A quantum computer - a new kind of computer far more powerful than any that currently exist - could be made today, say Thaddeus Ladd of Stanford University and co-workers. They have sketched a blueprint for a silicon quantum computer that could be built using current fabrication and measurement techniques1.

The microelectronics industry has decades of experience of controlling and fine-tuning the structure and properties of silicon. These skills would give a silicon-based quantum computer a head start over other schemes for putting one together.

Quantum and conventional computers encode, store and manipulate information as sequences of binary digits, or bits, denoted as 1s and 0s. In a normal computer, each bit is a switch, which can be either 'on' or 'off'.

In a quantum computer, switches can be on, off or in a superposition of states - on and off at the same time. These extra configurations mean that quantum bits, or qubits, can encode more information than classical switches.

That increase in capacity would, in theory, make quantum computers faster and more powerful. In practice it is extremely difficult to maintain a superposition of more than a few quantum states for any length of time. So far, quantum computing has been demonstrated with only four qubits, compared with the billions of bits that conventional silicon microprocessors handle.

Several quantum-computing demonstrations have used nuclear magnetic resonance (NMR) to control and detect the quantum states of atoms floating in solution. But this beaker-of-liquid approach is unlikely to remain viable beyond ten or so qubits.

Many researchers suspect that making a quantum computer with as many qubits as a Pentium chip has transistors will take the same kind of technology, recording the information in solid-state devices.

Chip off the old block

In 1998, Bruce Kane of the University of New South Wales in Australia showed that solid-state quantum computing was conceivable, but not practical. He suggested that atoms of phosphorus in crystalline films of silicon could store qubits that could be read and manipulated using NMR sensitive enough to detect single atoms2.

The device proposed by Ladd and his colleagues is similar, but more within the reach of current technical capabilities. They suggest that qubits could be encoded in an isotope of silicon called silicon-29, or 29Si.



http://www.nature.com/nsu/020617/images/silicon_160.jpg
The device is feasible without unrealistic advances in fabrication, measurement, or control technologies
Thaddeus Ladd, Stanford University



Ladd's group says that grid-like arrays of 29Si chains could be grown on the surface of the most abundant silicon isotope, 28Si. Microscopic magnetic rods laid down perpendicular to the chains could control the magnetic quantum states of 29Si.

Crucially, each qubit would be stored not just in a single 29Si atom but in many thousand copies, one in each 29Si chain. This would avoid the problem of making measurements on single atoms. The readout could be performed using magnetic resonance force microscopy, which detects the oscillations of a thin bridge in which the rows of silicon atoms are embedded.

The details are subtle, but the point, the researchers say, is that the device is feasible without "unrealistic advances in fabrication, measurement, or control technologies". All they have to do now is build it.


References
Ladd, T. D. et al. All-silicon quantum computer. Physical Review Letters, 89, 017901, (2002).
Kane, B. E.A silicon-based nuclear spin quantum computer. Nature, 393, 133, (1998).

CONTACT: Mark Eriksson; 608-263-2689; maeriksson@facstaff.wisc.edu

MADISON -- For the first time, University of Wisconsin-Madison scientists have designed a semiconductor-based device that can trap individual electrons and line them up, an advance that could bring quantum computing out of the gee-whiz world of scientific novelty and into the practical realm.

Professors Mark Eriksson and Bob Joynt ( physics), Max Lagally (materials science and engineering), and Dan van der Weide (electrical and computer engineering) have developed a new type of "quantum dot" device for holding electrons that can be scaled up to build a working quantum computer.

Made from tiny amounts of the same semiconductor materials used in today's computer chips, each quantum dot device contains just one infinitesimally small electron. When many of the devices are aligned, the electrons they house become usable quantum bits, or qubits, for computing.


"The first prerequisite to building a large computer is to have a lot of bits, and we think we have a way to get a lot of them," says Eriksson. "We've done some sophisticated simulations with this device that show the concept is very likely to work, and we're in the beginning stages of actually making the device."

Unlike the bits of classical, serial computers, which exist in either the 0 or 1 state, qubits can exist in more than one state at once. This elusive quality of their components frees quantum computers to calculate all the possible solutions to a problem simultaneously, instead of running through them one-by-one like their slower, serial counterparts.

This ability to "parallel process" means quantum computers hold tremendous number-crunching potential for certain tasks -- such as highly sophisticated data encryption and code-breaking -- that now defy even the most powerful computers.

The team's device uses layers of semiconductor materials and electrostatic forces -- the same forces that build up when you scuff across a carpet in winter -- to squeeze a single electron into place within each quantum dot. The design allows the alignment of a large number of dots, their captured electrons separated by a distance only one-one thousandth the width of a human hair. Eriksson emphasizes that researchers worldwide are trying to find the best way to harness subatomic particles for quantum computing. In fact, others have realized success in stringing a few quantum dots together.

"People often talk about quantum computing in the future tense, but that's not really right -- it exists today. People have solved simple problems with it, but in the future we want to address problems that can't be solved by any other means," says Eriksson.

With its potential for coupling hundreds of electrons, Eriksson believes the team's device could provide a quantum leap in that direction. "Our invention makes it more likely that quantum computing might actually be useful someday instead of a curiosity," he says.

Collaborators include postdoctoral researcher Mark Friesen (physics theory), staff scientist Don Savage and graduate student Paul Rugheimer (materials growth).

To read a preprint paper describing the technology, visit: http://xxx.lanl.gov/abs/cond-mat/0204035.

A patent on the technology has been filed by the Wisconsin Alumni Research Foundation, a non-profit organization that manages the intellectual property of the UW-Madison.

PDF Technical Information (Design and proof of concept for silicon-based quantum dot) (http://xxx.lanl.gov/ftp/cond-mat/papers/0204/0204035.pdf)

Link to article... (http://www.news.wisc.edu/releases/view.html?id=7674)