4 August 2004--Researchers have long been trying to develop quantum computers based on the same semiconductor technologies that have so successfully powered conventional computers. Now, after years of exploration, two groups have begun to connect the dots--literally.
CONNECTING THE DOTS
A scanning electron microscope photo shows a piece of gallium arsenide, viewed from above, with metal electrodes on top. Negative voltages applied to the electrodes repel the electrons underneath. The pattern of the electrodes cause two tiny puddles of electrons (quantum dots) to form, side by side, in the center of the image, where the electrodes meet
The dots in this case are quantum dots. They are nanoscale structures built within semiconducting materials that hold tiny puddles of electrons, which give each dot a collective quantum mechanical property called spin. The dots' spins, which can be either up or down, represent bits of quantum information, or qubits. Because quantum properties such as spin can exist in two states at once--being both up and down in the case of spin--computers using qubits can make many calculations simultaneously.
Separate groups of researchers at Duke University, in Durham, N.C., and at Harvard University, in Cambridge, Mass., have independently demonstrated how to connect quantum dots to form what may be the building blocks of a solid-state quantum computer.
The Duke and Harvard teams, which reported their work last April in the Physical Review Letters and Science , respectively, have shown how to make two quantum dots interact through the ghostly quantum connection known as entanglement. If two particles are entangled, when one is observed, fixing it into a particular state, the other is instantly fixed into a related state, regardless of how far apart the particles are. Einstein famously called it "spooky action at a distance." When the two quantum dots are entangled, the quantum states of their spins become inextricably linked to each other, an essential feature for quantum computations.
Peter Shor, a theorist who came up with a quantum computing algorithm for defeating encryption schemes, says the Duke and Harvard experiments are "very promising early steps." But he cautions that to build a quantum computer, it will be necessary to have a large number of these dots working together in a reliable way. "To give an analogy, this is like the first operation of a transistor," says Shor, a mathematics professor at the Massachusetts Institute of Technology, in Cambridge. "To get a quantum computer, we need to put many of these together and perform [calculations] reasonably fast and reasonably reliably."
In the past several years, other groups have built quantum computer prototypes using molecules in solutions or ions trapped by lasers and electric fields that were capable of performing simple, yet remarkable, quantum computations. But these prototypes, which required roomfuls of lasers, magnets, and other equipment, were limited in the number of qubits they could handle. They were also sensitive to interference from the environment, such as stray photons, that could disrupt the spins and introduce errors in the calculations.
Semiconductor technology could offer a better way. Because it can integrate a huge number of components in small areas, semiconductor-based quantum computers may be both more scalable and reliable. The Duke and Harvard researchers say these machines could start to fulfill the promise of quantum computers--machines that, among other things, would be able to factorize very large numbers. It's a feat that could make most cryptography systems useless, due to their dependence on the difficulty of such calculation.