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Pulsed laser synchronizes electron spin

09 Jan 2008

Quantum computers are a step closer thanks to research into electron spin in semiconductor quantum dots.

Pulsed laser light is helping researchers in the US to achieve coherent spin of an ensemble of electrons localized in quantum dots (QDs). Causing electrons to spin in the same direction is an important step towards developing optics-based quantum computing and quantum information processing. (Science 317 DOI: 10.1126)

"Localized electron spin is known as the quantum bit, which is the basic building block for solid-state-based quantum devices," Alexander Efros, a researcher at the Naval Research Laboratory (NRL), told optics.org. "The coherent manipulation of electron spins localized in QDs could enable functionality of all solid-state devices for quantum information processing and quantum computing."

The spin of an electron can take two values and so replace the 0 and 1 of a classical digital bit. However, such a quantum unit can also take all values in between, which means it can hold much more information than a classical one. "The use of such quantum bits can make certain computer calculations exponentially more efficient than those used in a standard computer," commented Efros.

Until now, the major problem has been that the electron spins in different QDs are never identical. "The electron spin precession frequencies in an external magnetic field are different from each other due to small variations of the QD shape and size," commented Efros. "This makes coherent control and manipulation of electron spins in an ensemble of QDs impossible."

In a bid to synchronize the electron spin, the NRL team used periodic resonant excitation from a pulsed laser to drive almost the whole ensemble of electron spins (>90%) to precess coherently. The excitation laser fired 1.5 ps pulses at a rate of 75.6 MHz at an ensemble of InGaAs/GaAs QDs. The structure contained 20 layers of QDs, each with a dot density of about 1010cm-2.

"The nuclear contribution to the electron spin precession acts constructively by focusing the electron spin precession in different QDs to a few precession modes," explained Efros. "The precession modes are controlled by the laser excitation protocol, instead of acting as a random perturbation of electron spins."

Future efforts will focus on demonstrating all coherent single q-bit operations using an ensemble of charged quantum dots. "Another important use for quantum computing will be the demonstration of a quantum-dot gate operation," concluded Efros. "The macroscopic coherent precession of the electron spin ensemble will allow scientists to study several optical coherent phenomena, such as electromagnetically induced transparency and slow light."

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