Polariton Fluids as Quantum Field Theory Simulators on Tailored Curved Spacetimes

Quantum fields in curved spacetime exhibit a wealth of effects like Hawking radiation from black holes. While quantum field theory in black holes can only be studied theoretically, it can be tested in controlled laboratory experiments. In experiments, a fluid going from sub- to supersonic speed creates an effectively curved spacetime for the acoustic field, with a horizon where the speed of the fluid equals the speed of sound. The challenge to test predictions like the Hawking effect in such systems lies in the control of the spacetime curvature and access to the field spectrum thereon. Here, we create tailored stationary effective curved spacetimes in a polaritonic quantum fluid of light in which either massless or massive excitations can be created, with smooth and steep horizons and various supersonic fluid speeds. Using a recently developed spectroscopy method we measure the spectrum of collective excitations on these spacetimes, crucially observing negative energy modes in the supersonic regions, which signals the formation of a horizon. Control over the horizon curvature and access to the spectrum on either side demonstrates the potential of quantum fluids of light for the study of field theories on curved spacetimes, and we discuss the possibility of investigating emission and spectral instabilities with a horizon or in an effective Exotic Compact Object configuration.