Christian Thalmann

A team led by ETH Professor Ataç Imamoglu (1) has succeeded in defining the spin state of an electron in a quantum dot with a very small error rate. One day these are intended to form the components of a quantum computer. The results are published in the current issue of Science.

Artificial atoms

The intention is that the “bits” of a quantum computer will be able to adopt not only the values 0 or 1 but also complex superpositions of these states. A very promising candidate for this kind of component is the quantum dot: This is a tiny piece of semiconductor material measuring only 20 nanometres that accommodates a single free electron. Because of the extreme spatial restrictions, the semiconductor creates sharply demarcated energy levels in which the electron can reside, similar to those in an atom. This is why quantum dots are also called “artificial atoms”. The spin (angular momentum) of the electron displays the behaviour that a quantum bit is required to have.

Because of the energy level, the electron can be “pumped” into a required spin state by irradiation with laser light having a suitable wavelength and polarisation. However, this state generally decays much faster than it can be formed, because the electron interacts with the numerous atomic nuclei in the semiconductor, and occasionally exchanges with another electron from the exterior via the tunnelling effect. Research in Imamoglu’s group has now shown that both of these interfering effects can be decisively reduced at particular magnetic field strengths and potentials. Under these conditions, the electron spin can be controlled with a reliability of 99.8%.

Participation of various department

According to Imamoglu, ETH interdisciplinary collaborations were decisive in the success of this work. The FIRST Lab (2) at Hönggerberg, a world leader in nanotechnology, was given the task of manufacturing the tiny structures. Nor was there any lack of brainpower: the QSIT (Quantum Systems for Information Technology) innovation initiative (3) supplied the required intellectual support.