A New Method for Controlling Quantum-Bound States in Superconducting Devices

Researchers successfully controlled the quantum mechanical properties of Andreev bound states in graphene-based bilayer Josephson junctions using a gate voltage. Their research was published in Physical inspection lettersThe research team consists of Professors Gil-Ho Lee and Gil Young Cho from the Department of Physics, Pohang University of Science and Technology (POSTECH) in South Korea, in collaboration with Dr. Kenji Watanabe and Dr. Takashi Taniguchi from the National Institute of Materials Science (NIMS) in Japan.

Superconductors are materials that, under certain conditions, such as extremely low temperatures or high pressures, exhibit zero electrical resistance. When a very thin normal conductor is placed between two superconductors, a supercurrent flows through the normal conductor due to the proximity effect, in which superconductivity extends to the normal conductor. This device is known as a Josephson junction.

In a normal conductor, new quantum states called Andreev bound states are created, which are crucial for mediating the flow of supercurrent.

The number of energy levels in Andreev bound states, which determines the electrical properties of the Josephson junction, depends on the ratio of the conduction channel length (the length of a normal conductor) to the superconducting coherence length (the length along which the superconducting state can be maintained in a normal conductor).

When the conduction channel is short and the number of Andreev bound state levels is limited to a pair, the system is said to be in the short junction limit. Conversely, if there are more than two pairs, it is called the long junction limit.

In this study, the research team used gate voltage to control the quadratic energy dispersion of bilayer graphene as well as the superconducting coherence length in real time. Using tunneling spectroscopy developed in their previous work, they observed the real-time change in Andreev bound states at different gate voltages and confirmed that the experimental results were consistent with theoretical predictions.

Geon-Hyoung Park, lead author and researcher at the POSTECH Quantum Information Devices Research and Education Center, stated: “We observed Andreev bound states in the long boundary of the Josephson junction, a phenomenon mainly observed in the short boundary of the Josephson junction. We envision that the number of energy levels can be easily tuned using gate voltage alone, offering potential applications in various fields such as quantum computing and high-precision quantum sensors.”