Lasers can lengthen quantum bit memory by 1,000 times

A quantum dot is a semiconductor nanostructure that is onecandidate for creating quantum bits.

The scientists, including the University of Michigan'sDuncan Steel, used lasers to elicit a previously undiscovered natural feedbackreaction that stabilizes the quantum dot's magnetic field, lengthening thestable existence of the quantum bit by several orders of magnitude, or morethan 1,000 times.

The findings are published in the June 25 edition of Nature.

Because of their ability to represent multiple statessimultaneously, quantum computers could theoretically factor numbersdramatically faster and with smaller computers than conventional computers. Forthis reason, they could vastly improve computer security.

"In our approach, the quantum bit for informationstorage is an electron spin confined to a single dot in a semiconductor likeindium arsenide. Rather than representing a 0 or a 1 as a transistor does in aclassical computer, a quantum bit can be a linear combination of 0 and 1. It'ssort of like hitting two piano keys at the same time," said Steel, aprofessor in the Department of Physics and the Robert J. Hiller Professor ofElectrical Engineering and Computer Science.

"One of the serious problems in quantum computing isthat anything that disturbs the phase of one of these spins relative to theother causes a loss of coherence and destroys the information that was stored.It is as though one of the two notes on the piano is silenced, leaving only theother note."

Spin is an intrinsic property of the electron that isn'trotation, but is more like magnetic poles. Electrons are said to have spin upor down, which represent the 0s and 1s.

A major cause of information loss in a popular class ofsemiconductors called 3/5 materials is the interaction of the electron (thequantum bit) with the nuclei of the atoms in the quantum dot holding theelectron. Trapping the electron in a particular spin, as is necessary inquantum computers, gives rise to a small magnetic field that couples with themagnetic field in the nuclei and breaks down the memory in a few billionths ofa second.

By exciting the quantum dot with a laser, the scientistswere able to block the interaction of these magnetic fields. The laser causesan electron in the quantum dot to jump to a higher energy level, leaving behinda charged hole in the electron cloud. This hole, or space vacated by anelectron, also has a magnetic field due to the collective spin of the remainingelectron cloud. It turns out that the hole acts directly with the nuclei andcontrols its magnetic field without any intervention from outside except thefixed excitation by the lasers to create the hole.

"This discovery was quite unexpected," Steel said."Naturally occurring, nonlinear feedback in physical systems is rarelyobserved. We found a remarkable piece of physics in nature. We still have othermajor technical obstacles, but our work shows that one of the major hurdles toquantum computers that we thought might be a show-stopper isn't one,"Steel said.