Electron Traps That Compute

ETH Zurich quantum physicists have developed a semiconductor system that can be used for quantum computing if need be. They “grew” a gallium arsenide crystal. On top of that they applied two layers of indium-gallium arsenide from which tiny bubbles, the quantum dots, formed. The blobs in the second layer grew directly above those in the first layer. Lucio Robledo, first author of a paper published in Science, says “This kind of dot is like an artificial atom only bigger, and two superimposed dots constitute an artificial molecule.”

The Quantum Photonics Group researchers of ETH Zurich led by Ataç Imamoglu finally succeeded in populating these quantum dots with single electrons and were able to manipulate them with lasers and analyse their properties. The physicists determined exactly how many electrons were present in one of their semiconductor system’s quantum dots. Above all, however, they were able to imprison the charged particles in them individually.

Electrons as bits

Each electron in turn has a particular spin, i.e. it rotates in one direction around its own axis and is thus rather like a quantum magnet with quantum-mechanical properties. Research in theoretical and experimental quantum physics has focused for many years on gaining a better understanding of these properties and control over them.

Using the electron spin to carry encoded information was also already suggested several years ago. The information elements in a normal computer are bits with values of zero or one. This is not so with quanta, which can occupy both states simultaneously.

This means an electron has two different spin orientations at the same time. Jeroen Elzerman, a co-author of the study, stresses that “This is one of the fundamental mysteries of the quantum world.” However, he says this enables numerous computing operations to be performed simultaneously and allows a computer’s speed to be increased many times over.

Optical control

The Quantum Photonics Group researchers finally used two coupled quantum dots to study their semiconductor system, because these govern one another reciprocally. The state of one dot influences that of the one above it, and vice versa. On top of that, the ETH Zurich physicists were able to control these states optically from the outside, i.e. by excitation with a laser. Robledo says “We found a way to make quantum dots interact with one another and to communicate in a controlled fashion.” The controlled interaction presented in the study could be a suitable way to carry out fundamental quantum operations.

This optical manipulation of quantum dot spins is an important step forward for the Quantum Photonics Group researchers. For example they were able to set an electron’s spin state in a particular direction with high reliability, and also read it out again. The physicists were also able to couple individual quantum dots to optical nano-resonators.

Scale-up capability unsolved

Despite these impressive successes, Ataç Imamoglu hesitates to regard quantum dots as the most promising route to quantum computers, because a large amount of physics at the nano-scale still needs to be deciphered. In addition the architecture of a quantum computer would have to be expandable in a modular way as with a conventional computer – by which he means transistors as the structural element of chips – to enable thousands more to be added to these two quantum bits. The researchers still need to find a solution to this challenge facing quantum dots first of all.