The well-known Yukawa and Efimov potentials are two different mediated interaction potentials. The first one arises in quantum field theory from the exchange of virtual particles. The second one is mediated by a real particle resonantly interacting with two other particles. This Letter shows how two impurities immersed in a Bose-Einstein condensate can exhibit both phenomena. For a weak impurity-boson attractive interactionattraction with the condensate, the two impurities form two polarons that interact through a weak Yukawa attraction mediated by virtual excitations. For a resonant attraction with the condensate, the exchanged excitation becomes a real boson and the mediated interaction changes to a strong Efimov attraction that can bind the two polarons. The resulting bipolarons turn into in-medium Efimov trimers made of the two impurities and one boson. Evidence of this physics could be seen in ultracold mixtures of atoms.
The well-known Yukawa and Efimov potentials are two different mediated interaction potentials. The first one arises in quantum field theory from the exchange of virtual particles. The second one is mediated by a real particle resonantly interacting with two other particles. This Letter shows how two impurities immersed in a Bose-Einstein condensate can exhibit both phenomena. For a weak impurity-boson attractive interactionattraction with the condensate, the two impurities form two polarons that interact through a weak Yukawa attraction mediated by virtual excitations. For a resonant attraction with the condensate, the exchanged excitation becomes a real boson and the mediated interaction changes to a strong Efimov attraction that can bind the two polarons. The resulting bipolarons turn into in-medium Efimov trimers made of the two impurities and one boson. Evidence of this physics could be seen in ultracold mixtures of atoms.
We investigate the zero-temperature properties of an impurity particle interacting with a Bose-Einstein condensate (BEC), using a variational wave function that includes up to two Bogoliubov excitations of the BEC. This allows one to capture three-body Efimov physics, as well as to recover the first nontrivial terms in the weak-coupling expansion. We show that the energy and quasiparticle residue of the dressed impurity (polaron) are significantly lowered by three-body correlations, even for weak interactions where there is no Efimov trimer state in a vacuum. For increasing attraction between the impurity and the BEC, we observe a smooth crossover from atom to Efimov trimer, with a superposition of states near the Efimov resonance. We furthermore demonstrate that three-body loss does not prohibit the experimental observation of these effects. Our results thus suggest a route to realizing Efimov physics in a stable quantum many-body system for the first time.
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.
We consider two heavy particles immersed in a Bose-Einstein condensate in three dimensions and compute their mutual interaction induced by excitations of the medium. For an ideal Bose gas, the induced interaction is Newtonian up to a shift in distance, which depends on the coupling strength between impurities and Bosons. For a real BEC, we find that, on short distances, the induced potential is dominated by three-body physics of a single Boson bound to the impurities, leading to an Efimov potential. At large distances of the order of the healing length, a Yukawa potential emerges instead. In particular, we find that both regimes are realized for all impurity-boson couplings and determine the corresponding crossover scales. The transition from the real to the ideal condensate at low gas parameters is investigated.
Since the Efimov effect was introduced in 1970, a detailed theoretical understanding of Efimov physics has been developed in the few-body context. However, it has proven to be challenging to describe the role Efimov-type correlations play in many-body systems such as quenched or collapsing Bose-Einstein condensates (BECs). To study the impact the Efimov effect can have in such scenarios, we consider a light impurity immersed in a weakly interacting BEC, forming a Bose polaron. In this case, the higher-order correlations are localized around the impurity, making it more feasible to develop a theoretical description. Specifically, we employ a Gaussian state variational Ansatz in the reference frame of the impurity, capable of both capturing the Efimov effect and the formation of the polaron cloud. We find that the Efimov effect leads to a cooperative binding of bosons to the impurity and the formation of a many-body bound state. As a result, the polaron is not the ground state, but rendered a metastable excited state which can decay into these Efimov clusters. While this decay is slow for small interaction strengths, it becomes more prominent as the attractive scattering length increases, up to the point where the polaron becomes completely unstable. This critical scattering length can be interpreted as a many-body shifted Efimov resonance, where the scattering of two excitations of the bath with the polaron can lead to bound state formation. Compared to the few-body case, the resonance is shifted to smaller attractive scattering lengths due to the participation of the polaron cloud in the cooperative binding process. This corresponds to an intriguing scenario of polaron-assisted chemistry, where many-body effects lead to enhanced signal of the chemical recombination process, which can be directly probed in state-of-the-art experiments.
Quasiparticles and their interactions are a key part of our understanding of quantum many-body systems. Quantum simulation experiments with cold atoms have in recent years advanced our understanding of isolated quasiparticles, but so far they have provided limited information regarding their interactions and possible bound states. Here, we show how exploring mobile impurities immersed in a Bose-Einstein condensate (BEC) in a two-dimensional lattice can address this problem. First, the spectral properties of individual impurities are examined, and in addition to the attractive and repulsive polarons known from continuum gases, we identify a new kind of quasiparticle stable for repulsive boson-impurity interactions. The spatial properties of polarons are calculated showing that there is an increased density of bosons at the site of the impurity both for repulsive and attractive interactions. We then derive an effective Schrödinger equation describing two polarons interacting via the exchange of density oscillations in the BEC, which takes into account strong impurity-boson two-body correlations. Using this, we show that the attractive nature of the effective interaction between two polarons combined with the two-dimensionality of the lattice leads to the formation of bound states - i.e. bipolarons. The wave functions of the bipolarons are examined showing that the ground state is symmetric under particle exchange and therefore relevant for bosonic impurities, whereas the first excited state is doubly degenerate and odd under particle exchange making it relevant for fermionic impurities. Our results show that quantum gas microscopy in optical lattices is a promising platform to explore the spatial properties of polarons as well as to finally observe the elusive bipolarons.
Impurities immersed in a Bose-Einstein condensate (BEC) can interact indirectly through the exchange of Bogoliubov excitations. These impurities, which form dressed quasiparticles known as Bose polarons due to their interaction with the BEC, can pair up to form a bound state called a bipolaron, via an induced interaction. Previous studies on induced interactions have focused primarily on cases with an isotropic excitation spectrum. In this work, we investigate the properties of induced interactions and bipolarons mediated by anisotropic Bogoliubov excitations using field theory. Taking a BEC with spin-orbit coupling as an example, we show that the induced interaction becomes anisotropic. Notably, a double-minima feature appears in the induced interaction in momentum space due to the exchange of roton excitations. Additionally, we calculate the binding energy and wave functions of these bipolarons induced by anisotropic interactions. Unlike previously studied bipolarons formed through the exchange of isotropic phonon excitations, we identify an alternative type of bipolaron whose wave functions feature a double-peak structure under strong impurity-boson interactions. Our work extends the theory of induced interactions from isotropic to anisotropic systems, and it reveals the novel features in both the induced interactions and bipolarons arising from BEC with an unconventional excitation spectrum.