Quantum computer design lights dots

By Eric Smalley, Technology Research News

Figuring out how to hit atoms and subatomic particles with thousands of laser pulses or radio waves while keeping them isolated from the environment is one of the main thrusts of quantum computing research.

Another important focus is making quantum computers that can be manufactured relatively cheaply rather than cobbled together with expensive laboratory equipment.

A team of researchers in Italy has proposed a scheme for building quantum computers using microscopic specks of semiconductor and ultrafast lasers that could achieve both goals.

The scheme is one of several proposals based on using quantum dots, which are pieces of semiconductor that are usually no larger than a few hundred atoms across. Quantum dots are often referred to as artificial atoms or macroatoms because they corral small numbers of electrons. They have the potential to be mass-produced because they are made using the same processes as today's computer chips.

"The idea is to use quantum hardware fully compatible with current microelectronics technology," said Fausto Rossi, an associate professor of physics at the Polytechnic Institute of Torino.

But this quality could make it harder to achieve the other goal of squeezing in enough laser pulses before the quantum bits, or qubits, decohere, or come out of the quantum state of superposition due to interactions with the environment. Practical quantum computers made from quantum dots will likely have qubits based on electrical charge, and charge-based qubits decohere quickly.

The Italian scheme's qubit is made from an exciton, which is an electron and a hole in a temporarily stable orbit around each other. Holes are positively charged gaps where negatively charged electrons can reside.

The researchers propose to get around the decoherence limitation by using only ultrafast lasers to perform logic operations on the qubits. Other quantum dot quantum computing schemes use radio waves or magnetic fields, either alone or with ultrafast lasers. Laser's can be pulsed on the order of picoseconds, which is millions of times faster than radio waves or magnetic fields. A picosecond is one trillionth of a second. A picosecond is to one second as one second is to 31,709 years.

Because excitons can survive in the required quantum mechanical state of superposition for nanoseconds or even microseconds, which are thousands or millions of times longer than the pulses, the scheme could allow for the many thousands of laser pulses that will make up the computational operations needed for useful quantum algorithms.

The principal drawback to the Italian researchers' scheme is the lack of a method for addressing individual qubits. Because the quantum dots have to be spaced more closely than the wavelengths of light, the researchers can't use light to observe individual qubits, said Rossi.

The researchers are considering getting around the problem by using a cellular automata scheme instead of attempting to address each qubit individually. If quantum dots are spaced closely enough, the position of electrons in one quantum dot determines the position of electrons in the adjacent dot, which allows information to be transferred along a series of dots.

The goal of the researchers' project is to demonstrate basic quantum computing operations on a two-qubit prototype within three years, said Rossi. "If this [works], then we will start thinking about practical issues like large-scale integration and scalability," he said.

Most researchers say they believe that practical quantum computers are at least 20 years away.

Rossi's research colleagues were Eliana Biolatti, Rita C. Iotti and Paolo Zanardi of the Italian National Institute for Material Physics. They published the research in the December 25, 2000 Physical Review Letters. The research was funded by the European Commission's Future and Emerging Technologies program.

Timeline:   <3 years, 20 years
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Quantum Information Processing with Semiconductor Macroatoms," Physical Review Letters, December 25, 2000


February 21, 2001

Page One

Artificial cells make mini lab

PDA interface keeps a low profile

Software sifts text to sort Web sites

Light drives microscopic metal gears

Quantum computer design lights dots


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