High-fidelity all-microwave CZ gate with partial erasure-error detection via a transmon coupler

TL;DR

This paper presents an all-microwave controlled-Z gate for superconducting qubits that achieves high fidelity, suppresses residual interactions, and incorporates partial erasure-error detection to enhance quantum error correction.

Contribution

The authors propose and experimentally demonstrate a novel all-microwave CZ gate using a transmon coupler with multi-path coupling, reducing residual ZZ interactions and enabling erasure-error detection.

Findings

Achieved high-fidelity CZ gate with suppressed residual ZZ interactions.

Implemented erasure-error detection by measuring the coupler state post-gate.

Maintained small net transverse interaction while increasing coupling speed.

Abstract

Entangling gates between neighboring physical qubits are essential for quantum error correction. Implementing them in an all-microwave manner simplifies signal routing and control apparatus of superconducting quantum processors. We propose and experimentally demonstrate an all-microwave controlled-Z (CZ) gate that achieves high fidelity while suppressing residual ZZ interactions. Our approach utilizes a fixed-frequency transmon coupler and multi-path coupling, thereby sufficiently reducing the net transverse interaction between data transmons to suppress residual ZZ interactions. The controlled phase arises from the dispersive frequency shift of the $|gf\rangle$$\unicode{x2013}$$|eg\rangle$ transition between the coupler and one of the data transmons conditioned on the state of the other data transmon. Driving the transitions at the midpoint of two dispersively shifted resonance frequencies induces state-dependent geometric phases to achieve the CZ gate. Crucially, with this scheme, we can maintain a small net transverse interaction between two data transmons while increasing the coupling between the data and coupler transmons to accelerate the CZ-gate speed. Additionally, we measure the coupler state after the gate to detect a subset of decoherence-induced failures that occur during the gate operation. These events constitute erasure errors with known locations, enabling erasure-aware quantum error-correcting codes to improve future logical qubit performance.