Quantum field theory (QFT) in curved spacetimes predicts the amplification of field excitations and the occurrence of classical and quantum correlations, as in the Hawking effect for example. This raises interest in experiments in which the curvature of spacetime can be controlled and amplification measured, as in fluids going from subsonic to supersonic speeds where acoustic excitations are effectively trapped inside an acoustic horizon. Quantum fluctuations of the acoustic field are predicted to yield entangled emission across the horizon, as in black holes. Here, we introduce such a QFT simulator in a one-dimensional polaritonic fluid of light. We demonstrate the unique tunability of our system by engineering smooth and steep horizons, which respectively have quasithermal, but weak, and strong Hawking radiation. We measure the spectrum on either side of the horizon and evidence the excitation of negative energy waves in fluids of light for the first time. Notably, we explicitly show that, beyond phononic excitations as in other systems, our simulator also supports excitations with a massive, relativistic dispersion. In the future, quantum optics techniques offer the possibility to measure entanglement in unexplored regimes, giving insight in this outstanding prediction of relativistic QFT.

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• Exciton polariton
• Polaritons
• Quantum field theory
• Quantum simulation
• Polariton condensate
• Superfluids