  Subvacuum effects arise in quantum field theory when a classically positive
quantity, such as the local energy density, acquires a negative renormalized
expectation value. Here we investigate the case of states of the quantized
electromagnetic field with negative mean-squared electric field, and their
effects on the propagation of light pulses in a nonlinear dielectric material
with a nonzero third-order susceptibility. We identify two distinct signatures
of the subvacuum effect in this situation. The first is an increase in the
speed of the pulse, which is analogous to the superluminal light propagation in
gravity which can arise from negative energy density. This increase in speed
leads to a phase shift which might be large enough to observe. The second
effect is a change in the frequency and power spectra of the pulse. We identify
a specific measure of the modified spectra which can signal the presence of a
negative mean squared electric field. These ideas are implemented in the
particular example of a wave guide filled with a nonlinear dielectric material.
