creating qubits that interact and stay in superposition long enough to make themselves useful will preoccupy quantum-computing researchers for years to come. Still, practical payoffs are emerging as scientists exploit the phenomena behind quantum computing in related fields.
At NIST, whose institutional mission includes setting standards for the measurement of time, Wineland’s interest in quantum computation predated even Shor’s algorithm. “We were starting to think up ways quantum entanglement could be used to improve the signal-to-noise ratio in atomic clocks,” he explains. “We knew there was a quantum entanglement state that could improve the clock, and the ideas of quantum computation showed how to make it.” Roughly speaking, today’s atomic clocks work by taking the average of simultaneous readings of the oscillating magnetic fields of more than a million cesium atoms; quantum entanglement could reduce the time needed to compute that average and enhance precision by allowing many readings to be taken concurrently.
The Jet Propulsion Laboratory’s Dowling adds that quantum entanglement may provide a better way to synchronize earthbound clocks with those in space. At present, ground-and-space synchronization, which is most often done by radio, is thrown off, albeit minutely, by atmospheric refraction and other effects. Because entangled photons are linked on a quantum level, they are immune to these physical disturbances. “It would be a really big deal to knock those [effects] out,” Dowling says. He suggests sending entangled particles to the sites to be synchronized. Measuring one particle would instantaneously set the other one to “ticking,” Dowling says. Having calibrated their clocks to the ticking particles, operators would know the clocks were in agreement.
Lest anyone think that quantum-scale refinements of time measurement are of academic interest only, it should be noted that atomic timekeeping is the basis of geographic positioning systems, satellite tracking technologies, and mobile communications networks, which are synchronized by the second. “History has shown forever that whenever there’s a better clock, it gets used,” Wineland says. “It’s a good bet that trend will continue.”
Scientists in industry, meanwhile, are looking for ways to bootstrap quantum computing by linking it with conventional technologies in which they have more experience. Last year Hewlett-Packard forged a $2.5 million working alliance with Gershenfeld and Chuang to, as HP Labs senior scientist Philip Kuekes says, “combine our respective expertise.” HP is intrigued, for instance, by the possibility of transmitting quantum bits via ordinary fiber-optic lines-thousands of kilometers of which lie installed but underutilized around the country. “That’s actually quite interesting,” Kuekes says. The long-distance transmission of quantum information, enhanced by the characteristics of quantum entanglement, would allow correspondents to share code keys without fear of their being compromised. That means, he adds, that “one of the things that might happen quite early is quantum cryptography.”
Although, as research has shown, qubits can be transmitted over fiber-optic lines, the transmissions work for no more than tens of kilometers at a time. Sending qubits across continents or oceans, Kuekes says, would require a system of quantum switches and repeaters analogous to the solid-state versions that help move data throughout the Internet. These would amount to simple quantum computers equipped with error-correcting software that could compensate for the inevitable loss of superposition among many of the traveling qubits. Development of this software is one of the main thrusts of HP Labs’ research.
In a scientific example of the child’s being father to the man, applied research has thrown off some ancillary benefits even in the parent science of quantum mechanics. The tools needed to perfect quantum computing, it turns out, also help demonstrate particle behavior that physicists, until now, have posited only in theory.
“There’s a beautiful flowing-back the other way,” says John Preskill, a professor of theoretical physics at Caltech. “Interest in quantum computing has inspired a lot of interesting science. We’re a long way from a crash program in engineering, but we’re entering a new era in condensed-matter physics.”
This is largely a result of quantum computing’s requirement that qubits be controlled and measured with unprecedented precision. “The tradition in condensed-matter physics has been to do experiments on ensembles,” that is, huge quantities of atoms whose quantum behavior can be identified statistically, Preskill says. “You don’t ordinarily measure the behavior of single electrons.”
In particular, Wineland’s experiments at NIST, Preskill says, have given physicists an unparalleled window on the behaviors of individual particles. The breakdown of qubits into classical ones or zeroes, for example, is a phenomenon that, in the past, scientists could only infer by observing whole clouds of electrons or photons. The clouds’ average signal would indicate whether some particles had changed their quantum state, “but you wouldn’t really see the individual particle behavior,” Preskill says. “It really is a new type of experiment.”
Preskill, like others in the field, cautions that many questions need to be answered and critical problems resolved before quantum computing can advance beyond its current elementary applications. “Whether this field will still look this exciting in 10 years, I can’t say,” he admits. “But for now, the field feels fresh and new.” Once again.