  Bright squeezed vacuum, a promising tool for quantum information, can be
generated by high-gain parametric down-conversion. However, its frequency and
angular spectra are typically quite broad, which is undesirable for
applications requiring single-mode radiation. We tailor the frequency spectrum
of high-gain parametric down-conversion using an SU(1,1) interferometer
consisting of two nonlinear crystals with a dispersive medium separating them.
The dispersive medium allows us to select a narrow band of the frequency
spectrum to be exponentially amplified by high-gain parametric amplification.
The frequency spectrum is thereby narrowed from (56.5 +- 0.1) to (1.22 +- 0.02)
THz and, in doing so, the number of frequency modes is reduced from
approximately 50 to 1.82 +- 0.02. Moreover, this method provides control and
flexibility over the spectrum of the generated light through the timing of the
pump.
