In a very cold gas of light atoms, a small number of heavy impurity atoms float around. They are 22 times heavier than their neighbors, so they barely move, while the light atoms around them form something like a sea. When a heavy atom is illuminated by a laser, it starts to oscillate between two states, like a pendulum. The rate of these oscillations is called the Rabi frequency.
The scientists varied the light intensity and observed how the oscillation frequency changed. It turned out that the ratio of the measured frequency to that of a single atom grows according to a power law: the stronger the light, the higher the ratio, and the growth is described by an exponent that depends on whether the impurity attracts or repels its surroundings. This behavior was predicted many years ago by the Anderson orthogonality catastrophe effect. Previously, it had only been seen in idealized calculations at zero temperature and with an infinitely heavy impurity, but here it appeared in a real experiment at finite temperature. This means the effect is robust enough to be used as a tool for studying quantum liquids.
The Rabi frequency without the Fermi sea was about 7.62 kHz, and with the Fermi sea under weak excitation it dropped by almost half. The Fermi energy in the experiment is 31 kHz, corresponding to a temperature of 1.49 microkelvin.
In Stanislaw Lem's novel "Solaris," the ocean responds to researchers' actions in unpredictable ways. Here, an impurity immersed in a Fermi sea responds to coherent excitation with a power law: a collective response that cannot be reduced to a sum of individual reactions.