Double-Transmon Coupler: Fast Two-Qubit Gate with No Residual Coupling for Highly Detuned Superconducting Qubits

Abstract

Although two-qubit entangling gates are necessary for universal quantum computing, they are notoriously difficult to implement with high fidelity. Recently, tunable couplers have become a key component for realizing high-fidelity two-qubit gates in superconducting quantum computers. However, it is still difficult to achieve tunable coupling free of unwanted residual coupling—in particular, for highly detuned qubits, which are desirable for mitigating qubit-frequency crowding or errors due to crosstalk between qubits. We thus propose a kind of tunable coupler that we call a double-transmon coupler, because it is composed of two transmon qubits coupled through a common loop with an additional Josephson junction. By controlling the magnetic flux in the loop, we can achieve not only fast high-fidelity two-qubit gates but also no residual coupling during the idle time, where computational qubits are highly detuned fixed-frequency transmons. The proposed coupler is expected to offer an alternative approach to higher-performance superconducting quantum computers.

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