Why Is Horizon Quantum Holdings ( $HQ ) Stock Down -28.38% Today? (Find out why any stock moved today on Tickeron.) HQ shares plunged roughly 28.4% in Monday trading, falling to about $10.90 from Friday's closing price of $15.22. The move extends a volatile stretch for the Singapore-based quantum software infrastructure company, which remains effectively pre-revenue. $ARQQ $BRUN $QNC $AEVA $BIRD