Demonstration of quantum volume 64 on a superconducting quantum computing system

Abstract: We improve the quality of quantum circuits on superconducting quantum computing systems, as measured by the quantum volume, with a combination of dynamical decoupling, compiler optimizations, shorter two-qubit gates, and excited state promoted readout. This result shows that the path to larger quantum volume systems requires the simultaneous increase of coherence, control gate fidelities, measurement fidelities, and smarter software which takes into account hardware details, thereby demonstrating the need to continue to co-design the software and hardware stack for the foreseeable future.

• The paper demonstrates a quantum volume of 64 by integrating advanced compiler optimization, dynamical decoupling, and efficient two-qubit gate sequences.
• Methodologies such as binary integer programming for qubit routing and pulse-efficient SU(4) decomposition were key to enhancing gate fidelity and reducing circuit depth.
• These innovations yield improved coherence times, reduced gate errors, and higher measurement fidelity, marking a significant milestone in superconducting quantum computing.

Quantum Volume 64: Advancements in Superconducting Quantum Computing

The paper "Demonstration of quantum volume 64 on a superconducting quantum computing system" offers an in-depth analysis and demonstration of enhancing the quantum volume (QV) to 64 on IBM's superconducting quantum systems. The work significantly describes advancements in circuit quality for superconducting quantum computing systems achieved through a multifaceted approach that integrates software and hardware optimizations.

The central achievement discussed in this paper is the realization of a quantum volume of 64, which represents a notable enhancement in the computational capability of the tested quantum system, specifically ibmq_montreal, a 27-qubit IBM Quantum Falcon processor. Quantum volume is a comprehensive benchmark reflecting various facets of quantum system performance, such as coherence times, gate errors, and measurement fidelities. Achieving QV64 implies a considerable improvement in system capabilities, prompting discussions about the confluence of improved coherence, advanced compiler algorithms, gate fidelities, and measurement techniques.

Key Advances and Methodologies

The achievement of QV64 was realized through several critical innovations and optimizations. These include:

1. Improvements in the Qiskit Compiler: The authors introduced a novel binary integer programming approach to optimize qubit layout and routing, significantly improving gate fidelity by reducing gate counts and optimizing circuit depth. Additionally, a pulse-efficient SU(4) decomposition technique was employed, leveraging the native hardware gate directions, thus reducing the number of required operations.
2. Implementation of Dynamical Decoupling (DD): This technique addressed errors introduced during idle qubit times. By employing a structured $X_p - X_m$ sequence, the study demonstrated reductions in decoherence-related errors, reflecting in higher circuit fidelity.
3. Shorter Two-Qubit Gates: The methods incorporated changes in the two-qubit gate sequences by focusing on utilizing a "direct" cross-resonance gate instead of the standard echoed cross-resonance gate. This resulted in a significant reduction in gate duration, contributing to enhanced computational speeds and improved overall fidelity.
4. Excited State Promoted (ESP) Readout: Enhancements in readout fidelity were achieved by promoting excited qubit states before measurement, which improved the separation in measurement signals and reduced assignment errors compared to conventional techniques.

Implications and Future Directions

The results presented in this paper have notable implications for the future of quantum computing. These advancements not only suggest a more significant quantum computational capacity but also advocate for the necessity of simultaneous advancements in both hardware and software. The need for a synergized co-design approach, where innovations in the computational processing stack can be complemented by those in algorithms and compiler design, is highlighted as vital for evolving towards more fault-tolerant and practical quantum computing architectures.

The methodological advancements in quantum gate compilation, dynamical decoupling, and readout methodologies provide a template for future improvements. As quantum systems scale in complexity, incorporating and refining these techniques will be essential to maintain and improve operational fidelity and efficiency.

Conclusion

This paper's successful demonstration of a QV of 64 using a superconducting quantum computing system represents a significant milestone. The breadth of advancements reported—from compiler improvements to physical gate execution optimizations—underscores the multifaceted nature of research necessary to push the boundaries of quantum technology. As researchers continue to address the noise and error challenges inherent in current quantum systems, the methodologies and results from this work will serve as a valuable foundation. The journey towards higher quantum volumes and more capable quantum computers is ongoing, with the promise of new breakthroughs in both practical performance and theoretical understanding.