Quantum computers are expected to solve problems that no classical machine can solve, with potential applications spanning several fields. Realizing this potential hinges on performing quantum operations accurately enough to sustain error correction. One of the dominating error sources in superconducting quantum processors is the two-qubit gate. Tunable couplers make these gates fast while keeping idling qubits decoupled. However, the typical design, the single-transmon coupler, cannot fully cancel the residual ZZ interaction for qubits detuned by more than their anharmonicity [1]. The double-transmon coupler (DTC) avoids this limitation, so qubits can stay fixed-frequency and highly detuned [1]. In simulation, the DTC can also reduce the next-nearest-neighbor coupling by approximately one order of magnitude compared to the single-transmon coupler [2]. Devices built on this idea have reached CZ-gate fidelities of 99.90% [3] and decoherence-limited parametric iSWAP gates [4].

In DiCarlo lab, we always work to improve the overall performance of our superconducting quantum processors. We are looking for a motivated Master’s student (QIST or Applied Physics) to join us in designing our next generation of couplers. Below is a brief description of the masters end project (MEP) that we offer.

Project description

You will develop simulations of the DTC to predict the unwanted residual interactions, the robustness of its cancellation point, and the fidelity of two-qubit gates. You will use them to map the design parameter space and translate your parameters into a device layout, then deliver the design to our fabrication team.

Should chip fabrication be completed within your project timeframe, you will participate in its cryogenic characterization. If you have a strong background in quantum mechanics and programming and are interested in this challenge, please contact Yuejie Xin (Y.Xin-2@tudelft.nl, daily supervisor) to learn more.

For information about our lab and publications: https://qutech.nl/lab/dicarlo-lab-welcome/.

References and further reading

[1] Goto, Double-transmon coupler: fast two-qubit gate with no residual coupling for highly detuned superconducting qubits, Phys. Rev. Applied 18, 034038 (2022).

[2] Kubo, Ho, and Goto, High-performance multiqubit system with double-transmon couplers, Phys. Rev. Applied 22, 024057 (2024).

[3] Li et al., Realization of high-fidelity CZ gate based on a double-transmon coupler, Phys. Rev. X 14, 041050 (2024).

[4] Tiwari et al., High-fidelity iSWAP gate with double transmon coupler, arXiv:2604.27080 (2026).

[5] Li et al., Capacitively shunted double-transmon coupler realizing bias-free idling and a high-fidelity CZ gate, Phys. Rev. Applied 23, 064069 (2025).