Universality in fidelity-based quantum metrology

Abstract

We consider the problem of identifying the quantum spin states that are the optimal sensors of a given transformation averaged over all possible orientations of the spin system. Our geometric approach to the problem is based on a fidelity criterion and is entirely general, encompassing both unitary transformations (such as rotations and squeezing) and non-unitary transformations (such as Lorentz boosts). This formalism leads to a universality result: There exists a zero-measure subset of states that will be optimal sensors for certain transformations and the worst sensors for others, and this set does not depend on the transformation under consideration. In other words, some spin states are simply the best (or worst) sensors, regardless of what they detect.