Quantum metrology at level anti-crossing

TL;DR

This paper demonstrates that universal, optimal measurement strategies for parameter estimation in two-level systems with level anti-crossing can be designed, achieving ultimate quantum precision regardless of the parameter or temperature conditions.

Contribution

It introduces a method to construct parameter-independent measurement schemes for optimal quantum estimation in two-level systems with anti-crossing.

Findings

Universal optimal strategies are achievable regardless of parameter value.

Optimal estimation can be maintained at high temperatures depending on Hamiltonian structure.

Dynamical strategies do not improve estimation precision.

Abstract

We address parameter estimation in two-level systems exhibiting level anti-crossing and prove that universally optimal strategies for parameter estimation may be designed, that is, we may find a parameter independent measurement scheme leading to the ultimate quantum precision independently on the nature and the value of the parameter of interest. Optimal estimation may be achievable also at high temperature depending on the structure of the two-level Hamiltonian. Finally, we show that no improvement is achievable by dynamical strategies and discuss examples of applications.