Session C05: Polarons and Impurities

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The study of quantum many-body systems has unveiled remarkable new physics upon the inclusion of impurities, with the polaron - a hybridization between an electron and a lattice phonon - a paradigmatic example. Though firmly rooted in the phenomenology of solid-state physics, interest in analogue models of polaron physics by immersion of impurities in Bose-Einstein condensates (BECs) has grown recently [J. Tempere et al., Phys. Rev. B 80 (2009)]. In turn, vortices in BECs have been shown to be producible in a controlled manner [J. R. Abo-Shaeer et al., Science 292, 5516 (2001)], whose stability is founded on a topological invariant quantizing the angular momentum of the BEC. In this work, we investigate the problem of an impurity bounded to a single vortex in a quasi two-dimensional (2D) BEC in the mean-field regime, obtaining approximate analytical solutions for the bound-state spectrum; in particular, we show that there is a tunable, finite number of bound states [JEHB, H. Ter\c{c}as, arXiv:1910.08434 (accepted in Phys. Rev. A)]. Such a vortex-impurity system provides a potentially new paradigm for polaron physics in quasi-2D BECs, as well as a promising scheme for the realization of qubits due to the topological stability of the vortex [JEHB, H. Ter\c{c}as, (in preparation)].   

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We investigate the radio-frequency spectroscopy of impurities interacting with a quantum gas at finite temperature. In the limit of a single impurity, we show that introducing (or injecting) an impurity into the medium is equivalent to ejecting an impurity that is initially interacting with the medium since the ``injection'' and ``ejection'' spectral responses are simply related to each other by an exponential function of frequency. Thus, the full spectral information for the quantum impurity is contained in the injection spectral response, which can be determined using a range of theoretical methods, including variational approaches. Our results for the contact of a mobile impurity are in excellent agreement with recent experiments and we find that the finite-temperature behaviour is qualitatively different compared to the case of infinite impurity mass.   

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Understanding the behavior of a spin impurity strongly-interacting with a Fermi sea is a long-standing challenge in many-body physics. For short-range interactions and zero temperature, most theories predict a first-order phase transition between a polaronic ground state and a molecular one. We study this question with an ultracold Fermi gas, utilizing a novel high-sensitivity Raman spectroscopy probing technique that allows us to isolate the quasiparticle contribution [1-3]. We find that for increasing interactions, there is a smooth transition from a polaronic to a molecular response, with no evidence of a first-order phase transition. We determine the polaron energy, molecule binding energy, and the contact parameter. The later follows the molecular branch, in contrast to the prediction that it will have a clear change of behavior as the ground state changes its nature. The emerging physical picture is of a smooth transition between polarons and molecules and coexistence of both in the region around the expected phase-transition. [1] C. Shkedrov, Y. Florshaim, G. Ness, A. Gandman, and Y. Sagi, PRL 121, 093402 (2018). [2] C. Shkedrov, G. Ness, Y. Florshaim, and Y. Sagi, PRA 101, 013609 (2020). [3] G. Ness, C. Shkedrov, Y. Florshaim, and Yoav Sagi, arXiv:2001.10450 (2020).   

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Hybrid systems of ultracold ions and atoms are characterized by relatively long-range interactions which compete with the other length scales in the system. This opens the possibility to study new regimes of polaron physics. We show that a single ion immersed in a degenerate Bose gas can acquire very large effective mass, binding multiple bosons in a cluster state. The presence of the ion induces correlations in the gas which can be interpreted as effective interaction. We also discuss the properties of an ion crystal with an atomic impurity, where the polaron is formed by dressing with the lattice distortion. We show that the system can be described in terms of an extended Hubbard-Holstein model with exotic long-ranged interactions.   

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A Rydberg molecule, formed by the binding of one or more ground-state atoms to a Rydberg atom, can be destroyed by interactions involving the Rydberg core ion and a neutral ground-state atom within the Rydberg electron orbit. Previous studies with rubidium and strontium have measured such loss rates in dense gases($\sim10^{14}$cm$^{-3}$) and have attributed the destruction to two main processes: associative ionization resulting in a Rb$_2^+$/Sr$_2^+$ ion formation and dissociation due to L-changing collisions[1,2]. Short lifetimes($\sim1\mu$s) were measured at low principal quantum numbers ($n$) for both Rb and Sr, whereas an increased lifetime($\sim10\mu$s) was reported at high $n$($\sim$140) for Rb. This behavior in Rb was attributed to the presence of a p-wave shape resonance in e-Rb scattering. Here we present a study of the lifetime of Sr Rydberg molecules (Sr does not possess a p-wave shape resonance) in a dense BEC by monitoring the evolution of the Rydberg population using field ionization. The total decay rate and the competition between two main destruction channels is investigated for different $n$, 49$\leq n\leq$150, and BEC densities, $0.8\times10^{14}$cm$^{-3}\leq\rho\leq3\times 10^{14}$cm$^{-3}$. [1] Phys. Rev. A 96, 042702(2017), [2] Phys. Rev. X 6, 031020(2016).   

