Figure 1:
X rays measure squeezed phonon states in a crystal of bismuth. At time t_{1}, a laser pulse excites electrons in bismuth from the valence to the conduction band. The change in electron density in the conduction band causes the interatomic bonds to soften and “squeezes” the thermally excited phonons. The mean-square displacements of the bismuth atoms, denoted by the size of the circles, oscillates in time at a frequency that is twice that of the phonon. The lower the spread in the displacements, the larger the spread in momenta (and vice versa), which corresponds to a squeezed state. The intensity of an x-ray diffraction peak from the bismuth lattice planes, measured at time t_{2}, depends on the amplitude of the mean-square displacements and can therefore measure the squeezed state.