  A Heisenberg uncertainty relation is derived for spatially-gated electric and
magnetic field fluctuations. The uncertainty increases for small gating sizes
which implies that in confined spaces the quantum nature of the electromagnetic
field must be taken into account. Optimizing the state of light to minimize the
electric at the expense of the magnetic field, and vice versa should be
possible. Spatial confinements and quantum fields may alternatively be realized
without gating by interaction of the field with a nanostructure. Possible
applications include nonlinear spectroscopy of nanostructures and optical
cavities and chiral signals.
