Quantum state tomography via non-convex Riemannian gradient descent

Ming-Chien Hsu

En-Jui Kuo

Wei-Hsuan Yu

Jian-Feng Cai

Min-Hsiu Hsieh

出版日期

April 15, 2024

研究中心

量子計算研究所

發表資訊

Physical Review Letters, vol. 132, Art. 240804, 2024. DOI: 10.1103/PhysRevLett.132.240804 scholars.ncu.edu.tw +15 link.aps.org +15 semanticscholar.org +15

內容目錄

The recovery of an unknown density matrix of large size requires huge computational resources. The recent Factored Gradient Descent (FGD) algorithm and its variants achieved state-of-the-art performance since they could mitigate the dimensionality barrier by utilizing some of the underlying structures of the density matrix. Despite their theoretical guarantee of a linear convergence rate, the convergence in practical scenarios is still slow because the contracting factor of the FGD algorithms depends on the condition number κ of the ground truth state. Consequently, the total number of iterations can be as large as O(κ√ln(1ε)) to achieve the estimation error ε. In this work, we derive a quantum state tomography scheme that improves the dependence on κ to the logarithmic scale; namely, our algorithm could achieve the approximation error ε in O(ln(1κε)) steps. The improvement comes from the application of the non-convex Riemannian gradient descent (RGD). The contracting factor in our approach is thus a universal constant that is independent of the given state. Our theoretical results of extremely fast convergence and nearly optimal error bounds are corroborated by numerical results.