  It is shown that the quantum jumps in the photon number n from zero to one or
more photons induced by backaction evasion quantum nondemolition measurements
of a quadrature component x of the vacuum light field state are strongly
correlated with the quadrature component measurement results. This correlation
corresponds to the operator expectation value <xnx> which is equal to one
fourth for the vacuum even though the photon number eigenvalue is zero. Quantum
nondemolition measurements of a quadrature component can thus provide
experimental evidence of the nonclassical operator ordering dependence of the
correlations between photon number and field components in the vacuum state.
