The 'fish-eye' lens is an unusual piece of glass: its refractive index changes from the center to the edge so that any ray emitted from any point is focused to the point directly opposite. Making such a lens from ordinary glass is nearly impossible, but physicists found a workaround. In a cloud of atoms cooled to billionths of a degree above absolute zero, sound waves behave like light in a lens. The speed of sound in such a cloud can be smoothly varied from point to point, bending the waves as needed.
The experiment was carried out with a Bose-Einstein condensate — a state of matter where thousands of atoms merge into a single quantum wave. They excited sound pulses in it and tracked how their wave fronts converged at the focus. The observations matched the calculations: waves emitted at one point indeed gathered at the opposite point. This is the first realization of an acoustic 'fish-eye' in a quantum gas.
This approach opens the door to simulating curved spaces and unusual optical effects using sound. Instead of expensive and complex optical devices — a cloud of atoms that can be controlled with magnetic fields and lasers.
In a Bose-Einstein condensate, sound travels at just a few millimeters per second, a hundred thousand times slower than in air.