  We study spectral properties of quantum radiation of ultimately short
duration. In particular, we introduce a continuous multimode squeezing operator
for the description of subcycle pulses of entangled photons generated by a
coherent-field driving in a thin nonlinear crystal with second order
susceptibility. We find the ultrabroadband spectra of the emitted quantum
radiation perturbatively in the strength of the driving field. These spectra
can be related to the spectra expected in an Unruh-Davies experiment with a
finite time of acceleration. In the time domain, we describe the corresponding
behavior of the normally ordered electric field variance.
