Driven-dissipative quantum systems can undergo transitions from stationary to dynamical phases, reflecting the emergence of collective nonequilibrium behavior. We study such a transition in a Bose-Einstein condensate coupled to an optical cavity and develop a cavity-assisted Bragg spectroscopy technique to resolve its collective modes. We observe dissipation-induced synchronization at the quasiparticle level, where two rotonlike modes coalesce at an exceptional point. This reveals how dissipation microscopically drives collective dynamics and signals a precursor to a dynamical phase transition.

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• Atoms, ions, & molecules in cavities
• Bose-Einstein condensates
• Cavity quantum electrodynamics
• Dynamical phase transitions
• Open quantum systems & decoherence
• Optical lattices & traps
• Synchronization