  We report terahertz spectroscopic measurements of quantum spin dynamics in
the spin-1/2 honeycomb magnet Na$_2$Co$_2$TeO$_6$ as a function of applied
magnetic field with different terahertz polarizations. Distinct field
dependencies of the resolved spin dynamics are identified in three regimes,
which are separated by two critical fields at $B_{c1}\approx 7$ and
$B_{c2}\approx 10$ T. A polarization selective continuum is observed in the
intermediate phase, featuring spin fluctuations of a proximate quantum spin
liquid.
