Echo in the Fermi Sea: How a Heavy Atom Responds to Light ✛ ✎

Original: "Anderson orthogonality scaling in the Rabi-driven heavy Fermi polaron"

· Michael Rautenberg, Tobias Krom, Eugen Dizer, Olivier Bleu, Eleonora Lippi, Tilman Enss, Manfred Salmhofer, Lauriane Chomaz, Matthias Weidemüller

arXiv:2609.09129v1 · 2026-09-08 · arXiv non-exclusive · independent analysis · 2 min

Quantum Gases Quantum Physics

In an ultracold gas, an impurity atom responds to excitation with a power law predicted half a century ago.

In an ultracold cloud of 180,000 lithium atoms, about a thousand cesium atoms drift. They are 22 times heavier than their neighbors, which makes them nearly motionless among the fast-moving particles. When such a heavy atom absorbs light, it starts oscillating between two states, and the rate of these oscillations is called the Rabi frequency. But in the midst of the Fermi sea, this frequency behaves differently than for a single atom.

The Fermi sea is a gas of particles that fill all available energy levels up to a certain limit, much as water fills a pool to the brim.

The experimenters excited the impurity with a laser and tracked how fast it oscillated. They found that the stronger the excitation, the larger the ratio of the measured frequency to that of a single atom, and it grows according to a power law. The exponent depends on the strength of the interaction between the impurity and its surroundings: one value for attraction, another for repulsion. This is the signature of Anderson's orthogonality catastrophe, an effect in which even a weak perturbation forces the entire Fermi sea to rearrange.

Anderson's orthogonality catastrophe is the collective response of the Fermi sea to a local perturbation: the state of the system before and after the perturbation become orthogonal, meaning they do not overlap.

Previously, this effect had only been seen in idealized calculations at zero temperature and with an infinitely heavy impurity. Here, the temperature was a quarter of the Fermi temperature, and the impurity mass was finite, yet the power law still emerged. This shows that the catastrophe is not fragile but rather quite robust. Numerical models at finite temperature reproduce the observed scaling, although they describe the damping rate of the oscillations less accurately.

An impurity in the Fermi sea behaves like an echo: if you shout louder, the response comes back not just stronger but with a different tonal color. This dependence makes the impurity a sensitive probe of the collective properties of a quantum liquid. By controlling it with light, one can study how the Fermi sea responds to local perturbations and test many-body theories in the laboratory.

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.

\frac{\Omega}{\Omega_0} \propto \left(\frac{\hbar\Omega_0}{E_F}\right)^{\alpha/(2-\alpha)}

The ratio of the Rabi frequency to the bare one depends on the excitation strength as a power law with an exponent determined by the scattering phase shift

\alpha = \frac{\delta(k_F)^2}{\pi^2}

The exponent is expressed in terms of the scattering phase shift at the Fermi surface

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Echo in the Fermi Sea: How a Heavy Atom Responds to Light ✛ ✎