Nature Nanotech. Novel quantum bits discovered in semiconductor structures
A team of German and Chinese researchers has succeeded in creating a type of quantum bit in semiconductor nanostructures. Using a special energy conversion, the researchers have created a superposition state in a quantum dot (a tiny region of a semiconductor) in which an electron hole has two different energy levels at the same time.
Such superposition states are the basis of quantum computing. However, excitation of this state requires large free-electron lasers capable of emitting light in the terahertz range. In addition, this wavelength is too long to focus the beam onto tiny quantum dots.
The German-Chinese team has now achieved excitation using two fine-tuned short-wavelength laser pulses.
The research team headed by Feng Liu of Zhejiang University in Hangzhou, together with a research group led by Dr. Arne Ludwig of Ruhr-Universität Bochum, as well as other researchers from China and the United Kingdom, report their findings in the July 24, 2023 online issue of the journal Nature Nanotechnology.
"Coherent control of a high-orbital hole in a semiconductor quantum dot."
The research team utilized the radiative Rauscher effect. In this process, an electron recombines with a hole, partly releasing energy in the form of a single photon and partly transferring energy to another electron. The same process can be observed in electron vacancies (in other words, lost electrons).
In 2021, a team of researchers succeeded for the first time in exclusively exciting the radiative Oechs process in semiconductors.
In the current project, the researchers demonstrated that the radiation-ohmic process can be coherently driven: they used two different laser beams whose intensities were in a specific ratio to each other. With the first laser beam, they excited electron-hole pairs in quantum dots, producing quasiparticles consisting of two holes and one electron. Using the second laser beam, they triggered the radiative Oechs effect, which boosts a hole to a series of higher energy states.
Radiative Ohmic emission from a positively charged QD
The team used fine-tuned laser pulses to create a superposition between the hole's ground state and higher-energy state; in this way, the hole exists in both states at the same time. This superposition is the basis of the quantum bit, which, unlike conventional bits, exists not only in the 0 and 1 states, but also in a superposition of these two states.
Hans-Georg Babin, under the guidance of Dr. Arne Ludwig from the Chair of Applied Solid State Physics headed by Prof. Andreas Wieck, fabricated the high-purity semiconductor samples used for the experiment at the Ruhr-Universität Bochum. In the process, the researchers improved the homogeneity of the ensemble of quantum dots and ensured the high purity of the produced structures.
These measures also facilitated experiments by Chinese partners working with Jun-Yong Yan and Feng Liu.
Rabi oscillations in high orbital holes
Ramsey interference
Direct measurement of single-well relaxation kinetics
Hole release time versus energy separation
An optical scheme demonstrated in this study allows for the coherent control of a high orbital cavity by an excited Russo-hysteresis process with a fidelity close to unity. The coherence of the Rochelle process is demonstrated by the observed Rabi oscillations and Ramsey interference. The coherence time is mainly limited by the cavity relaxation time. This scheme simplifies the requirements for entering the "orbital state" at terahertz intervals, thus opening a new avenue for deeper understanding of the carrier dynamics of quantum emitters and the creation of "orbital"-based quantum photonic devices. This opens a new avenue for deeper understanding of the carrier dynamics of quantum emitters and the creation of orbital-based quantum photonic devices.
By applying this scheme to positively charged QDs, for example, we were able to observe the phonon bottleneck effect and the single-phonon assisted cascade relaxation process.
The team says, "Our results are a key step in the study of various quantum optical phenomena in solid-state multi-orbital cascade systems. Examples include excited Raman transitions between two orbital states and the generation of orbital-frequency entanglement between charge carriers and photons."
In addition, pulse-driven excited Rauscher processes can be used to eliminate excited QDs at high speed and high fidelity, providing a new mechanism for achieving ultrafast all-optical switching and super-resolution imaging. "In addition to QDs, our approach has the potential to be extended to coherently control other quantum emitters: including two-dimensional materials and colloidal nanostructures - where the ohmic process plays a key role in exciton recombination dynamics."
[1] https://www.eurekalert.org/news-releases/996626
