Broadband detection of squeezed vacuum: A spectrum of quantum states

We demonstrate the simultaneous quantum state reconstruction of the spectral modes of the light field emitted by a continuous-wave degenerate optical parametric amplifier. The scheme is based on broadband measurement of the quantum fluctuations of the electric-field quadratures and subsequent Fourier decomposition into spectral intervals. Applying the standard reconstruction algorithms to each bandwidth-limited quantum trajectory, a "spectrum" of density matrices and Wigner functions is obtained. The recorded states show a smooth transition from the squeezed vacuum to a vacuum state. In the time domain we evaluated the first-order correlation function of the squeezed output field, showing good agreement with theory.