Session P07: Quantum gases in low dimensions

An observation of Tomonaga-Luttinger liquid in one-dimensional interacting Bose gases

It is a long standing challenge to observe quantum liquids in low-dimensional quantum systems. In this talk, I will present a fundamental understanding of universal low energy physics in one dimensional quantum systems, showing the origin of the Tomonaga-Luttinger liquid. Then I will further discuss an experimental observation of Tomonaga-Luttinger liquid in the Lieb-Linger model. \\ \noindent [1] Yang, et. al., Phys. Rev. Lett. 119, 165701 (2017)\\ \noindent [2] Jiang, et. al., Chinese Physics. B 24, 050311 (2015)\\ \noindent [3] Cheng, et. al., Phys. Rev. B 97, 121111(R) (2018)\\   

Strong-coupling polarons in one dimensional Bose condensates

We discuss the interaction of a mobile impurity with a surrounding Bose-Einstein condensate (BEC) forming a Bose polaron. For high BEC-impurity couplings, the standard approach of expanding around a homogeneous condensate does not account adequately for condensate deformations and thus becomes inappropriate. We present an alternative approach taking into account the condensate deformation already on the mean-field level, for which analytic solutions can be obtained in 1D [1]. The calculated polaron energy and condensate depletion show excellent agreement with quantum Monte-Carlo simulations and improves on approaches based on expansions around an undepleted BEC [2]. Finally, we discuss potential problems in the calculation of a proper polaron mass. \newline \newline \newline [1] M. Schecter, D. Gangardt, A. Kamenev, New J. Phys. 18 (2016) 065002 \newline [2] F. Grusdt, G. Astarkarchik, E. Demler, New J. Phys. 19 (2017) 103035   

Observation of interparticle interaction effects in a Bose-Einstein condensate confined in a 1D optical lattice

Ultracold atoms in optical lattices provide a versatile analogue for solid-state systems and are also of practical interest for atom interferometry and precision measurements. While in many cases the single-particle band structure is well understood, unexpected or even counterintuitive behavior of the system can arise when the lattice depth decreases and becomes comparable to the mean-field interactions. The system then enters a highly nonlinear regime, which adds complexity to determining the lattice depth experimentally and can give rise to loop structures in the dispersion. We demonstrate nonlinear effects in a Bose-Einstein condensate in a 1D optical lattice where the mean-field energy is on the order of the lattice coupling strength by observing the results of Rabi oscillations and Bloch oscillations in the system. The current status and future directions of this work are discussed.   

Adiabatic Theory of Three Bosons under Transverse Confinement using Hypervectorial Coordinates

Anisotropic confinement is used to create low-dimensional quantum gases, and its role in modifying few-body collisions is fundamental for the system's lifetime and stability. Rotational symmetry is broken, so the usual techniques for obtaining adiabatic potentials are not easily applicable. Thus theories involving transverse confinement usually assume a linear geometry. Recently, Zundel \textit{et al.}$^{1}$ measured the three-body recombination rates of confined $^{87}$Rb and saw a cubic energy dependence near threshold as predicted$^{2}$. Yet questions remain; in particular, isotropic phenomena, such as the first few Efimov trimers, must persist where confinement is weak. We adopt hypervectorial coordinates$^{3}$ and derive adiabatic surfaces, from which the adiabatic curves under confinement can be found. We focus on 3 bosons with J$_{z}$ = 0 and show how the even-J$^{+}$ curves evolve into a manifold of quasi-1D curves attached to transverse modes, with emphasis on J = 0 Efimov potentials that a pure-1D theory does not capture. \\ \noindent $^{1}$L. A. Zundel \textit{et al.}, PRL \textbf{122,} 013402 (2019). \\ $^{2}$N. P. Mehta \textit{et al.}, PRA \textbf{76,} 022711 (2007). \\ $^{3}$S. T. Rittenhouse \textit{et al.}, J. Phys. Chem. A. \textbf{113,} 15016 (2009).   

