# Fast Reconstruction of High-qubit Quantum States via Low Rate Measurements

## TL;DR

This paper introduces a fast, efficient quantum state reconstruction method for up to 12 qubits using low measurement rates, leveraging quantum compressive sensing and an optimized ADMM algorithm.

## Contribution

It presents a novel, scalable approach for reconstructing high-qubit quantum states with minimal measurements and computational effort, extending applicability to various entangled states.

## Key findings

- Reconstructs 10-12 qubit states in seconds with less than 1% measurements
- Applicable to pure, mixed, and entangled states like GHZ, W, and cluster states
- Demonstrated effectiveness with experimental ion trap data up to 8 qubits

## Abstract

Due to the exponential complexity of the resources required by quantum state tomography (QST), people are interested in approaches towards identifying quantum states which require less effort and time. In this paper, we provide a tailored and efficient method for reconstructing mixed quantum states up to $12$ (or even more) qubits from an incomplete set of observables subject to noises. Our method is applicable to any pure or nearly pure state $\rho$, and can be extended to many states of interest in quantum information processing, such as multi-particle entangled $W$ state, GHZ state and cluster states that are matrix product operators of low dimensions. The method applies the quantum density matrix constraints to a quantum compressive sensing optimization problem, and exploits a modified Quantum Alternating Direction Multiplier Method (Quantum-ADMM) to accelerate the convergence. Our algorithm takes $8,35$ and $226$ seconds respectively to reconstruct superposition state density matrices of $10,11,12$ qubits with acceptable fidelity, using less than $1 \%$ of measurements of expectation. To our knowledge it is the fastest realization that people can achieve using a normal desktop. We further discuss applications of this method using experimental data of mixed states obtained in an ion trap experiment of up to $8$ qubits.