Eur. Phys. J. D 39, 321-327 (2006)
https://doi.org/10.1140/epjd/e2006-00097-y

Quantum relative states

GAP-Optique, Université de Genève, 1211 Genève, Switzerland

Corresponding authors: ^a nicolas.gisin@physics.unige.ch - ^b sofyan.iblisdir@physics.unige.ch

Received: 14 October 2005
Revised: 20 January 2006
Published online: 5 May 2006

Abstract

We study quantum state estimation problems where the reference system with respect to which the state is measured should itself be treated quantum mechanically. In this situation, the difference between the system and the reference tends to fade. We investigate how the overlap between two pure quantum states can be optimally estimated, in several scenarios, and we re-visit homodyne detection. uantum information