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Molecules Build A Bridge To Spintronics

The prospect of a new generation of devices that harness the spin of electrons has moved closer following a recent experiment in the US.

Min Ouyang and David Awschalom of the University of California at Santa Barbara have transferred electron spins across molecular 'bridges' between quantum dots for the first time. Even better, the pair found that they could transfer the spins most effectively at room temperature (M Ouyang and D Awschalom 2003 Sciencexpress 1086963).

Conventional electronic devices manipulate the flow of electronic charge, but spintronic devices would also exploit the intrinsic angular momentum or spin of electrons. Several proposals to build a so-called solid-state quantum computer rely on using electron spins as "quantum bits".

To make such devices work it is necessary to trap electrons and protect their spins from outside influences. The obvious way to do this is to store the electrons on quantum dots - tiny islands of a semiconductor material embedded in another semiconductor with a different band gap. But until now, physicists have failed to transfer spins between quantum dots, a key feature of any quantum computer.

The Santa Barbara team has overcome this hurdle by building structures made of alternate layers of 7-nm and 3.4-nm cadmium selenide quantum dots. The dots are linked by chain-like organic molecules, which both bind the array together and act as channels for the transfer of spin. Ouyang and Awschalom start by using a ultrashort circularly-polarized laser pulse to get the electron spins pointing in the right direction, followed by linearly-polarized pulse to measure the degree of electron polarization at a later time.

Red pulses are used to polarize the electrons in the larger quantum dots, and green pulses are used for the smaller dots. However, when Ouyang and Awschalom fire red pulses at their assembly, followed by green pulses, they find that the small quantum dots absorb far less green light than they do in experiments in which only green pulses are used. According to the pair, this shows that the spins in the large quantum dots had migrated across the molecular bridges to the small quantum dots.

Moreover, the efficiency of the process jumps from 12% at very low temperatures to 20% at room temperature. Together with the simplicity of their assembly process, Ouyang and Awschalom believe that these advantages could make their technique an important step towards a practical spintronic device.

Katie Pennicott is a science writer living in Bristol, UK