Scalable Quantum Error Mitigation with Phase Cycling Corrects Decoherence Overestimation in Noisy Quantum Systems

Quantum technologies currently face a significant hurdle in the form of decoherence and control errors, which limit their potential to outperform classical computers. Weibin Ni, Zhijie Li, and Guanyu Qu, along with their colleagues, address this challenge by developing a new approach to quantum error mitigation that improves the reliability of dynamical decoupling, a common technique for suppressing decoherence. Their work reveals that previously reported improvements in decoherence times were often overestimated due to susceptibility to control errors, a problem they solve by introducing Hadamard phase cycling. This method effectively eliminates errors arising from faulty control, scales well with increasingly complex quantum circuits, and accurately measures decoherence times in diverse systems including electron spins and trapped ions, paving the way for more robust and practical quantum technologies.

Pulse Sequences Extend Qubit Coherence Times

Scientists have developed methods to significantly extend the coherence of qubits, the fundamental building blocks of quantum computers. This research focuses on improving the fidelity and duration of quantum information storage by manipulating qubit states with precisely timed pulses. The team investigated different pulse sequences, including constant pulse (CP-m) and uniformly distributed delay (UDD-m) sequences, and compared the effectiveness of two phase cycling techniques, two-phase cycling (TPC) and high-phase cycling (HPC). These experiments were conducted on three distinct qubit technologies: superconducting transmon qubits, trapped ion qubits, and diamond nitrogen-vacancy (NV) centers.

The research demonstrates that HPC consistently outperforms TPC in maintaining qubit coherence and fidelity, particularly when using longer pulse sequences. HPC proves more robust to systematic errors introduced during qubit manipulation, preserving state fidelity above 99. 3% for up to 16 inversion pulses in superconducting qubits. Quantum state tomography confirms the superior quality of states prepared using HPC, with similar improvements observed in trapped ion qubits and significantly extended coherence times in diamond NV centers when using HPC with optimized pulse sequences like Carr-Purcell-Meiboom-Gill (CPMG) and UDD. These findings reveal that HPC is a superior technique for enhancing qubit performance across various platforms. By enabling more precise control over quantum states, this method contributes to the development of more robust and scalable quantum computers, providing a practical approach to mitigating the effects of noise and imperfections in quantum systems.

Hadamard Phase Cycling Mitigates Decoherence Overestimation

Scientists have introduced Hadamard phase cycling, a novel quantum error mitigation technique designed to address the problem of overestimated decoherence times in noisy quantum systems. This method improves dynamical decoupling, a technique used to protect quantum information, by classifying qubit dynamics and designing equivalent quantum circuits with varying phases. By selectively extracting desired, error-free dynamics through averaging, Hadamard phase cycling effectively eliminates outputs generated by erroneous dynamics and scales linearly with circuit depth. Experiments conducted on solid-state electron spin qubits, diamond nitrogen-vacancy centers, trapped ions, and superconducting transmon qubits demonstrate the effectiveness of this technique.

Hadamard phase cycling accurately acquires decoherence times and preserves state fidelity during dynamical decoupling, showcasing its broad applicability and allowing for a more accurate assessment of qubit performance. This research highlights the potential of scalable quantum error mitigation to facilitate the development of robust quantum technologies with noisy hardware. By mitigating control errors, scientists can achieve more accurate measurements and improve the performance of qubits, bringing us closer to realizing the potential of quantum computing.

Hadamard Phase Cycling Corrects Decoherence Estimates

Scientists have achieved significant enhancement of qubit coherence times using Hadamard phase cycling, a new quantum error mitigation technique applied to dynamical decoupling. Standard dynamical decoupling methods can overestimate decoherence times due to unmitigated control errors, leading to inaccurate assessments of qubit performance. This research demonstrates that by separating desired and undesired echoes, using a modified CPMG sequence, undesired echoes decay much faster than desired echoes. This work introduces a method to accurately measure and enhance coherence times for ensembles of electron spin qubits and nitrogen-vacancy centers in diamond.

Benchmarking on trapped ions and superconducting qubits yielded near-quantitative effective state fidelity. The team constructed phase configurations of quantum circuits using Hadamard phase cycling, effectively eliminating outputs generated from erroneous dynamics and achieving linear scaling with circuit depth. This research demonstrates a high degree of error mitigation and enhances the accuracy of decoherence time measurements. By accurately measuring and enhancing coherence, scientists can improve the performance of qubits and advance the development of quantum technologies. This technique represents a balance between complexity, effectiveness, and specific task demands, potentially improving the sensitivity of nanoscale magnetic resonance and facilitating the development of more robust quantum computing and magnetic resonance imaging techniques.