Measuring the phase variance of light

The results of experiments designed to measure the operational phase cosine and sine variances of weak states of light disagree with the variances pre dicted by canonical phase formalisms. As these variances are fundamental ma nifestations of the quantum nature of phase, it is important to be able to measure the canonical variances also. A recent suggestion to do so, based o n the use of a two-component probe, involves the di? cult preparation of ex otic states of light which have not yet been produced. In this paper we sho w how the variances can be measured with simple coherent state inputs. The retrodictive formalism of quantum mechanics provides useful insight into th e physics involved.