We provide a new expression of the quantum Fisher information(QFI) for a general system. Utilizing this expression, the QFI for a non-full rank density matrix is only determined by its support. This expression can bring convenience for a infinite dimensional density matrix with a finite support. Besides, a matrix representation of the QFI is also given.
Cite
@article{arxiv.1312.6910,
title = {Quantum Fisher information for density matrices with arbitrary ranks},
author = {Jing Liu and Xiao-Xing Jing and Wei Zhong and Xiaoguang Wang},
journal= {arXiv preprint arXiv:1312.6910},
year = {2014}
}