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Rydberg dressing, the admixing of a Rydberg state into the ground state of an atom, has been proposed as a way to introduce laser-controlled long-range interactions in an ultracold gas. The resulting soft-core interaction potentials have been measured with localized atoms [1,2] and used to simulate spin models in optical lattices [3]. An exciting prospect is the preparation of a Rydberg-dressed quantum gas with a sufficiently long lifetime to observe motional effects. We report on the realization of Rydberg-dressing of lithium-6 atoms in an optical lattice using single photon excitation. We measure the interactions between the dressed atoms and study the lifetime of the gas and its dependence on the geometry of the sample. The experiments are conducted in a quantum gas microscope, which allows the use of small atom numbers to ameliorate the effect of collective loss processes. The light mass of lithium-6 facilitates exiting the frozen gas regime while the fermionic nature of the species opens the door to new avenues in the quantum simulation of Hamiltonians with long-range interactions. [1] Y.-Y. Jau et al, Nat. Phys. 12, 71 (2016) [2] J. Zeiher et al, Nat. Phys. 12, 1095 (2016) [3] J. Zeiher et al, Phys. Rev. X 7, 041063 (2017)   

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In a mixture of a condensed Bose gas (BEC) and spin polarized degenerate Fermi gas (DFG), fermions can mediate interaction between bosons, leading to an effective long-range interaction between the bosons, analogous to Ruderman--Kittel--Kasuya--Yosida (RKKY) interaction in solids. We used Ramsey spectroscopy to measure frequency shifts of the bosons' hyperfine levels due to interactions with fermions. We isolated the frequency shift related to mediated interaction from shifts caused by direct collision of fermions and bosons. Our measurement showed an increase of spin-spin interaction between bosons by a factor of $\eta = 1.43 \pm 0.06^{\mathrm{stat}}\pm 0.17^{\mathrm{sys}}$ in the presence of the DFG, providing a clear evidence of spin-spin fermion mediated interaction. This interaction can be tuned with a boson-fermion Feshbach resonance. Fermion mediated interactions can potentially give rise to interesting new magnetic phases and extend the Bose-Hubbard model when the atoms are placed in an optical lattice.   

Time-resolved correlated dynamics of Polarons and evidence for thermalization

We unravel the time-resolved dynamics of either fermionic or bosonic impurities immersed in a harmonically trapped Bose Einstein condensate utilizing pump-probe spectroscopy. A pump pulse transfers the impurities from a noninteracting to the polaronic state whilst the probe pulse allows to directly monitor their nonequilibrium dynamics. Coherent attractive or repulsive Fermi and Bose polarons and their induced-interactions are detected in the probe spectra for moderate attractive and repulsive interspecies interactions. Strikingly, for strong repulsions an orthogonality catastrophe occurs independently of the impurities flavour and interactions. In this case and for long time scales a steady state is reached characterized by substantial coherence losses of the impurities which acquire an effectively large temperature. This steady state is related to an eigenstate thermalization which is found to be independent of the system’s characteristics.   

An ultracold heavy Rydberg system formed from ultra-long-range molecules bound in a stairwell potential

We propose a scheme to realize a \textit{heavy Rydberg system} (HRS), a bound pair of oppositely charged ions, from a gas of ultracold atoms. The intermediate step to achieve large internuclear separations is the creation of a unique class of ultra-long-range Rydberg molecules bound in a stairwell potential energy curve. Here, a ground-state atom is bound to a Rydberg atom in an oscillatory potential emerging due to attractive singlet $p$-wave electron scattering. The utility of our approach originates in the large electronic dipole transition element between the Rydberg- and the ionic molecule, while the nuclear configuration of the ultracold gas is preserved. The Rabi coupling between the Rydberg molecule and the heavy Rydberg system is typically in the MHz range and the permanent electric dipole moments of the HRS can be as large as one kilo-Debye. We identify specific transitions which place the creation of the heavy Rydberg system within immediate reach of experimental realization.