Bipolarons in a Bose-Einstein Condensate

Abstract

Mobile impurities in a Bose-Einstein condensate form quasiparticles called polarons. Here, we show that two such polarons can bind to form a bound bipolaron state. Its emergence is caused by an induced nonlocal interaction mediated by density oscillations in the condensate, and we derive using field theory an effective Schrödinger equation describing this for an arbitrarily strong impurity-boson interaction. We furthermore compare with quantum Monte Carlo simulations finding remarkable agreement, which underlines the predictive power of the developed theory. It is found that bipolaron formation typically requires strong impurity interactions beyond the validity of more commonly used weak-coupling approaches that lead to local Yukawa-type interactions. We predict that the bipolarons are observable in present experiments, and we describe a procedure to probe their properties.

Many-body quantum dynamics and induced correlations of Bose polarons

“…impurity system within this interaction regime. Similarly, for g BI <−2, á ñ  r using the effective potential is significantly larger than the corresponding MB result, which can be attributed to the prominent role of induced interactions in the formation of the bipolaron state [10].…”

“…, where á ñ  r become approximately constant, the impurities come very close with respect to one another. Here, the corresponding ( ) ( ) r  x x , 2 1 2 shrinks along its anti-diagonal and its diagonal becomes elongated (see figure 1(b 1 )), which is indicative of a bound state having formed between the impurities known as a bipolaron state [8,10,94].…”

“…provides the probability of finding at time t a pseudospin- boson at location x 1 and a second one at x 2 [65,66]. Most importantly, it allows us to monitor the two-body spatially resolved dynamics of the impurities and infer whether they move independently or correlate with each other [8,10,42]. Figure 7 shows ( ) ( ) r  x x t , ; is deformed along its diagonal (figures 7(a 4 ), (a 6 )) or its anti-diagonal (figure 7(a 5 )) indicating that the impurities tend to be slightly apart or at the same location respectively.…”

Critical slowdown of non-equilibrium polaron dynamics

“…The dots indicate when the damping rate crosses the long time limit, defining the formation time t f . The colored dashed lines in panel (b) show the asymptotic rate (28). The used mass ratio is n = = m m 1 B…”

“…Experiments on imbalanced Fermi gases [8][9][10][11][12] provide an ideal quantum simulation platform for the Fermi polaron [13][14][15]. At the same time, the possibility to realize so-called Bose polarons in atomic Bose-Einstein condensates (BECs) [16,17] has raised further questions and ushered in new theoretical investigations [18][19][20][21][22][23][24][25][26][27][28][29][30], expanding our understanding of quantum impurity physics. The properties of the Bose polaron is arguably closer to the generic solid-state polaron, since the surrounding BEC has a linear low energy dispersion in analogy with acoustic phonons in a solid.…”