Anderson's orthogonal catastrophe is one of the paradoxical effects in many-body physics: a local perturbation in a Fermi gas makes the many-body wave function orthogonal to the unperturbed one. In ultracold atomic gases, this effect has long remained elusive: thermal noise masked its signatures in the spectra of heavy polarons. This work stands alongside studies of coherent impurity control, but unlike neighboring experiments with equal masses, it uses a record mass ratio between impurity and bath for alkali mixtures (about 22), bringing the system closer to the infinitely heavy impurity limit, where the catastrophe is most pronounced.
They prepared a degenerate Fermi gas of 180,000 lithium-6 atoms and placed about a thousand cesium-133 atoms in a noninteracting state. Using momentum-transfer-free Raman spectroscopy, they coherently transferred impurities into a state with tunable interactions, varying the excitation strength over a wide range, from 0.2 to 3 Fermi energies. The magnetization evolution was recorded via absorption, extracting the Rabi frequency and the damping rate of oscillations. The temperature was maintained at a quarter of the Fermi temperature.
The ratio of the Rabi frequency to the bare one grows with excitation strength following a power law, whose exponent quantitatively agrees with the prediction of the orthogonal catastrophe for different interactions. For example, for an interaction parameter of −0.53(5) the exponent was 0.53(5), and for +0.51(5) it was 0.51(5). The power-law regime is observed from thermal energy up to the Fermi energy, without the plateau characteristic of mobile polarons. The damping rate of oscillations depends nonmonotonically on excitation strength: it first increases, then decreases, with the maximum on the repulsive branch being higher than on the attractive one.
This work shows that Anderson's orthogonal catastrophe is accessible in the dynamics of coherently controlled impurities even at finite temperature and finite mass. Unlike neighboring studies that used impurities with masses close to that of the bath, here a mass ratio of 22 is achieved for the first time, making the system nearly static and allowing direct comparison with the theory for an infinitely heavy impurity. This expands the toolbox of quantum many-body simulation: local coherent control becomes a probe of the collective response of the Fermi liquid.
Further lowering the temperature and increasing the mass ratio could make the power law even more pronounced. It would be interesting to explore the time-dependent orthogonal catastrophe and transport phenomena where this effect plays a key role. Coherent impurity control opens the way to studying dynamically screened interactions and spin exchange.
The results are important for ultracold atomic physics, condensed matter physics, and quantum technologies, where polarons and the orthogonal catastrophe manifest in spectroscopy and transport.
Perform measurements at lower temperatures and with heavier impurities to approach zero temperature and infinite mass. Investigate the dependence on detuning and test predictions for the ratio of frequency to damping.
Anderson's orthogonal catastrophe remains one of the unsolved problems in many-body physics: its manifestations under realistic conditions (finite temperature, finite mass) are not fully understood. This work shows that dynamical probes can bypass thermal masking, but the quantitative discrepancy between measured and calculated damping indicates a gap in the theory of decoherence.
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What the work does
This study explores the coherent excitation of a heavy Fermi polaron in a mixture of lithium-6 and cesium-133. The Rabi frequency was measured as a function of excitation strength, revealing a power-law dependence with an exponent consistent with the Anderson orthogonality catastrophe prediction. Numerical simulations at finite temperature reproduce the observed scaling.
Where its strength is
Achieving a record impurity-to-environment mass ratio of about 22 brings the system close to the infinitely heavy impurity limit, enabling a direct comparison between experiment and the theory of the orthogonality catastrophe. The wide range of excitation strengths and the finite temperature demonstrate the robustness of the effect beyond idealized conditions.
Where it could go next
Worth trying
Why this may matter
This work paves the way for using coherently controlled impurities as probes of quantum many-body dynamics, and the proposed directions could deepen our understanding of the orthogonality catastrophe under realistic conditions.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.50, against a usual 0.64. It looks like few have drilled nearby.
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.