Multi-parameter estimation: global, local and sequential strategies

Quantum metrology is one of the technologies from which we expect a quantum superiority over classical strategies in the near future. For single-parameter estimation, a clear advantage for global strategies based on entanglement can be proven. However, there exist different answers to the question whether global strategies are better than separable strategies for multi-parameter estimation. The putative inconsistency is based on the investigation of different problem settings as well as different interpretations of the word 'separable' leading to different estimation strategies. In this paper, we discuss these different strategies for the special case of estimating a complete set of linear independent global variables of a sensor network (Eldredge et al 2018 Phys. Rev. A 97, 042337, Proctor et al 2017 arXiv:1702.04271) and outline the advantages and disadvantages of the different strategies.