Figure 13: Light field in a vacuum quantum state . Left panel (a) features a typical result one
could get measuring the (normalized) electric field strength of the light wave in a vacuum state as a
function of time. Right panel (b) represents a phase space picture of the results of measurement. A red
dashed circle displays the error ellipse for the state that encircles the area of single standard
deviation for a two-dimensional random vector of measured light quadrature amplitudes. The
principal radii of the error ellipse (equal in vacuum state case) are equal to square roots of the
covariance matrix eigenvalues, i.e., to .
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