Bose Polarons in the Strongly Interacting Regime

Abstract

When an impurity is immersed in a Bose-Einstein condensate, impurity-boson interactions are expected to dress the impurity into a quasiparticle, the Bose polaron. We superimpose an ultracold atomic gas of 87 Rb with a much lower density gas of fermionic 40 K impurities. Through the use of a Feshbach resonance and RF spectroscopy, we characterize the energy, spectral width and lifetime of the resultant polaron on both the attractive and the repulsive branches in the strongly interacting regime. The width of the polaron in the attractive branch is narrow compared to its binding energy, even as the two-body scattering length formally diverges.The behavior of a dilute impurity interacting with quantum bath is a simplified yet nontrivial many-body model system with wide relevance to material systems. For example, an electron moving in an ionic crystal lattice is dressed by coupling to phonons and forms a quasiparticle known as a Bose polaron (see Fig. 1a) that is an important paradigm in quantum many-body physics [1]. Impurity atoms immersed in a degenerate bosonic or fermionic atomic gas are a convenient experimental realization for Bose or Fermi polaron physics, respectively. Recent theoretical work [2-9] has explored the Bose polaron case, and the ability to use a Feshbach resonance to tune [10] the impurity-boson scattering length a IB opens the possibility of exploring the Bose polaron in the strongly interacting regime [11][12][13][14]. Experiments to date [15][16][17][18][19][20] have focused on the weak Bose polaron limit. The Bose polaron in the strongly interacting regime is interesting in part because it represents step towards understanding a fully strongly interacting Bose system. While a IB can be tuned to approach infinity, the boson-boson scattering length a BB can still correspond to the meanfield limit. A dilute impurity interacting very strongly with a Bose gas that is otherwise in the mean-field regime is, on the one hand, something more difficult to model and to measure than a weakly interacting system. On the other hand it is theoretically more tractable, and empirically more stable than a single-component "unitary" Bose gas in which a BB diverges and thus every pair of atoms is strongly coupled [21].Our experiment employs techniques similar to those used in recent Fermi polaron measurements [22][23][24][25]. However, there are important differences between the Bose polaron and the Fermi polaron. From a theory point of view, the Bose polaron problem involves an interacting superfluid environment and also has the possibility of three-body interactions [14], both of which are not present for the Fermi polaron. And on the experimental side, both three-body inelastic collisions and the relatively small spatial extent of a BEC (compared to that of the impurity gas) create challenges for measurements of the Bose polaron. This work, in parallel with work done at Aarhus [26], describes the first experiments performed on Bose polarons in the strongly interacting regime. In our case, the impurity is fermi...

Theory of excitation of Rydberg polarons in an atomic quantum gas

“…The Efimov effect arises for resonant short-range two-body interactions in three dimensions. In previous experiments studying Bose polarons close to a Feshbach resonance it was found that these Efimov states do not strongly influence the polaron physics [7,8]. In our case the multimolecular states are not related to the Efimov effect, and dimer, trimer, etc.…”

“…So far it has been experimentally verified that the inclusion of this term is relevant for the observation of Bose polarons [7,8], where the impurity-Bose interaction can be modeled by a potential that supports only a single, weakly bound two-body molecular state. In the present work we encounter a new type of impurity problem where the impurity is dressed by large sets of molecular states that have ultra-long-range character.…”

“…Polarons play important roles in the conduction in ionic crystals and polar semiconductors [1], spin-current transport in organic semiconductors [2], dynamics of molecules in superfluid helium nanodroplets [3][4][5], and collective excitations in strongly interacting fermionic and bosonic ultracold gases [6][7][8].…”

Bose-Einstein-condensate polaron in harmonic trap potentials in the weak-coupling regime: Lee-Low-Pines–type approach

“…It should be noted that the interaction energy form = 0 is equivalent to the second-order perturbation due to virtual boson excitations, because f n,nt,m;u = 0 for m = 0 (see Eq. (29) with 1 r 2 u = m f ω f t |m| ) and hence only m = 0 states for boson excitations contribute to the summation in (34). This, in turn, leads to no drag effect η = 0 form = 0 via (31), as will be seen again later.…”

“…We present numerical results for the properties of a polaron in the ground and low-lying excited states, employing the parameter values that are used in the experiment for the boson-fermion mixture of 87 Rb bosons in a BEC and 40 K impurity fermions [34]:…”