Accessible information of a general quantum Gaussian ensemble

Abstract

Accessible information, which is a basic quantity in quantum information theory, is computed for a general quantum Gaussian ensemble under certain “threshold conditions.” It is shown that the maximizing measurement is Gaussian, constituting a far-reaching generalization of optical heterodyning. This substantially extends the previous result concerning the gauge-invariant case even for a single bosonic mode. A simple sufficient condition is provided that implies the threshold condition for a general Gaussian ensemble. The tightness of the sufficient condition is demonstrated in the single-mode case.