[Paper Review] Long-distance transmon coupler with CZ gate fidelity above $99.8\%$

Fabian Marxer, Antti Vepsäläinen+37 more|arXiv (Cornell University)|Aug 19, 2022

Quantum Information and Cryptography45 references5 citations

TL;DR

This paper presents a long-distance tunable coupler architecture using a floating transmon coupler and waveguide extenders to enable high-fidelity CZ gates between superconducting qubits separated by at least 2 mm. By mediating couplings through waveguides instead of direct capacitive coupling, the design maintains >50 MHz coupling strength, reduces non-nearest-neighbor interactions, and enables individual qubit readout resonators and Purcell filters, achieving a CZ gate fidelity of 99.81 ± 0.02%.

ABSTRACT

Tunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures. Here, we demonstrate a tunable qubit-qubit coupler based on a floating transmon device which allows us to place qubits at least 2 mm apart from each other while maintaining over 50 MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter needed for fast high-fidelity readout. In addition, the large qubit-qubit distance reduces unwanted non-nearest neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. Using the proposed flexible and scalable architecture, we demonstrate a controlled-$Z$ gate with $(99.81 \pm 0.02)\%$ fidelity.

Motivation & Objective

• To overcome the limitations of short qubit distances in tunable coupler architectures that lead to high spurious non-nearest-neighbor coupling.
• To enable scalable quantum processor designs by increasing physical separation between qubits while maintaining strong, tunable coupling.
• To reduce crosstalk from control lines and accommodate individual qubit readout resonators and Purcell filters by increasing qubit spacing.
• To achieve high-fidelity two-qubit gates (specifically CZ) in a system with large qubit-qubit distances and minimal static ZZ interaction.
• To demonstrate that waveguide-mediated coupling can maintain high coupling strength and gate fidelity over long distances, enabling improved chip layout flexibility.

Proposed method

• Implement a floating transmon coupler that is tunable via external flux, enabling dynamic control of qubit coupling.
• Use waveguide extenders to mediate coupling between the coupler and qubits instead of direct capacitive coupling, decoupling coupling strength from qubit distance.
• Design a quasi-lumped-element circuit model with effective lumped-element capacitances arising from waveguide extenders to simulate coupling strengths.
• Employ a flip-chip architecture where qubits and coupler are on a bottom chip, and control lines cross over on a top chip via long perpendicular transmission lines.
• Simulate and optimize crosstalk by calculating capacitance coupling ratios between components and transmission lines at various crossing positions.
• Use numerical simulations to evaluate non-nearest-neighbor coupling (e.g., g13) and identify low-crosstalk regions for control line routing.

Experimental results

Research questions

• RQ1 Can a tunable coupler maintain high coupling strength (>50 MHz) over qubit separations of at least 2 mm?
• RQ2 Does waveguide-mediated coupling significantly reduce spurious non-nearest-neighbor interactions compared to direct capacitive coupling?
• RQ3 Can individual qubit readout resonators and Purcell filters be integrated in a 2 mm-separated qubit architecture without compromising gate fidelity?
• RQ4 What is the achievable CZ gate fidelity in a long-distance coupler architecture with minimal static ZZ interaction?
• RQ5 How does the coupler's flux tuning behavior and coherence affect gate fidelity, especially near the sweet spot?

Key findings

• The proposed coupler design maintains a coupling strength of over 50 MHz between the coupler and each qubit at a qubit-qubit distance of 1960 µm.
• The measured CZ gate fidelity reaches 99.81 ± 0.02%, exceeding the 99.8% threshold and demonstrating high-fidelity two-qubit operation.
• Simulations show that increasing qubit-qubit distance significantly reduces spurious next-nearest-neighbor coupling (g13), improving gate selectivity.
• The waveguide-mediated coupling architecture allows multiple control lines to cross over the structure with minimal crosstalk, as confirmed by capacitance coupling ratio simulations.
• The effective decoherence times for the coupled modes are calculated using weighted averages of uncoupled qubit and coupler coherence times, supporting the high gate fidelity.
• A sharp dephasing feature at 3.8 GHz is observed and attributed to a spectrally active two-level system (TLS), but it does not significantly degrade gate performance.