# Quantum-inspired sublinear algorithm for solving low-rank semidefinite programming

## TL;DR

This paper introduces a quantum-inspired, sublinear-time algorithm for solving low-rank semidefinite programs, enabling efficient computation of solutions and spectral decompositions with applications to quantum state learning.

## Contribution

It presents a novel sublinear-time algorithm for low-rank SDP solving using sampling techniques, including new methods for spectral decomposition and matrix exponential approximation.

## Key findings

- Algorithm runs in time $O(m ext{poly}( ext{log} n, r, 1/\varepsilon))$
- First sampling-based spectral decomposition method introduced
- Application demonstrated in quantum state learning

## Abstract

Semidefinite programming (SDP) is a central topic in mathematical optimization with extensive studies on its efficient solvers. In this paper, we present a proof-of-principle sublinear-time algorithm for solving SDPs with low-rank constraints; specifically, given an SDP with $m$ constraint matrices, each of dimension $n$ and rank $r$, our algorithm can compute any entry and efficient descriptions of the spectral decomposition of the solution matrix. The algorithm runs in time $O(m\cdot\mathrm{poly}(\log n,r,1/\varepsilon))$ given access to a sampling-based low-overhead data structure for the constraint matrices, where $\varepsilon$ is the precision of the solution. In addition, we apply our algorithm to a quantum state learning task as an application. Technically, our approach aligns with 1) SDP solvers based on the matrix multiplicative weight (MMW) framework by Arora and Kale [TOC '12]; 2) sampling-based dequantizing framework pioneered by Tang [STOC '19]. In order to compute the matrix exponential required in the MMW framework, we introduce two new techniques that may be of independent interest: $\bullet$ Weighted sampling: assuming sampling access to each individual constraint matrix $A_{1},\ldots,A_{\tau}$, we propose a procedure that gives a good approximation of $A=A_{1}+\cdots+A_{\tau}$. $\bullet$ Symmetric approximation: we propose a sampling procedure that gives the \emph{spectral decomposition} of a low-rank Hermitian matrix $A$. To the best of our knowledge, this is the first sampling-based algorithm for spectral decomposition, as previous works only give singular values and vectors.

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Source: https://tomesphere.com/paper/1901.03254