  A first-order quantum chromodynamics (QCD) phase transition (PT) may take
place in the protocompact star (PCS) produced by a core-collapse supernova
(CCSN). In this work, we study the consequences of such a PT in a non-rotating
CCSN with axisymmetric hydrodynamic simulations. We find that the PT leads to
the collapse of the PCS and results in a loud burst of gravitational waves
(GWs). The amplitude of this GW burst is $\sim30$ times larger than the
post-bounce GW signal normally found for non-rotating CCSN. It shows a broad
peak at high frequencies ($\sim2500-4000$ Hz) in the spectrum, has a duration
of $\lesssim5 {\rm ms}$, and carries $\sim3$ orders of magnitude more energy
than the other episodes. Also, the peak frequency of the PCS oscillation
increases dramatically after the PT-induced collapse. In addition to a second
neutrino burst, the GW signal, if detected by the ground-based GW detectors, is
decisive evidence of the first-order QCD PT inside CCSNe and provides key
information about the structure and dynamics of the PCS.
