In order to make a unified treatment for estimation problems of a very small noise or a very weak signal in a quantum process, we introduce the notion of a low-noise quantum channel with one noise parameter. It is known in several examples that prior entanglement together with nonlocal output measurement improves the performance of the channel estimation. In this paper, we study this "ancilla-assisted enhancementa" for estimation of the noise parameter in a general low-noise channel. For channels on two level systems we prove that the enhancement factor, the ratio of the Fisher information of the ancilla-assisted estimation to that of the original one, is always upper bounded by 3/2. Some conditions for the attainability are also given with illustrative examples.
|Journal||Physical Review A - Atomic, Molecular, and Optical Physics|
|Publication status||Published - 2005 Nov 1|
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics