Monday, December 19, 2005

Quantum Hardware

An innovative "ion trap" on a semiconductor chip could lay the foundation for mass production of quantum chips.

By Kate Greene

The quantum computer is the poster-technology for the next generation of computing, mainly because the computational power of such a machine could far exceed that of conventional computers. For instance, a quantum computer could quickly determine all the factors of a 300-digit number -- a feat that's impossible even for all of the world's supercomputers combined.

Such a capability isn't just a number-crunching trick either: factoring is fundamental to cryptography -- and governments handsomely support research that could keep their information safe.

Despite grant money and the work of corporate, university, and government research groups -- at IBM, Hewlett Packard, and the National Institute of Standards and Technology (NIST), to name a few -- quantum computing has remained primarily in the lab (see Harnessing Quantum Bits, March 2003). Recently, however, researchers at the University of Michigan have pushed quantum computing closer to the real world with the fabrication of a key component, an "ion trap," on a common semiconductor chip. This ion trap is so important because it can host a quantum bit, or "qubit," the most fundamental element in quantum computing. The advance suggests one way that the hardware for quantum computing could be mass-produced.

"We, along with a small handful of other research groups across the world, are working hard to see how far ion trap technology can be pushed," says Christopher Monroe, professor of physics at the University of Michigan in Ann Arbor, and leader of the ion trap study, which was described in the December 11 issue of Nature

Ion-trap technology uses electric and magnetic fields to isolate a charged particle from its environment -- a prerequisite for exploiting the temperamental quantum properties of electrons. Although ion traps are just one technology for building a quantum computer, they have the longest history -- the first trap was built in Monroe's lab in 1995 -- and they've advanced the furthest.

All quantum computation exploits the quantum nature of an electron's spin or a photon's polarity. Quantum theory dictates that until these properties are actually observed, they are indeterminate: the spin of an electron, for instance, can be "up" or "down," or a combination of the two.

Therefore, in a quantum computer, an electron's spin can represent a 1 and a 0 simultaneously, forming a qubit. Only when the electron's spin is observed, nominally after a computation is complete, does a qubit correspond to a definitive value of 1 or 0. Because qubits compute with both values at once, the processing power of a quantum computer doubles with each additional qubit: two qubits can do the work of four conventional bits; three qubits, the work of eight, and so on.