superconducting chip to different places on the globe.

Ultimately, the anatomy of future quantum computers — whether they are based on one material or many, and which ones — may depend on how many qubits the device needs, which in turn depends on what problem it is intended to tackle.

A small quantum computer with dozens of qubits, for instance, could potentially solve some outstanding mysteries in science. It could probably simulate materials that are too complicated to be understood with today’s computers — revealing how high-temperature superconductors work, for instance.

It’s not unreasonable to think that a quantum computer based on a single kind of qubit could accomplish this and similar tasks. Atomic qubits have successfully simulated run-of-the-mill magnets that can already be understood using classical computers. And the quantum lifetimes of superconducting qubits, while still lousy, have steadily improved to the point that some people have proposed using them for simulations as well.

“We’re close to doing some quantum simulations that we could never do classically,” says physicist Chris Monroe, who works with atom qubits at the Joint Quantum Institute. “That’ going to happen pretty soon, I think.”

But other applications, such as code breaking, will certainly require devices much bigger and more powerful, with a million or more qubits — though no one knows the exact number for sure.

What a device that complex will end up looking like, and how many approaches get mixed and matched to make it, is still anyone’s guess.