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...

We study the dynamics of a dilute Bose gas at zero temperature following a sudden quench of the scattering length from a noninteracting Bose condensate to unitarity (infinite scattering length). We apply three complementary approaches to understand the momentum distribution and loss rates. First, using a time-dependent variational ansatz for the many-body state, we calculate the dynamics of the momentum distribution. Second, we demonstrate that, at short times and large momenta compared to those set by the density, the physics can be well understood within a simple, analytic two-body model. We derive a quantitative prediction for the evolution of Tan's contact, which increases linearly at short times. We also study the three-body losses at finite densities. Consistent with experiments, we observe lifetimes which are long compared to the dynamics of large momentum modes.Ultracold atomic physics offers unique opportunities to study strongly correlated systems due to the tunability of the interatomic interaction, parameterized by the s-wave scattering length, a, via Fano-Feshbach resonances [1]. Particularly interesting are quantum gases at unitarity, where a is much larger than any other length scale in the system. Such systems are predicted to exhibit universal behaviour, which depends only on the mean interparticle separation. Here the physics is highly nonperturbative, with no obvious small parameter. Investigations to date have predominantly focused on the Fermi gas, where three-body recombination is naturally suppressed by statistical repulsion [2]. Over the last decade, a general consensus seems to have emerged on many issues surrounding the unitary Fermi gas [3][4][5][6][7]. Theoretical understanding of the unitary Bose gas is far less developed. Although experiments in the quantum degenerate regime have been able to measure beyond mean field effects, such as the famous Lee-Huang-Yang correction [8,9] for values of na 3 7 × 10 −3 (n being the three dimensional number density), progress towards a unitary Bose gas with na 3 ≫ 1 is hampered by the catastrophic scaling of three-body loss in the system. At zero temperature, in the dilute gas, na 3 ≪ 1, with a ≫ r 0 where r 0 is the van der Waals length, the three-body recombination constant scales universally as L 3 ∝ a 4 /m [10][11][12][13][14]. This a 4 scaling renders any adiabatic transfer from the weakly interacting limit to the unitary limit impossible [15,16]. As a → ∞, although remaining dilute compared to the van der Waals length, the long-range aspects of Efimov physics become important [11][12][13][14]. One approach to limiting losses has emerged by considering non-degenerate unitary Bose gases [17,18], where the thermal de Broglie wavelength, λ T , can provide a small parameter nλ 3 T ≪ 1, and low-recombination regimes exist [19].A brazen new approach adopted in a recent experiment [20] utilizes an effectively diabatic quench in the scattering length to unitarity, with the initial gas temperature deeply degenerate. Although dimensional analysis requir...

We investigate dynamical three-body correlations in the Bose gas during the earliest stages of evolution after a quench to the unitary regime. The development of few-body correlations is theoretically observed by determining the two-and three-body contacts. We find that the growth of three-body correlations is gradual compared to two-body correlations. The three-body contact oscillates coherently, and we identify this as a signature of Efimov trimers. We show that the growth of three-body correlations depends nontrivially on parameters derived from both the density and Efimov physics. These results demonstrate the violation of scaling invariance of unitary bosonic systems via the appearance of log-periodic modulation of three-body correlations.

We investigate the dynamics of a homogenous Bose-Einstein condensate (BEC) following a sudden quench of the scattering length. Our focus is the time evolution of short-range correlations via the dynamical contact. We compute the dynamics using a combination of two- and many-body models, and we propose an intuitive connection between them that unifies their short-time, short-range predictions. Our two-body models are exactly solvable and, when properly calibrated, lead to analytic formulae for the contact dynamics. Immediately after the quench, the contact exhibits strong oscillations at the frequency of the two-body bound state. These oscillations are large in amplitude, and their time average is typically much larger than the unregularized Bogoliubov prediction. The condensate fraction shows similar oscillations, whose amplitude we are able to estimate. These results demonstrate the importance of including the bound state in descriptions of diabatically-quenched BEC experiments.Comment: 10 pages, 5 figures, updated reference

A quantum-field-theory description of photoemission by a laser-driven single-electron wave packet is presented. We show that, when the incident light is represented with multimode coherent states then, to all orders of perturbation theory, the relative phases of the electron's constituent momenta have no influence on the amount of scattered light. These results are extended using the Furry picture, where the (unidirectional) arbitrary incident light pulse is treated nonperturbatively with Volkov functions. This analysis increases the scope of our prior results in [Phys. Rev. A 84, 053831 (2011)], which demonstrate that the spatial size of the electron wave packet does not influence photoemission.

