A New Type Of Quantum Bits In Semiconductor Nanostructures

In a semiconductor nanostructure, researchers have created a superimposed quantum state that would be suitable as a basis for quantum computing. The trick: two finely tuned lasers.

A German-Chinese research team has succeeded in realizing a quantum bit in a semiconductor nanostructure. Using a special energy transition, the researchers created a superimposed state in a quantum dot – a narrowly defined area of ​​the semiconductor – in which an electron hole had two different energy levels at the same time. Such superposition states are the basis for quantum computing. Excitation of the state usually requires a laser that emits light in the terahertz range. However, this wavelength is too long to be able to focus the beam onto the tiny quantum dot. The German-Chinese group has now succeeded in excitation with two finely tuned short-wave lasers.

The team led by Feng Liu from Zhejiang University in Hangzhou reports on the results together with a group led by Dr. Arne Ludwig from the Ruhr University Bochum and other researchers from China and Great Britain in the journal " Nature Nanotechnology ", published online on July 24, 2023.

Lasers trigger radiant Auger process
The team made use of what is known as the radiant Auger process: an electron falls from a higher to a lower energy level and in doing so releases its energy partly in the form of a single light particle, partly it transfers the energy to another electron. The same process can also be observed with electron holes - missing electrons. In 2021, a research team succeeded for the first time in specifically stimulating the radiating Auger process in a semiconductor.

In the current work, the researchers showed that the radiative Auger process can be manipulated coherently: they used two different laser beams whose intensities were in a specific ratio to one another. With the first laser, they raised an electron-hole pair in the quantum dot to a higher energy level. This is how a quasiparticle was formed from two holes and an electron. With a second laser, they triggered the radiant Auger process to bring one of the holes into even higher energy states.

Two states at once
Using finely tuned laser pulses, the team created a superposition between the hole ground state and the higher energy state. So the electron hole existed in both states at the same time. Such overlays are the basis for quantum bits, which, unlike conventional bits, not only exist in the states "0" and "1", but also in overlays of them.

The high-purity semiconductor samples for the experiment were produced by Hans-Georg Babin at the Ruhr University in Bochum under the direction of Dr. Arne Ludwig at the Chair of Solid State Physics of Prof. Dr. Andrew Wieck. Among other things, the ensemble homogeneity of the quantum dots was increased and attention was paid to the high purity of the structures produced. These measures made it easier for the Chinese partners around Jun-Yong Yan and Feng Liu to carry out the experiments.