  Decays of $B^0_s \to D_s^- \pi^+$ allow to determine the oscillation
frequency $\Delta m_s$ between the $B^0_s$ particle and
$\kern0.18em\overline{\kern -0.18em B}{}^0_s$ antiparticle states with high
precision. It is a crucial input to constrain the CKM matrix and a
manifestation of the quantum nature of physics. A new measurement of this
frequency is presented, using a dataset corresponding to $6\,\mathrm{fb}^{-1}$
of $pp$ collisions, recorded by LHCb at a centre-of-mass energy of
$13\,\mathrm{TeV}$. The oscillation frequency is determined to be $\Delta m_s =
17.7683\pm0.0051(\mathrm{stat.})\pm0.0032(\mathrm{syst.})\,\mathrm{ps}^{-1}$
and a combination with previous LHCb measurements yields $\Delta
m_s^\mathrm{LHCb} = 17.7656\pm0.0057\,\mathrm{ps}^{-1}$.
