Bose polarons near quantum criticality

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Science  10 Apr 2020:
Vol. 368, Issue 6487, pp. 190-194
DOI: 10.1126/science.aax5850

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Immersing the impurities

In solids, moving electrons can polarize their environment, the ionic crystal lattice, creating quasi particles known as polarons. An analogous phenomenon can be studied in quantum gases by placing impurity atoms into a much denser gas of atoms of a different species. Yan et al. studied the behavior of Bose polarons, quasiparticles consisting of impurity atoms of potassium-40 immersed in a Bose gas of sodium-23 atoms. In the vicinity of a quantum critical point, the lifetime of the polarons followed the so-called Planckian scale.

Science, this issue p. 190


The emergence of quasiparticles in interacting matter represents one of the cornerstones of modern physics. However, in the vicinity of a quantum critical point, the existence of quasiparticles comes under question. Here, we created Bose polarons near quantum criticality by immersing atomic impurities in a Bose-Einstein condensate (BEC) with near-resonant interactions. Using radiofrequency spectroscopy, we probed the energy, spectral width, and short-range correlations of the impurities as a function of temperature. Far below the superfluid critical temperature, the impurities formed well-defined quasiparticles. Their inverse lifetime, given by their spectral width, increased linearly with temperature at the so-called Planckian scale, consistent with quantum critical behavior. Close to the BEC critical temperature, the spectral width exceeded the impurity’s binding energy, signaling a breakdown of the quasiparticle picture.

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