Squeezed states of the electromagnetic field have been generated by degenerate parametric down conversion in a subthreshold optical parametric oscillator. Reductions in photocurrent noise greater than 60% relative to the standard quantum limit have been observed in a balanced homodyne detector. A quantitative comparison with theory suggests that the observed noise reductions result from a field that in the absence of linear losses would be squeezed more than ten-fold. An explicit demonstration of the Heisenberg uncertainty principle for the electromagnetic field is made from the measurements, which show that the field state produced by the down conversion process is a state of minimum uncertainty. |