Advanced Quantum State Control in Bilayer Graphene for Quantum Computing

A team of researchers from the Pohang University of Science and Technology (POSTECH) in South Korea and the National Institute for Materials Science (NIMS) in Japan have made a significant breakthrough in controlling the quantum mechanical properties of Andreev-bound states in bilayer graphene-based Josephson junctions. This research was published in the journal Physical Review Letters.

Led by Professors Gil-Ho Lee and Gil Young Cho from POSTECH, the team, which also included Dr. Kenji Watanabe and Dr. Takashi Taniguchi from NIMS, focused on studying the behavior of superconductors, substances that exhibit zero electrical resistance under certain conditions. One of the key phenomena associated with superconductors is the proximity effect, where superconductivity extends into a normal conductor, allowing a supercurrent to flow through a thin normal conductor between two superconductors. This apparatus is known as a Josephson junction, and Andreev-bound states, novel quantum states that play a crucial role in mediating supercurrent flow, are found within the normal conductor.

The number of energy levels in the Andreev bound states is determined by the ratio of the “conduction channel length” (the length of the normal conductor) to the “superconducting coherence length” (the length along which the superconducting state can be maintained in the normal conductor). In the “short junction limit,” when the conduction channel is short, there are only two Andreev-bound state levels. On the other hand, the term “long junction limit” is used when there are more than two pairs.

In this study, the researchers utilized gate voltage to manipulate both the superconducting coherence length and the quadratic energy dispersion of bilayer graphene. By observing the changing Andreev bound states at various gate voltages in real-time using tunneling spectroscopy, they confirmed that their experimental results matched theoretical expectations.

The team’s findings are noteworthy because they have observed Andreev bound states in the long Josephson junction limit, a phenomenon that is predominantly seen in the short Josephson junction limit. This discovery opens up the potential for adjusting the number of energy levels using gate voltage alone, which could have implications in various fields such as quantum computing and high-precision quantum sensors.

The research was funded by several organizations, including the National Research Foundation of Korea, the Ministry of Science and ICT, the ITRC, the Air Force Office of Scientific Research, the Institute for Basic Science (IBS), the Samsung Future Technology Incubation Program, Samsung Electronics, the Basic Science Research Institute, the JSPS KAKENHI, and the World Premier International Research Center Initiative (WPI). The study was published in Physical Review Letters with the title “Andreev Bound States in Bilayer Graphene Josephson Junctions from Short to Long Junction Limits.”

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