The Nuts and Bolts of Qubits, Part 1

(If You Really Must Know)

By David H. Freedman|Friday, January 01, 1999
If only atoms were more cooperative. David Wineland is a member of one of the dozen or so teams of scientists who are wrestling with them in an effort to put quantum weirdness to work for the cause of faster--much, much faster--computing. Wineland and his colleagues at the National Institute of Standards and Technology use an electron beam to first strip a beryllium atom of an electron. Then they use a combination of laser beams and electromagnetic fields to hold the beryllium ion in place.

Once coaxed into this somnolent state, the ion can be tickled with another laser beam into two simultaneous quantum states--one in which the ion is vibrating at high energy, and the other in which it's vibrating at lower energy. The mixture of the two states can be thought of as a qubit, with, for example, the low-energy state being a 0 and the high-energy state being a 1.

After working on this for some four years now, Wineland can get two or more ions side by side acting as potential qubits. Unfortunately, there isn't really any interesting computing that can be done on a few qubits--or even a million qubits, for that matter--if the qubits aren't linked in some way. That's because computing logic requires that different pieces of data affect one another; for example, when two numbers are added to make a third, or a no is changed into a yes if two names match.

To achieve this sort of basic computer logic, Wineland recently added an important new wrinkle to the scheme. He uses the laser beam to cause the ions in his trap to swing rapidly back and forth in unison, creating yet another pair of superimposed quantum states: one in which the group of ions is swinging together, and the other in which they're at rest. The trick lies in the ability of the ions to go in and out of this group jig, which affects, and is affected by, the rapid vibrations of the individual ions. Thus the vibrations of one ion might in some cases cause the group to start swinging together, which might in turn cause one of the other ions to speed up or slow down its vibrations.

Think of a van full of people in which a few of the passengers start stomping their feet. This sets the van rocking, which inspires a new person to stomp but also scares the original stompers into stopping, which stops the van from rocking. That leads new people to stomp and--voila--an odd sort of processing occurs in the form of stompers being turned on and off via the van's rocking. In the same way, the vibrational states of the individual ions can be turned on and off via the swinging of the group, or vice versa. What's more, the exact determination of which ions are turned on and off by the rocking, and under what conditions, can be controlled simply by applying laser beams at different energies. "This gives us a general approach to quantum logic," says Wineland.

Well, at least in principle. In practice, Wineland has been battling a stream of problems. Some are minor, such as the tendency of lasers to fluctuate in intensity a bit too much to allow each ion to be precisely controlled. He's also having trouble getting more than two or three ions to sit still in his trap. But a two- or even three-qubit computer won't cut it; as with ordinary computers, it takes a lot of bits to deal with a significant amount of data. Still, Wineland predicts he'll make big strides in both problems over the next few years. "I don't see any fundamental roadblocks to eventually dealing with tens of ions," he says. "Maybe hundreds." That would be plenty to tackle some important real-world computing chores.
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