Researchers have achieved high gate fidelities up to 99.98% using a new double-transmon coupler. This development enhances quantum computing performance and supports the advancement toward fault-tolerant systems.
Researchers from the RIKEN Center for Quantum Computing and Toshiba have developed a quantum computer gate using a double-transmon coupler (DTC), a device previously proposed in theory to enhance the fidelity of quantum gates significantly. With this innovation, the team achieved a fidelity of 99.92% for a two-qubit device known as a CZ gate and 99.98% for a single-qubit gate.
This milestone, part of the Q-LEAP project, not only improves the performance of noisy intermediate-scale quantum (NISQ) devices but also lays the groundwork for fault-tolerant quantum computation through more effective error correction.
Enhanced Gate Fidelity With DTC
The DTC is a novel tunable coupler comprising two fixed-frequency transmons—a type of qubit designed to be less sensitive to noise caused by charge—connected through a loop containing an additional Josephson junction. This architecture addresses a critical challenge in quantum computing: achieving high-fidelity connections between qubits. High fidelity is crucial for reducing errors and increasing the reliability of quantum computations.
The DTC stands out by minimizing residual interactions while enabling fast, high-fidelity two-qubit gate operations, even for qubits with significant frequency differences (detuning). While single-qubit gates have reached fidelity levels of 99.9%, two-qubit gates have typically had error rates of 1% or more, primarily due to unwanted qubit interactions, such as the ZZ interaction. The DTC approach directly tackles these issues, representing a major advance in quantum gate technology.
Optimizing Quantum Error Correction
A key of the current work, published in Physical Review X, is the construction of a gate using state-of-the-art fabrication techniques using a type of machine learning known as reinforcement learning. This approach allowed the researchers to translate the theoretical potential of the DTC into practical application. They used this approach to achieve a balance between two types of remaining error—leakage error and decoherence error—that remained within the system, selecting a length of 48 nanoseconds as an optimal compromise between the two error sources. Thanks to this, they were able to achieve fidelity levels that are among the highest reported in the field.
Future Prospects in Quantum Technology
According to Yasunobu Nakamura, director of the RIKEN Center for Quantum Computing, “By reducing the error rates in quantum gates, we have made more reliable and accurate quantum computations possible. This is particularly important for the development of fault-tolerant quantum computers, which are the future of quantum computing.”
He continues, “This device’s ability to perform effectively with highly detuned qubits makes it a versatile and competitive building block for various quantum computing architectures. This adaptability ensures that it can be integrated into existing and future superconducting quantum processors, enhancing their overall performance and scalability. In the future, we plan to try to achieve a shorter gate length, as this could help minimize the incoherent error.”
Reference: “Realization of High-Fidelity CZ Gate Based on a Double-Transmon Coupler” by Rui Li, Kentaro Kubo, Yinghao Ho, Zhiguang Yan, Yasunobu Nakamura and Hayato Goto, 21 November 2024, Physical Review X.
DOI: 10.1103/PhysRevX.14.041050
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