Longitudinal coupling synthesis in superconducting qubits

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. (© 2024 Wiley‐VCH GmbH)