  Spectral components of continuous squeezed fields are entangled. In this
article we review and clarify this phenomenon by analyzing systematically the
relations between the correlations of modes filtered from stationary continuous
fields and the cross power spectrum between the operators of the corresponding
spectral components. Moreover, we study the specific spectral components that
are filtered in homodyne or heterodyne detections and their entanglement
properties. In particular, we establish the equivalence between two-mode
squeezing variance and logarithmic negativity for the spectral components of
continuous stationary fields, thereby demonstrating that the measurement of the
homodyne or heterodyne spectrum is, in fact, a direct measurement of the
logarithmic negativity between specific spectral modes. As an illustrative
example, we apply these concepts to the analysis of entanglement in
ponderomotive squeezing.
