  H$\alpha$ is a powerful tracer of accretion and chromospheric activity, which
has been detected in the case of young brown dwarfs and even recently in
planetary mass companions (e.g. PDS70 b and c). H$\alpha$ detections and
characterisation of brown dwarf and planet companions can further our knowledge
of their formation and evolution, and expanding such a sample is therefore our
primary goal. We used the Zurich IMaging POLarimeter (ZIMPOL) of the SPHERE
instrument at the Very Large Telescope (VLT) to observe the known
$38-72$M$_{\mathrm{J}}$ companion orbiting PZ Tel, obtaining simultaneous
angular differential imaging observations in both continuum and narrow
H$\alpha$ band. We detect H$\alpha$ emission from the companion, making this
only the second H$\alpha$ detection of a companion using the SPHERE instrument.
We used our newly added astrometric measurements to update the orbital analysis
of PZ Tel B, and we used our photometric measurements to evaluate the H$\alpha$
line flux. Given the estimated bolometric luminosity, we obtained an H$\alpha$
activity (log$\mathrm{(L_{H\alpha}/L_{bol}})$) between $-4.16$ and $-4.31$. The
H$\alpha$ activity of PZ Tel B is consistent with known average activity levels
for M dwarf of the same spectral type. Given the absence of a known gaseous
disk and the relatively old age of the system (24 Myr), we conclude that the
H$\alpha$ emission around PZ Tel B is likely due to chromospheric activity.
