  We discuss an example of a subvacuum effect, where a quantum expectation
value is below the vacuum level, and is hence negative. The example is the time
average of the mean squared electric field in a non-classical state where one
mode is excited. We give some specific examples of such states, and discuss the
lower bound on the squared field or its time average. We show when a lower
bound can be obtained by diagonalization of the squared electric field
operator, and calculate this bound. We also discuss the case of an instant time
mean squared electric field, when the operator cannot be diagonalized. In this
case, a lower bound still exists but is attained only by the limit of a
sequence of quantum states. In general, the optimum lower bound on the mean
squared electric field is minus one-half of the mean squared electric field in
a one photon state. This provides a convenient estimate of the subvacuum
effect, and may be useful for attempts to experimentally measure this effect.
