Quantum Physics


Herding Schrodinger’s Cats

Why are qubits like cats?

They’re strange and contradictory—and they don’t like to be herded.

Qubits are the essential elements of quantum computers. Conventional computers symbolize data as a series of ones and zeroes—binary digits known as bits. Quantum computers use quantum bits, or “qubits,” that don’t just toggle on or off like the transistors in conventional computers, but can be both on and off simultaneously. In principle, quantum computers can vastly outperform traditional computers. But actually building such a machine is a challenge akin to—you guessed it—herding cats.

To understand how quantum computers work, you can start by opening your eyes. If you see something, you pretty much know it’s there, right? However, if you can’t see something—if it’s hidden in a box, for instance, or it’s too small to see—you might very well imagine that it might be anywhere, or nowhere. This realm of uncertainty is more than just a fanciful idea; it is the backbone of quantum physics, and it is exactly the odd life that the elementary building blocks of the universe live if you aren’t looking. Atoms and subatomic particles live in states of flux known as “superpositions” where they can, for instance, exist in two or more places at once, or spin in two opposite ways simultaneously. However, once a particle gets disturbed by its surroundings, its superposition “collapses” so that it is in just one of the many possible states represented by the superposition.

Quantum computers are based on objects in superposition—those qubits that can read both “zero” and “one” simultaneously. The more qubits you “entangle,” or link together so that they operate in perfect unison, the more potential on/off combinations your quantum computer can run at the same time. A quantum computer with just 300 such qubits could run more calculations in an instant than there are atoms in the universe.

Superpositions are extraordinarily fragile, though. The easiest chunks of matter to coax into superpositions are usually very small, because their activity is easier to control, or very cold, because their low energy makes it unlikely they will interact with their environment—for instance, super-cooled rubidium or ytterbium atoms.

So far quantum computers are only capable of fairly rudimentary behavior, such as figuring out what numbers multiply together to get 15. But these are just toy versions of the powerhouses that quantum computers could one day become. Today, militaries, intelligence agencies, corporations and universities worldwide are competing to develop quantum computers that live up to that promise.

Conventional computers symbolize data as a series of ones and zeroes, binary digits known as bits. This code is conveyed via transistors, which are electronic switches that are flicked either on or off to represent a one or a zero, and is the basis for all the calculations associated with traditional computers. In contrast, quantum computers are based on objects in superpositions—quantum bits or “qubits” that don’t just work on or off, but both on and off simultaneously. The more qubits are “entangled” or linked together in a way where they operate in perfect unison, the more potential on-off combinations they can run at the same time. A quantum computer with just 300 such qubits could run more calculations in an instant than there are atoms in the universe.

“A quantum computer will allow humankind to perform tasks that are far beyond what the best classical supercomputer can do,” said physicist Matteo Mariantoni at the University of California at Santa Barbara.

Certain tasks thought impossible for regular computers could be accomplished quickly by quantum computers. For instance, a quantum computer could easily factor a number hundreds of digits long. This is a math problem that’s too difficult for even the best computers today, which is why online encryption of credit card numbers and passwords depends on it. The National Security Agency (NSA) and others in the intelligence community are therefore racing to build a quantum computer that’s up to the task before someone gets there first. “My bet is the first quantum computer will appear in a lab related to the NSA,” said Raymond Laflamme, executive director of the Institute for Quantum Computing at the University of Waterloo in Canada.

Quantum computers could be good for more than just hacking. Since they are quantum systems, they can be used to simulate other quantum systems, helping scientists investigate how complex molecules behave. Such work “could revolutionize the pharmaceutical industry,” Mariantoni said. Quantum simulations could also help “solve mysteries of physics,” such as superconductivity, the phenomenon where electrons zip without resistance through objects, said Markus Greiner, a physicist at Harvard University. In the process, such research could also help develop novel materials with fantastic, unforeseen new properties, he added.

Research teams across the world are pursuing a wide variety of different methods to create quantum computers. Qubits are being made from electrically charged atoms held in place by electrical fields, from photons of light, and from superconducting circuits, among many other architectures. An intriguing development in quantum computing are qubits that don’t need super-cold temperatures, but rather can exist at room temperature. For instance, impurities within diamond can stay in superposition because their placement within such a pure crystal insulates them from outside disturbances.

Although scientists have created basic working quantum computers with a few qubits for nearly 20 years now, more advanced versions with hundreds of qubits that can outperform classical computers will likely take decades to materialize. Superpositions are delicate and easily broken, and the problem of keeping them isolated gets harder with each additional qubit. Moreover, it’s not certain which architecture is optimal. “It’s not even clear there will be a winner. Maybe we’ll see hybrid devices that take advantage of several architectures, or a different approach altogether,” said Jeremy O’Brien, director of the Center for Quantum Photonics at the University of Bristol in England. “However, personally, I’m very confident that an all-optical approach with single photons is the leading one.”

Although it might seem as if development of quantum computers is slow, “Charles Babbage conceptualized and designed the first programmable computer in the 1830s,” Awschalom said. “Nevertheless, it took until the second half of the 20th century for a recognizable electronic computer to arrive.”

Still, benefits from spinoff technologies should appear long before a quantum computer more powerful than a conventional supercomputer does. For instance, research into quantum computers is now pioneering ways to resist hacking. When qubits are entangled, they stay in sync instantaneously as if they were one, even if they are at separate ends of the universe, a seemingly impossible connection Einstein dubbed “spooky action at a distance.” If anyone tried to eavesdrop on communications involving qubits, the disturbance would immediately be obvious. Such research is now helping develop a new kind of extraordinarily secure cryptography. “Those applications may outpace the development of a quantum computer that will break current cryptographic schemes, which is a good thing for information security,” Awschalom said.

The first quantum computers will probably live in labs or server farms. However, according to experimental physicist Ian Walmsley at the University of Oxford in England “As they get easier to build and designs get more sophisticated, I think we’ll see them in the office—maybe in the home.”

If you find it hard to imagine a quantum computer sitting on your desk, Laflamme suggests looking to history: “If you went back to the 1950s and asked if you really needed a computer in the house, I think you’d get a similar answer.”

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Charles Choi

    Charles Q. Choi has written for Scientific American, The New York Times, Wired, Science and Nature, among others. In his spare time, he has traveled to all seven continents, including scaling the side of an iceberg in Antarctica, investigating mummies from Siberia, snorkeling in the Galapagos, climbing Mt. Kilimanjaro, camping in the Outback, avoiding thieves near Shaolin Temple and hunting for mammoth DNA in Yukon.