Researchers at TU Delft have succeeded in shuttling electrons one by one through a chain and reading them out at the end of that chain, without disturbing their state during the process. This could represent an important step in the development of a quantum computer in the future. The Delft researchers published their results in Nature Nanotechnology on 04 January.

Spintronics
"This field – spintronics – is all about being able to store, transport and manipulate electrons and their spin with great precision," explains TU Delft researcher Tim Baart. "Put simply, an electron's spin is the direction of its rotation, either to the left or the right. You can use the spin of an individual electron to store information. The spin state then represents a digital ‘0’ or ‘1’. 

Spin transport of huge numbers of electrons simultaneously over relatively long distances is already possible, and controlled single electron transport through semiconductor materials is now also routine. But until now, no-one had succeeded in transporting single electrons over large distances while preserving their spin." 

CCD
"To achieve this, we made something that works very like a charge-coupled device (as used in CCD cameras, for example) but on a far smaller scale," continues Baart. "We can use a CCD at this scale to move a single electron around and even manipulate and maintain its spin state while shuttling it over large distances.

In a CCD, pockets of electrical charge are passed along a capacitor array in much the same way as buckets in a bucket brigade – a line of people passing along buckets of water to extinguish a fire. The pockets of charge arrive sequentially at the end of the array, where they are detected by a charge amplifier. This simple concept works excellently for CCD cameras with millions of pixels.

We call our version of this concept a ‘single-spin CCD’. This device can shuttle electrons one by one along a chain without disturbing their spin states and then read out the state at the end of the chain," says Baart, who carried out this research primarily with Mohammad Shafiei, shared first author of the article. 

Quantum dot array
In this case, the chain is made up of three quantum dots (which can be seen as artificial atoms, each of which is occupied by one electron). Just like in a CCD, the three electrons are pushed sequentially (by applying electric fields) to the end of the chain (where they are read out by another quantum dot). 

"We are also able to manipulate the three spins," says Baart, "and this enables us to 'write' information. Moreover, we have shown that shuttling an electron back and forth in the array hundreds of times, covering a cumulative distance of 80 µm, has a negligible influence on its spin. Extending this concept to much larger arrays offers potential for a wide range of applications, in particular for carrying out quantum calculations. After all, being able to control spin state is a crucial element in creating a large-scale quantum computer."

Article: Single Spin CCD, Nature Nanotecnology 
Authors: T.A. Baart, M. Shafiei, T. Fujita, C. Reichl, W. Wegscheider and L.M.K. Vandersypen.