Advances in quantum bit creation for semiconductor nanostructures

A German/Chinese research team has created a quantum superposition state in a semiconductor nanostructure that could serve as a basis for quantum computing. Using a special energy transition, the researchers created a superposition state in a quantum dot — a tiny area of the semiconductor — in which an electron hole simultaneously possessed two different energy levels. Such superposition states are fundamental for quantum computing. However, excitation of the state would require a large-scale free-electron laser that can emit light in the terahertz range. Additionally, this wavelength is too long to focus the beam on the tiny quantum dot. The research team has now achieved the excitation with two finely tuned short-wavelength optical laser pulses.

The team, led by Feng Liu from Zhejiang University in Hangzhou, together with a group led by Dr Arne Ludwig from Ruhr University Bochum, reported their findings in the journal Nature Nanotechnology. The team made use of the so-called radiative Auger transition. In this process, an electron recombines with a hole, releasing its energy partly in the form of a single photon and partly by transferring the energy to another electron. The same process can also be observed with electron holes — or missing electrons.

The researchers showed that the radiative Auger process can be coherently driven: they used two different laser beams with intensities in a specific ratio to each other. With the first laser, they excited an electron-hole pair in the quantum dot to create a quasiparticle consisting of two holes and an electron. With a second laser, they triggered the radiative Auger process to elevate one hole to a series of higher energy states.

The researchers used finely tuned pulses to create a superposition between the hole ground state and the higher energy state — therefore, the hole existed in both states simultaneously. Such superpositions are the basis for quantum bits, which exist not only in the states “0” and “1”, but also in superpositions of both.

Hans-Georg Babin produced the high-purity semiconductor samples for the experiment at Ruhr University Bochum. In the process, the researchers increased the ensemble homogeneity of the quantum dots and ensured the high purity of the structures produced. This facilitated the performance of the experiments by the Chinese partners working with Feng Liu and Jun-Yong Yan.

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