  The origin of quantum physics was the discovery of the base unit of
electromagnetic action $h$ by Max Planck in 1900 when he analyzed the
experimental results of the black body radiation. This permitted Albert
Einstein a few years later to explain the photoelectric effect by the
absorption of photons with an energy of $E = h \cdot f$. We exploit the
Planck-Einstein relation in a new type of fundamental spectroscopic
measurements of direct transitions between two states with energy differences
of about 10 neV and induced frequencies of a few MHz. Employing a Lamb-shift
polarimeter and a Sona transition unit, featuring a relatively simple magnetic
field configuration of two opposing solenoidal coils, we were able to determine
$f$ and measure $E$ independently. Only resonances corresponding to integer
multiples of Planck's constant $h$ were observed in our setup, which can very
well be explained quantitatively by the Schr\"odinger equation. This new method
beautifully demonstrates the quantization in the micro-cosmos and allows one to
measure the hyperfine splitting energies between the substates with $F=1$ and
$m_F = -1, 0, +1$ of metastable hydrogen atoms as function of a magnetic field
and, thus, to investigate the influence of QED corrections on the Breit-Rabi
diagram.
