  Impulsive optical excitation can generate both coherent and squeezed phonons.
The expectation value of the phonon displacement $<u_q>$ oscillates at the mode
frequency for the coherent state but remains zero for a pure squeezed state. In
contrast, both show oscillations in $<|u_q|^2>$ at twice the mode frequency.
Therefore it can be difficult to distinguish them in a second-order measurement
of the displacements, such as in first-order x-ray diffuse scattering. Here we
demonstrate a simple method to distinguish squeezed from coherent atomic motion
by measurement of the diffuse scattering following double impulsive excitation.
We find that femtosecond optical excitation generates squeezed phonons spanning
the Brillouin zone in Ge, GaAs and InSb. Our results confirm the mechanism
suggested in [Nature Physics 9, 790 (2013)].
