How ‘Kink state’ control might enable quantum electronics

Led by a dedicated team, these scientists have made strides in controlling the delicate processes essential for creating and operating sophisticated devices like advanced sensors and lasers. They’ve designed a switch that toggles kink states – specific electrical conduction pathways at the edges of semiconducting materials – on and off. The ability to manipulate kink states then allows for precise regulation of electron flow in quantum systems.

Jun Zhu, Team Leader and Professor of Physics at Penn State, explained the concept: “We envision the construction of a quantum interconnect network using the kink states as the backbone. Such a network may be used to carry quantum information on-chip over a long distance, for which a classical copper wire won’t work because it has resistance and therefore cannot maintain quantum coherence.”

The scientist’s finding, recently published in the Science academic journal, could potentially pave the way for further exploration of kink states and their role in electron quantum optics devices and quantum computers.

Zhu further explains: “This switch operates differently from a conventional switch, where the electrical current is regulated through a gate, similarly to traffic through a toll plaza. Here, we are removing and rebuilding the road itself.”

The research was centred around Bernal bilayer graphene, a material comprising two layers of atomically then carbon arranged such that the atoms in one later are misaligned with those in the other. This specific arrangement, when combined with an electric field, gives rise to unique electronic properties, including the ‘Quantum Valley Hall’ effect. This effect involves electrons occupying distinct ‘valley’ states based on their energy and momentum, moving in opposite directions.

Doctoral Student at Penn State, First Author, and Team Member, Ke Huang, noted: “The amazing thing about our devices is that we can make electrons moving in opposite directions not collide with one another — which is called backscattering — even though they share the same pathways. This corresponds to the observation of a ‘quantised’ resistance value, which is key to the potential application of the kink states as quantum wires to transmit quantum information.”

Prior to this recent development, Zhu’s lab had explored kink states to some extent, but achieving quantisation of the Quantum Valley Hall effect required enhanced electronic cleanliness. This was accomplished by integrating a graphite/hexagonal boron nitride stack as a global gate, which facilitated electron flow.

“The incorporation of a graphite/hexagonal boron nitride stack as a global gate is critically important to the elimination of electron backscattering,” says Huang, highlighting this as the study’s significant technical breakthrough.

Remarkably, the quantisation of kink states persisted even at higher temperatures, reaching several tens of Kelvin – something that quantum effects can rarely achieve. Zero Kelvin, which is the absolute absence of thermal energy, corresponds to -273.15°C.

Zhu enthuses: “Quantum effects are often fragile and only survive at cryogenic temperatures of a few Kelvin. The higher temperature we can make this work, the more likely it can be used in applications.”

The researchers experimentally tested the switch they build and found that it could quickly and repeatedly control the current flow. This added to the arsenal of kink state-based quantum electronics widgets that help control and direct electrons – valve, waveguide, beam splitter – previously built by the team at the Zhu lab.

Zhu concluded: “We have developed a quantum highway system that could carry electrons without collision, be programmed to direct current flow and is potentially scalable — all of which lays a strong foundation for future studies exploring the fundamental science and application potentials of this system. Of course, to realise a quantum interconnect system, we still have a long way to go.”

Now, Zhu and her lab’s next goal is the demonstrate how electrons behave like coherent waves when travelling on the kink state highways.

Other authors included Hailong Fu, a former postdoctoral scholar and Eberly Fellow in physics at Penn State, and a current Assistant Professor at Zhejiang University, China; and Kenji Watanabe and Takashi Taniguchi, both with the National Institute for Materials Science in Japan.

The U.S. National Science Foundation, the U.S. Department of Energy, the Penn State Eberly Research Fellowship, the Kaufman New Initiative of the Pittsburgh Foundation, the Japan Society for the Promotion of Science, and the World Premier International Research Initiative of Japan’s Ministry of Education, Culture, Sports, Science and Technology funded this research.