A quantum theoretical description of photoemission by a single laser-driven electron wave packet is presented. Energy-momentum conservation ensures that the partial emissions from individual momentum components of the electron wave packet do not interfere when the driving field is unidirectional. In other words, light scattering by an electron packet is independent of the phases of the pure momentum states comprising the packet; the size of the electron wave packet doesn't matter. This result holds also in the case of high-intensity multiphoton scattering. Our analysis is first presented in the QED framework. Since QED permits the second-quantized entangled electron-photon final state to be projected onto pure plane-wave states, the Born probability interpretation requires these projections to be first squared and then summed to find an overall probability of a scattering event. The QED treatment indicates how a semiclassical framework can be developed to recover the key features of the correct result.

We investigate the wave packet dynamics of a pair of particles that undergoes a rapid change of scattering length. The short-range interactions are modeled in the zero-range limit, where the quench is accomplished by switching the boundary condition of the wave function at vanishing particle separation. This generates a correlation wave that propagates rapidly to nonzero particle separations. We have derived universal, analytic results for this process that lead to a simple phase-space picture of the quench-induced scattering. Intuitively, the strength of the correlation wave relates to the initial contact of the system. We find that, in one spatial dimension, the $k^{-4}$ tail of the momentum distribution contains a ballistic contribution that does not originate from short-range pair correlations, and a similar conclusion can hold in other dimensionalities depending on the quench protocol. We examine the resultant quench-induced transport in an optical lattice in 1D, and a semiclassical treatment is found to give quantitatively accurate estimates for the transport probabilities.Comment: 11 pages, 7 figure

We study the stability of a quasi-one-dimensional dipolar Bose-Einstein condensate (dBEC) that is perturbed by a weak lattice potential along its axis. Our numerical simulations demonstrate that systems exhibiting a roton-maxon structure destabilize readily when the lattice wavelength equals either half the roton wavelength or a low roton subharmonic. We apply perturbation theory to the Gross-Pitaevskii and Bogoliubov de Gennes equations to illustrate the mechanisms behind the instability threshhold. The features of our stability diagram may be used as a direct measurement of the roton wavelength for quasi-one-dimensional geometries.It is widely believed that ultracold, gaseous samples of bosonic atoms or molecules possessing sufficiently large dipole moments will exhibit internal structure reminiscent of the roton in superfluid helium [1]. The basic phenomenology of the roton, in analogy with helium, is a local minimum in the quasiparticle dispersion ω(k). The existence of such a minimum is predicted to lead to a host of attendant phenomena in these dilute gases, including structured ground-state density profiles [2-4], reduced and anisotropic critical superfluid velocity [1,5,6], enhanced sensitivity to external perturbations [7], abrupt transitions in Faraday patterns [8,9], short-wavelength immiscibility phases [10], and strongly-oscillatory twobody correlations on the roton length scale [11]. Signatures of the roton in Bragg spectroscopy of trapped dipolar Bose-Einstein condensates (dBECs) have been calculated in Ref. [12]. While these many exciting predictions are in principle observable in current experiments with highly magnetic atoms [13][14][15], or in future experiments with electrically polar molecules [16][17][18], rotons remain to be seen directly in dBECs [19].In a dipolar condensate, the roton represents a mode of finite wavelength that has an anomolously low energy -hence the minimum in ω(k). The location of this minimum is given by a momentum approximately equal to /l t , where l t = /mω t denotes the harmonic oscillator length of the tightest confinement of the dBEC (usually along the polarization axis). Without this confinement, a homogeneous dBEC would be energetically unstable to collapse due to the attraction between dipoles that are aligned head-to-tail. In the presence of this confinement, the collapse is prevented by the zero-point energy introduced by the confinement, at least up until a critical dipole moment or density. When this critical parameter is exceeded, the condensate collapses in localized "clumps" of size λ rot ∼ l t , that is, via a dynamical instability into the roton mode [20]. For dBECs that are just barely stable, the energy minimum ω(k rot ) is achieved at the roton wave vector k rot = 2π/λ rot .A low-energy roton is therefore a mode that is linked intrinsically to condensate instability. This link suggests that the dBEC may respond nontrivially as an object of conventional spectroscopy, that is, that it would absorb and distribute energy differently from different wa...