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A team of researchers from Finland, Sweden, and Japan have developed a semiconductor component in which quantum information can be efficiently exchanged between electron spin and photons at room temperature and above. The new method, described in an article recently published in Nature Photonics, is based on amplifying the spin polarisation in a semiconductor structure made up of InAs quantum dots and nitrogen-containing semiconductor compounds. The component was fabricated by researchers at Tampere University using the molecular beam epitaxy (MBE) technique.
“Conventional electronics carry and store information using transistors that switch between states, controlled by electrons with a negative charge. This technology is seeing its limits in terms of processing speed and capacity, as well as energy needs (a major example is the huge increase of electricity demand attributed to data canters). Emerging quantum technologies address these challenges by employing the quantum states of photons or the spin of electrons for information processing. Photonic quantum systems are already having a strong impact in new applications areas, such as quantum computers, quantum cryptography, or quantum sensing. On the other hand, spintronics, which exploits the quantum states of electrons stored in their momentum, could offer the needed quantum interface to electronic world. Connecting photonic and electronic quantum platforms is instrumental for the advance of quantum technologies towards practical applications”, says Mircea Guina, professor in the Faculty of Engineering and Natural Sciences at Tampere University.
Semiconductor-based method works at room temperature
In recent decades, the advancements in spintronics have been based on the use of metals, which has enabled the storing of large amounts of data. However, the use of spintronics based on semiconductors would offer many advantages, first of all because the semiconductors form the backbone of today’s electronics and photonics.
“One important advantage of semiconductor-based spintronics is the possibility to convert the information that is represented by the spin state and transfer it to light, and vice versa. This technology, known as opto-spintronics, would make it possible to integrate information processing and storage based on spin into information transfer through light”, says Weimin Chen, professor at Linköping University, Sweden, who led the project.
According to the paper, a serious problem in the development of spintronics has been that electrons tend to switch and randomise their direction of spin when the temperature rises. This means that the quantum information coded by the electron spin states is lost. For practical use, one should be able to manipulate the electron spin and maintain its orientation at above room temperature. The highest electron spin polarisation at room temperature achieved previously was 60%. The combined efforts of the research teams from Tampere University, Linköping University, and Hokkaido University have enabled demonstration of electron spin polarisation greater than 90% and at room temperature.
The breakthrough discovery is based on an opto-spintronic nanostructure that was fabricated at Tampere University from layers of different semiconductor materials. It contains nanoscale regions called quantum dots exhibiting strong quantum confinement. First, electrons with different spins generated in the semiconductor are filtered by a specific layer, which allow to pass only one-type of spin. In fact, an important part of the study was focused on developing the spin filter layer – more specifically, the work involved engineering of GaAsN heterostructures placed in the proximity of InAs quantum dots. When a spin polarised electron passing through the filter impinges on a quantum dot, it is transformed back to light with a quantum state determined by the electron spin. To put in an application perspective, the system can be used to generate entanglement between the qubit spin states and single photon qubits used for quantum information transfer and processing.
Leveraging expertise from materials developed for novel solar cells and laser diodes
Similar materials (i.e. GaAsN and InAs QDs) are already being used in optoelectronic technology based on gallium arsenide, and the researchers believe that this can make it easier to integrate spintronics into existing electronic and photonic components.
“We are very happy that our long-term efforts to increase the expertise required to fabricate highly-controlled N-containing semiconductors is defining a new frontier in spintronics. So far, we have had a good level of success when using such materials for optoelectronics devices, most recently in high-efficiency solar-cells and laser diodes. Now we are looking forward to continuing this work and to unite photonics and spintronics, using a common platform for light-based and spin-based quantum technology,” says Professor Guina, head of the research team at Tampere University in Finland.
“We succeeded in constructing a spin-optimised nanostructure, because we have persistently developed the precise control of semiconductor structures and built an in-depth understanding of the related material science aspects. For many years now we have made it a mission to do our part to secure Finland’s position as a world-leader in emerging photonics technology,” Guina emphasises.
At TAU the research was funded by the ERC AdG project “AMETIST” and the Academy of Finland projects “QuantSi” and “Nanolight”. The work is also part of the Finnish Flagship on Photonics Research and Innovation (PREIN).