Quantum computing promises state-of-the-art computational power. Many problems that are computationally intractable by classical computing methods could be solved in a matter of minutes by quantum computers of sufficient size. While this undoubtedly poses many challenges for current cryptography methods,1 this could be a huge boon for complex mathematical modeling, including for weather and computational chemistry.2
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While larger and larger quantum computers are being demonstrated3,4, many of these are reliant on technologies that require very precise engineering, low temperatures, and incredible vibrational stability.
Finding architectures that are suitable for the creation of the qubits with sufficient noise suppression has been one of the key challenges to overcome in the development of larger systems.
One approach to building a quantum computer has been to use the polarization of light to encode information. We can create photons, particles of light, one at a time and in a controlled polarization state. Varying those states can be used to change the information carried by each qubit.
Depending on the relative orientation and motion of the electric and magnetic fields composing the light, a light wave can be considered to have linear, circular, or even helical polarization. Often, different optical materials must be used to generate the different light polarizations.
A team of researchers at Nagoya University has now found a way to create light-emitting diodes (LEDs) that will emit chiral valley polarized light and operate at room temperature.5 Previous approaches have relied on the use of very low temperatures and bulky magnets to generate similar polarizations so this represents a significant step towards optical architectures that could be incorporated into room temperature quantum computers.
The research team used tungsten disulfide on a sapphire substrate, coated in an ion-gel film as its starting material. Tungsten disulfide is a two-dimensional material that has been used in semiconducting materials but has recently gained interest for its ability to generate light at room temperature when an electrical charge is applied.6
What the team at Nagoya University found was that, when the charge was applied at -193 °C, circularly polarized light would be emitted from all regions of the material. However, as they continued to raise the temperature of the device, some regions would stop emitting the circularly polarized light whereas other regions would continue.
On the further characterization of the strain across the material, the team realized that the regions where the sapphire substrate was under the most strain, corresponded to those that would remain emissive at room temperature. The inhomogeneities in the strain forces had arisen during the synthesis process of the material.
To test this, the team then designed tungsten disulfide devices on plastic substrates and loaded these onto a bending stage. The stage could apply a controlled amount of force to the device while the electrical current was present.
The team observed that when the current and strain forces were present, chiral light would be generated and the direction of the circularly polarized light could be switched simply by changing the direction of the current. A device with this level of control and operation at room temperature is a significant step towards practical quantum computing – particularly as chiral light is thought to be able to carry large amounts of information.
Further Development in Strain Engineering and Electrode Design
While this work represents a significant step forward towards room-temperature quantum computing and in the creation of chiral emitters that do not require spin alignment during manufacturing, the team suggests that developments in strain engineering and electrode design are the next steps towards the creation of a high-performance room-temperature chiral LED.
A high-performance device must have sufficient brightness, efficiency, and, most importantly, quality of polarization.
Since the first two-dimensional monolayer semiconductors were found to be a single quantum emitter – a property essential for their use in quantum computing – these two-dimensional materials seem to offer many advantages over bulk semiconductors in terms of photon extraction efficiency.7
The challenge now will be to maintain the good polarization purity observed for the tungsten disulfide while finding ways to boost the efficiency of the device.
Other materials engineering challenges will be finding substrates with the optimal properties to provide the required strain on the semiconductor substrate to achieve the chiral emission at room temperature.
The ability to generate chiral light from LEDs may have an impact beyond quantum computing, including developing more efficient display devices.
Chiral molecules generate a specific response from circularly polarized light, and this can be used to perform spin-selective electron injection, which may overcome fundamental limitations in the efficiency of LEDs in comparison to fluorescent lamps.
References and Further Reading
- Mavroeidis, V., Vishi, K., Zych, M. D., & Jøsang, A. (2018) The impact of quantum computing on present cryptography. International Journal of Advanced Computer Science and Applications, 9(3), 405–414. https://doi.org/10.14569/IJACSA.2018.090354
- Lubasch, M., Joo, J., Moinier, P., Kiffner, M., & Jaksch, D. (2020) Variational quantum algorithms for nonlinear problems. Physical Review A, 101(1), 1–7. https://doi.org/10.1103/PhysRevA.101.010301
- Cho, A. (2020) IBM promises a 100 qubit quantum computer. Science Mag. [Online] Available at: https://www.sciencemag.org/news/2020/09/ibm-promises-1000-qubit-quantum-computer-milestone-2023., accessed October 2021
- IBM (2021) Quantum Computing. [Online] Available at: https://www.ibm.com/quantum-computing/, accessed October 2021
- Pu, J., Zhang, W., Matsuoka, H., Kobayashi, Y., Takaguchi, Y., Miyata, Y., Matsuda, K., Miyauchi, Y., & Takenobu, T. (2021) Room-Temperature Chiral Light-Emitting Diode Based on Strained Monolayer Semiconductors. Advanced Materials, 33(36), 1–9. https://doi.org/10.1002/adma.202100601
- Gu, J., Chakraborty, B., Khatoniar, M., & Menon, V. M. (2019) A room-temperature polariton light-emitting diode based on monolayer WS2. Nature nanotechnology, 14(11), 1024–1028. https://doi.org/10.1038/s41565-019-0543-6
- He, Y. M., Clark, G., Schaibley, J. R., He, Y., Chen, M. C., Wei, Y. J., Ding, X., Zhang, Q., Yao, W., Xu, X., Lu, C. Y., & Pan, J. W. (2015) Single quantum emitters in monolayer semiconductors. Nature Nanotechnology, 10(6), 497–502. https://doi.org/10.1038/nnano.2015.75