# Applicability of coupling strength estimation for linear chains of restricted access

## TL;DR

This paper investigates a method for estimating coupling strengths in restricted-access quantum linear chains, focusing on robustness against observational errors and spectral limitations to improve Hamiltonian identification.

## Contribution

The study extends Burgart et al.'s algorithm, analyzing its effectiveness and robustness in practical scenarios with noise and spectral truncation.

## Key findings

- Spectral distribution of the end state influences method applicability
- Reducing spectral truncation loss improves robustness
- Numerical simulations validate the method's effectiveness under noise

## Abstract

The characterization of an unknown quantum system requires the Hamiltonian identification. The full access to the system, however, is usually restricted, hindering the direct retrieval of relevant parameters, and a reliable indirect estimation is usually required. In this work, the algorithm proposed by Burgarth et al. [Phys. Rev. A 79, 020305 (2009)], which allows estimating the coupling strengths in a linear chain by addressing only one end site, is further investigated. The scheme is numerically studied for states with chain structure, exploring its applicability against observational errors including the limited signal-noise ratio and the finite spectral width. The spectral distribution of the end state is shown to determine the applicability of the method, and reducing the loss from truncated spectral components is critical to realizing the robust reconstruction of coupling strengths.