Achieving reliable connections between quantum bits, or qubits, remains a central challenge in building practical quantum computers, and researchers are continually seeking ways to improve the accuracy of these connections. Nikita S. Smirnov, Aleksei R. Matanin, and Anton I. Ivanov, all from Bauman Moscow State Technical University, alongside their colleagues, demonstrate a significant advance in this area by creating highly accurate two-qubit gates using superconducting transmon qubits. Their innovative approach combines a carefully designed tunable coupler with precisely shaped control pulses, resulting in a gate performance exceeding expectations and offering inherent protection against common sources of error. This method not only simplifies the calibration process but also suggests a clear pathway towards building larger, more stable quantum processors capable of tackling complex computational problems.
Superconducting Qubit Materials and Fabrication
This body of research comprehensively explores superconducting qubits and the field of quantum computing, focusing on the fundamental building blocks of quantum computers and the techniques used to create them. Investigations center on various qubit types, with transmon qubits receiving significant attention, alongside explorations of flux and phase qubits. A substantial portion of the research emphasizes materials science and fabrication techniques, with scientists refining Josephson junctions to improve their quality, reproducibility, and control. Precise control of thin film growth is paramount, alongside efforts to minimize defects and control interfaces to enhance qubit coherence, while researchers also investigate different substrate materials to reduce noise and optimize qubit properties.
Detailed characterization of qubit properties, including frequency, coherence times, and relaxation rates, is also a key focus, as is the development of techniques to precisely manipulate qubit states using microwave pulses. Beyond fabrication, scientists are developing methods to accurately control qubits, including optimizing microwave pulses for high-fidelity gates, calibrating qubit parameters, and suppressing noise through dynamical decoupling. Optimal control theory and continuous variable quantum computing are also being explored, alongside quantum error correction and mitigation, with research focusing on error detection and correction codes, error mitigation techniques, and surface codes. Some research also explores topological qubits, which offer inherent error protection.
The research extends to exploring how to utilize quantum computers, with investigations into variational quantum algorithms, quantum machine learning, quantum simulation, and applying quantum algorithms to solve optimization problems. Specific technologies under investigation include 3D integration to increase qubit density, cryogenic control electronics for operation at extremely low temperatures, fluxonium qubits with potentially improved coherence, and continuous variable quantum computing. The research demonstrates a strong presence of researchers from Russia and Eastern Europe, particularly focused on materials science and qubit fabrication, with a key trend being the emphasis on improving existing qubit technologies rather than exploring entirely new types, suggesting a focus on building practical quantum computers in the near term. Hybrid quantum-classical computing, error correction, and materials science are all identified as critical areas for advancement.
High Fidelity Gates with Tunable Couplers
Scientists have engineered a novel approach to achieving high-fidelity two-qubit gates using superconducting transmon qubits. The method involves a precisely controlled pulse delivery protocol, enabling the creation of arbitrary controlled-phase gates modulated by an independent arbitrary waveform generator. This system combines a tunable coupler design with bipolar flux-pulsing, resulting in a gate performance exceeding 99. 48% and offering inherent protection against low-frequency noise. The team optimized energy levels within the system to mitigate leakage to the coupler and suppress unwanted residual interactions, ensuring the fidelity of the gate operations. Numerical simulations, modeling the system as three qutrits, indicate that an error below 10 -5 is achievable, confirming the potential for highly accurate quantum computations. To validate scalability, scientists implemented the scheme on a high-fidelity four-qubit processor, demonstrating its applicability to more complex quantum circuits.
Fidelity Two-Qubit Gates Demonstrated
Scientists have achieved a significant advance in superconducting quantum computing by demonstrating high-fidelity two-qubit gates with exceptional performance. The team developed a novel scheme utilizing superconducting transmon qubits and a precise control pulse delivery protocol, enabling the creation of arbitrary controlled-phase gates governed by an independent arbitrary waveform generator. This approach combines the benefits of tunable coupler design and bipolar flux-pulsing, resulting in a gate fidelity exceeding 99. 9%. The breakthrough addresses key challenges in quantum computation by minimizing residual interactions between qubits and incorporating inherent protection against low-frequency noise, while maintaining ease of calibration.
Through careful optimization of system energy levels, researchers effectively mitigated leakage to the coupler and suppressed unwanted residual interactions, paving the way for more stable and reliable quantum operations. Numerical simulations of a three-qubit system indicate the potential to achieve error rates below a threshold critical for scalable quantum computation. Experimental validation on a four-qubit superconducting processor confirms the scalability of the proposed scheme, demonstrating simultaneous single-qubit gate fidelities exceeding 99. 925% and CZ gate fidelities exceeding 99.13%.
These results represent a substantial improvement over previous work and demonstrate the potential for building larger, more complex quantum circuits. The team achieved these high fidelities by carefully engineering the energy levels of the qubits and coupler, minimizing unwanted interactions and leakage errors, and optimizing pulse shapes for precise control. This advancement promises to accelerate the development of practical quantum computers capable of tackling complex computational problems.
Fidelity Two-Qubit Gate Demonstration
This research presents a new approach to creating high-fidelity two-qubit gates, essential for building scalable quantum computers. Scientists have developed a system using superconducting transmon qubits and a carefully designed control pulse delivery protocol, achieving a high-fidelity controlled-phase gate with a performance of 99. 48%. This gate benefits from minimal unwanted interactions between qubits and inherent protection against noise, while remaining relatively simple to calibrate. The key innovation lies in a unique qubit architecture employing bipolar pulses and tunable coupling, which suppresses errors arising from residual qubit interactions and leakage to unwanted energy levels.
Demonstrations on a four-qubit processor confirm the scalability of this approach, achieving average CZ-gate fidelity of 99. 26% and peak fidelity of 99. 34%. The team acknowledges that the fidelity of these gates is currently limited by decoherence and short-timescale pulse distortions, suggesting that improvements in coherence times or pulse shaping could further enhance performance.
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🗞 High-fidelity two-qubit gates with transmon qubits using bipolar flux pulses and tunable couplers
🧠ArXiv: https://arxiv.org/abs/2509.04965