  The electromagnetic-vacuum-field fluctuations are intimately linked to the
process of spontaneous emission of light. Atomic emitters cannot probe
electric- and magnetic-field fluctuations simultaneously because electric and
magnetic transitions correspond to different selection rules. In this paper we
show that semiconductor quantum dots are fundamentally different and are
capable of mediating electric-dipole, magnetic-dipole, and electric-quadrupole
transitions on a single electronic resonance. As a consequence, quantum dots
can probe electric and magnetic fields simultaneously and can thus be applied
for sensing the electromagnetic environment of complex photonic nanostructures.
Our study opens the prospect of interfacing quantum dots with optical
metamaterials for tailoring the electric and magnetic light-matter interaction
at the single-emitter level.
