Posted by: Kenji Hall on May 7, 2007
Building a quantum computer can be a tricky business. That’s because physicists are still in the early stages of figuring out how to control the tiny circuits inside these futuristic machines. Unlike with the semiconductors in your average desktop or laptop PC, the so-called quantum bits, or qubits, behave in unpredictable ways. Ideally, physicists hope to string together a bunch of qubits so that each is in one of two energy states, or both—a 1 or 0, much like the bits in an ordinary computer. Tame enough connected qubits and you would have a machine capable of cranking out calculations too complex for today’s supercomputers.
But scientists have yet to figure out how. Recently, a team from NEC Corp., the Japan Science and Technology Agency and Japan’s Institute of Physical and Chemical Research (RIKEN) offered one possible solution. In the May 4 issue of the weekly journal Science, they connected qubits in such a way that each can be individually controlled, a first. “We verified that it works at the end of last year, in late November,” says project leader Jaw-Shen Tsai, at NEC’s Nano Electronics Research Laboratories, in Tsukuba, north of Tokyo.
Typically, when a current is applied to a qubit its neighbors tend to react as well. This makes controlling qubits extremely difficult. In explaining how all this works, Tsai uses arrows on a round-faced surface. Beam a microwave at a qubit, for instance, and the arrow representing the qubit's spin state goes from pointing upward to downward, while its neighbor does the exact opposite. Often, this "coupling" doesn't have the desired effect, such as when the arrows are pointing right or left but should be pointing up-down.
In Tsai & Co.'s experiments, each qubit--which, when viewed with an electron microscope, resembles a mousetrap--is made of a superconductive aluminum material and measures a mere 5 microns (one millionth of a meter) by 3 microns. Tsai's team of 14 (including MIT researcher Seth Lloyd) made two qubits work in sync by sandwiching an extra one in between them. The extra qubit's only function was to act like a gatekeeper: When activated, it's open letting energy flow freely between the two qubits on either side; when deactivated it closes and stops the flow.
The result is a system that is working near its "optimal operation point." The term refers to the time a quantum signal stays active. "Right now you can get something like a few microseconds (one millionth of a second) of coherence," says Tsai. "If coherence is gone you don't have a signal." In many cases, at the optimal operation point, the signal between two qubits dies. He adds: "If you look through the literature, there are many groups proposing many things. And only a few of them function at the optimal point." One challenge for Tsai's group and others is stretching the signal time to milliseconds, or one thousandth of a second.
Sound short? Turns out, it's long enough for the type of calculations or simulations of molecular-level reactions that scientists hope these special computers will be doing in the future.
Quantum computing especially intrigues Tsai because he has long studied an area of physics that could yield a solution: the Josephson effect, or how a current flows across weak superconducting materials. Tsai wants to see if the rules he's studied hold true in a bigger system--a computer, for instance. Yet despite his progress, he admits that a lot of "fine tuning" is still needed. "To make this thing work you have to control the qubit energy of these three qubits well," he says. Then he would have to show that a system 10 times the size also works in much the same fashion. "We're not sure how reproducible this is," he says.
So how much progress has really been made? One way of measuring that is to compare it to the timeline of current computers. "With transistors, there were material problems: Silicon had too many impurities. Materials scientists figured out how to get rid of the impurities. We have the same thing. We make qubit gates with aluminum and we know it's not the best thing. Maybe we need help from materials scientists. For now we stick to aluminum because it's easier to make," he says. In other words, the equivalent of the chip for a quantum computer hasn't even been invented yet.
So it's not surprising that Tsai thinks a working computer is a "long way off." It would take 30 or 40 qubits to have a quantum computer that's "useful," he says, adding: "I don't know if I'll see it happen in my lifetime." Maybe not, but at least he will have contributed to the theoretical debate about how to build one.