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arXiv:2208.10094 · Nature Physics

Direct manipulation of a superconducting spin qubit strongly coupled to a transmon qubit

Marta Pita‐Vidal, Arno Bargerbos, Rok Žitko, Lukas Johannes Splitthoff, Lukas Grünhaupt, Jaap J. Wesdorp, Yu Liu, Leo P. Kouwenhoven, Ramón Aguado, Bernard van Heck, Angela Kou, Christian Kraglund Andersen

Published May 22, 2023• 119 citations
arXivHTMLPDF
DOI
10.1038/s41567-023-02071-x

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Superconducting transmon qubits with strong anharmonicity and insensitivity to offset charge are highly desirable for low-error implementation. In this work we propose a c-axis junction, comprising triplet superconductors, and set at a relative twist angle. Invoking spin-orbit coupling and spin polarization, which are known to occur in the material platform of choice, we examine the resulting transmon Hamiltonian. This junction allows for direct control of the single and double Cooper pair tunneling strength, and most remarkably, an anomalous magnetic flux -- i.e. a phase offset equivalent to magnetic flux, yet in zero magnetic field. Having control over these three parameters -- single and double pair tunneling and anomalous flux -- allows for optimal design of the transmon qubit. Interestingly, in this architecture, the anomalous flux is determined by the twist angle of the junction, thereby offering a novel zero-field functionality. Our key results rely on symmetry arguments, for concreteness we demonstrate the implementation of our concept using a model of moiré graphene-based c-axis junctions.
Longitudinal coupling, which generates entanglement without energy exchange, has extensive applications in quantum computing and quantum simulation. However, achieving available direct and flexible longitudinal couplings between highly coherent superconducting qubits is challenging. In this study, a method is developed to achieve direct and flexible longitudinal couplings between superconducting qubits, including the direct longitudinal coupling between capacitively shunted flux qubits (C‐shunt flux qubits) and that between transmon qubits. Herein, first, a variant of the prototype C‐shunt flux qubit, the concentric C‐shunt flux qubit, is introduced. It is demonstrated that the large mutual inductance between concentric C‐shunt flux qubits produces a longitudinal coupling strength up to 21 MHz. It is also demonstrated that the method can be used to realize a strong longitudinal coupling between two transmon qubits. In the findings, it is demonstrated that it is possible to achieve a flexible and efficient direct longitudinal coupling between superconducting qubits, and open up new possibilities for the development of quantum gate operation methods as well as quantum simulation methods.