What is a broadband squeezed state?

A broadband or two-mode (nondegenerate) squeezed state is the natural two-mode analog of a single-mode (degenerate) squeezed state. The squeezing is a result of correlations between photons in the two modes. Two-mode squeezed states are produced by the same kind of physical process and the same kinds of physical device that produce single-mode squeezed states (e.g., a parametric amplifier), simply by moving away from degeneracy. They are to be contrasted with another kind of two-mode state, one produced by separately squeezing two single modes. The latter are produced by a different kind of physical process and different physical devices from those that produce two-mode squeezed states. States that are products of two single-mode squeezed states do not exhibit squeezing, for the photons in the two modes are not correlated. There is a formal sense, however, in which these different kinds of two-mode states are (unitarily) equivalent. This formal equivalence tells one that, to achieve the desired squeezing by separately squeezing two modes, one would have to use a frequency-converting device before and after squeezing the two modes separately. This process produces the required correlations, hence the squeezing, but it Is clearly not the natural way to obtain a broadband squeezed state.

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