Measurement of 3-body losses in a quasi-1D $^6$Li gas near a $p$-wave Feshbach resonance

$P$-wave interactions are known to lead to intriguing quantum phenomena such as $p + ip$ topological superfluids and Majorana fermions. However, the experimental detection of these phenomena in ultracold atomic gases remains a challenge due to the severe atom losses from three-body recombination collisions near the p-wave Feshbach resonance in a 3D atomic gas. It has been recently predicted\footnote{Lihong Zhou and Xiaoling Cui, Phys. Rev. A 96, 030701 (2017).} that such effects could be suppressed by introducing 1D confinement, thus leading to the formation of p-wave atom pairs. To study the stability of atom pairs, we will measure the three-body loss rate with spin-polarized $^6$Li atoms in the Zeeman ground state $\vert f=\frac{1}{2}, m_f=\frac{1}{2} \rangle$. With a two-dimensional compensated optical lattice, we can introduce a quasi-1D confinement. We will report the result of the three-body loss rate measurement in different dimensions.   

The p-wave contact of one-dimensional Fermi gases

Fermions with p-wave interactions, and other odd exchange parities, are of long-standing interest, but have also proven to be experimentally challenging to explore in both materials and ultracold gases. In contrast to broad s-wave resonances, a high closed-channel fraction of the p-wave Feshbach dimer is inevitable because the dimer is "stuck" behind a millikelvin centrifugal barrier. Recently, it has been conjectured that 1D odd-wave collisions might have a large open-channel fraction [1]. Since there is no rotation associated with odd-wave collisions in 1D, the antisymmetric collision will occur without a centrifugal barrier. Such a dimer would have a large spatial extent, much like 3D s-wave dimers. We report on work to test this conjecture in fermionic potassium. We use rf spin-flips to a weakly interacting state to measure the contact, the central quantity in a set of universal relations recently discovered for 3D p-wave gases [2]. By comparing with theoretical predictions of the p-wave contact [3] we aim to elucidate the nature of 1D p-wave physics near a Feshbach resonance, which is potentially a universal regime as in unitary 3D s-wave interactions. [1] Phys. Rev. A 96, 030701 (2017) [2] Nature Physics 12, 530 (2016) [3] Phys. Rev. A 98, 023605 (2018)   

Observation of Josephson Oscillations in a Homogeneous 2D Fermi-Gas

The Josephson Effect is one of the hallmark properties of superconductors. In solid state systems, Josephson junctions have been used for a wide range of applications, such as SQUIDs, single electron transistors and superconducting qubits. In this talk, we report on the first realization of a Josephson junction in a quasi-two-dimensional quantum gas. The atoms are confined in a homogeneous box-potential, which is split by a thin barrier created via a digital micromirror device (DMD). We initialize Josephson oscillations by imprinting a phase difference between the two sides using a second DMD and then observe the oscillations in amplitude and relative phase between the two sides. We perform such measurements for different barrier heights and for different interaction strengths. Since the Josephson Effect relies on the existence of phase coherence across a tunneling barrier this is a perfect tool to study the coherence properties of fermionic Berezinskii-Kosterlitz-Thouless superfluids across the BEC-BCS crossover. The flexibility of our setup also makes it an excellent platform to investigate fluxons, Shapiro resonances and tunnel junction arrays.   

The spin Drude weight of the XXZ chain

New techniques in the context of quantum integrable systems have raised exciting possibilities for the study of transport phenomenon. The spin Drude weight, a portion of the spin current that survives to infinite times, at zero magnetic field in a 1D XXZ Heisenberg spin chain in the paramagnetic regime has had a long and contentious history, however it seems to be amenable to these new techniques. Previous results have indicated that the spin Drude weight is a nowhere continuous (or fractal) function of the anisotropy, however this original derivation has some problematic steps. By exhausting the Mazur bound we are able to determine the value of the spin Drude weight and avoid some of the problematic steps of the original derivation, however our result was found to be equivalent to the previously obtained formula. This equivalence supports the fractal dependence on anisotropy of the spin Drude weight. We consider the consequences this has for spin transport and how this might be realized in an experimental context.