Quantum states can be precisely controlled with the help of tiny carbon rings measuring only a few nanometres in size. This is made possible by a class of rarely utilized electromagnetic dipoles called toroidal moments. Using computer simulations, physicists at Martin Luther University Halle-Wittenberg (MLU) have now found a way to generate and control these nanostructures without any loss. The findings were published in the journal “npj Computational Materials” and create new opportunities for quantum computer technology.
In physics, there are two well-known types of dipoles: Electric dipoles generate electric signals, such as those found in batteries and antennas. Magnetic dipoles, like a charged coil or a bar magnet, are created through moving charges or permanent magnets. These traditional dipoles are joined by a third class of charge-current distributions which, up until now, have been difficult to replicate at the molecular level: toroidal dipoles. “You can picture it like this: A coil bearing an electric current encloses a magnetic field which disappears outside the coil. Connecting the ends of the coil creates a toroidal system that is electrically neutral and generates no external electric or magnetic fields,” explains physicist Professor Jamal Berakdar at MLU, who conducted the study together with Dr Arkamita Bandyopadhyay.
Even though researchers knew that stable toroidal moments could exist, it was unclear how to generate and control them at the nano level; problems arise when they are reduced down to the nanoscale. “Conventional toroidal coils work well as long as they are large enough – for example, when they have a radius measuring one centimetre. However, if the coil is too small, the current does not flow efficiently in the circuit and there are high losses,” explains Arkamita Bandyopadhyay.
Researchers at MLU have used computer simulations to demonstrate how toroidal moments can be generated in so-called nanotori. These are ring-shaped structures made up of carbon atoms that look like tiny doughnuts. When a constant electric field is applied to these structures, the electrons move in a 3D vortex around the ring, thereby forming a toroidal moment. “We use computer simulations to show how toroidal moments can be generated without loss at the nanoscale, as well as be controlled, excited and switched,” says Jamal Berakdar.
The findings of the study open up new possibilities in the field of quantum computing. One example is being able to precisely control superconductors through which current can flow with virtually no loss. Existing methods often require magnetic or electric fields that, at the nanoscale, are very difficult to focus. These not only affect the superconductor but also excite other nearby particles. This can lead to signal noise or high energy consumption. “This problem can be circumvented by utilising toroidal moments in carbon nanotori as they can directly alter quantum mechanical phases,” concludes Bandyopadhyay.