The optimization of measurement for n samples of pure sates are studied. The error of the optimal measurement for n samples is asymptotically compared with the one of the maximum likelihood estimators from n data given by the optimal measurement for one sample.
IntroductionRecently, there has been a rise in the necessity for studies about statistical estimation for the unknown state, related to the corresponding advance in measuring technologies in quantum optics. An investigation including both quantum theory and mathematical statistics is necessary for an essential understanding of quantum theory because it has statistical aspects [1,2]. Therefore, it is indeed important to optimize the measuring process with respect to the estimation of the unknown state. Such research is known as quantum estimation, and was initiated by Helstrom in the late 1960s, originating in the optimization of the detecting process in optical communications [1]. In classical statistical estimation, one searches the most suitable estimator for which probability measure describes the objective probabilistic phenomenon. In quantum estimation, one searches the most suitable measurement for which density operator describes the objective quantum state.Contained among important results are three estimation problems. The first is of the complex amplitude of coherent light in thermal noise and the second is of the expectation parameters of quantum Gaussian state. The former was studied by Yuen and Lax [3] and the latter by Holevo [2]. These studies discovered that heterodyning is the most suitable for the estimation of the complex amplitude of coherent light in thermal noise. The third is a formulation of the covariant measurement with respect to an action of a group. It was studied by Holevo [2,4]. In the formulation, he established a quantum analogue of Hunt-Stein theorem.Quantum estimation, was first used in the evaluation of the estimation error of a single sample of the unknown state as it had advanced in connection with the optimization of the measuring process in optical communications. Thus early studies were lacking in asymptotic aspects, i.e. there were few researches with respect to reducing the estimation error by quantum correlations between samples.Recently, studies about the estimation of the unknown state are attracting many physicists [5,6,7,8]. Some of them were drawn by the variation of the measuring precision with respect to the number of samples of the unknown state [9,10].Nagaoka [11] studied, for the first time, asymptotic aspects of quantum estimation. He paid particular attention to the quantum correlations between samples of the unknown state, and studied the relation between the asymptotic estimation and the local detection of a one-parameter family of quantum states.In the early 1990s, Fujiwara and Nagaoka [12,13,14] studied the estimation problem for a multiparameter family consisting of pure states. They pioneered studies into the estimation problem of the complex amplitude of noiseless coherent light. The research f...