Session P5: Transport and Spatial Dynamics

Coherent Matterwave Emission from an Atomtronic Transistor

We investigate matterwave emission from a triple-well ``transistor'' atomic potential consisting of a ``source'' well treated as a reservoir having fixed chemical potential and temperature, a narrow ``gate'' well, and a ``drain'' well coupled to the vacuum. The ground state of the gate well is occupied by a Bose-Einstein condensate having large chemical potential along with the first excited state of the potential. These lower states are coupled to the two highest lying bound states lying near the top of the barriers separating the gate from the other two wells. We show that the energy level separations of the two lower states and the two upper states can be made degenerate by design of the Gaussian barrier widths and separation. When degenerate, the two pairs of states are strongly coupled by phonon exchange. We seek a self-consistent solution for the coupling between the high-lying states and the ground state pair, which occurs due to stimulated absorption and emission of phonons. In steady-state, coupling of the upper states leads to matter wave emission such that the emission of the two states is mutually coherent. The output from the transistor is thus an intensity-modulated matterwave whose frequency is approximately equal to the ground-state trap frequency.   

Non-linear superflow of a unitary Fermi gas through a quantum point contact

Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors, or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However, for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. We studied resonantly interacting Fermi gases of $^6$Li atoms connected by a tunable, ballistic quantum point contact, finding a nonlinear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.   

Transport in a capacitive ultracold atomtronic circuit

A recent NIST experiment~\footnote{J.G.\ Lee, et al.,arXiv:1506.08413 (2015)} studied the transport of a gaseous Bose--Einstein condensate (BEC) confined in an atomtronic ``dumbbell'' circuit. The optically created condensate potential consisted of a tight harmonic potential in the vertical direction confining the BEC to a horizontial plane. The horizontal potential consisted of two cylindrical wells separated by a channel produced by a harmonic oscillator potential transverse to the line joining the wells. The BEC, formed in the ``source'' well, was released to flow toward the ``drain'' well. We modeled this system with the Gross-Pitaevskii (GP) equation and found good agreement with the data provided that the channel potential is carefully reproduced. The GP simulations show behavior, not detectable in the experiment, that atoms can jump out of the dumbbell area after filling up the drain well. We describe the GP evolution of this system with a model RCL circuit having a time--dependent resistance. This resistance exhibits a strong connection to the time--dependence of the atom loss in the drain. We also studied and present the dependence of the $R$ and $L$ parameters of this model circuit on the channel shape.   

Crossover from classical to quantum spin diffusion in a nondegenerate gas

We study the crossover from classical to quantum diffusion by observing the equilibration of longitudinal spin domains in a trapped 87Rb sample just above quantum degeneracy. By controlling the degree of spin coherence in the domain wall, we can dramatically alter the relaxation dynamics of the system. Coherence in the domain wall leads to transverse-spin-mediated longitudinal spin diffusion that is slower than classical predictions, as well as altering the domains' oscillation frequency. We also investigate an instability in the longitudinal spin dynamics as the longitudinal and transverse spin components couple, and a conversion of longitudinal spin to transverse spin is observed, leading to longer lived coherent spin oscillations.   

Transport of a lattice gas under continuous measurement

The act of measurement has a profound consequence on a quantum system. While this backaction has hitherto been discussed as a limitation to the precision of measurements, it is increasingly being appreciated that measurement backaction is a powerful means of quantum control. We have previously demonstrated that backaction from position measurement can modify the coherent tunneling rate of a lattice gas through the Quantum Zeno effect [1]. By suitably designing measurement landscapes we can control the transport properties of the lattice gas. We describe a quantitative study of lattice gas dynamics under continuous quantum measurement in the context of a quantum to classical transition where the atom dynamics goes from a quantum walk at low measurement strengths to classical diffusion at high measurement strengths. We further discuss the prospect of using disorder measurement landscapes to realize a new form of Anderson localization. \\[4pt] [1] Y. S. Patil, S. Chakram and M. Vengalattore, Phys. Rev. Lett. 115, 140402 (2015)   

Signatures of spatial inversion asymmetry of an optical lattice observed in matter-wave diffraction

The structure of a two-dimensional honeycomb optical lattice potential with small inversion asymmetry is characterized using coherent diffraction of $^{87}$Rb atoms. We demonstrate that even a small potential asymmetry, with peak-to-peak amplitude of $\leq 2.3\%$ of the overall lattice potential, can lead to pronounced inversion asymmetry in the momentum-space diffraction pattern. The observed asymmetry is explained quantitatively by considering both Kaptiza-Dirac scattering in the Raman-Nath regime, and also either perturbative or full-numerical treatment of the band structure of a periodic potential with a weak inversion symmetry breaking term. Our results have relevance both for the experimental development of coherent atom optics and also for the proper interpretation of time-of-flight assays of atomic materials in optical lattices.   

Symmetry-broken momentum distributions induced by matter-wave diffraction during time-of-flight expansion of ultracold atoms

The information about quantum gas systems is still commonly inferred from time-of-flight measurements. Here, we demonstrate that interaction during the time-of-flight expansion can strongly alter the measurement of the initial atomic momentum distribution. We discuss the observation of symmetry-broken momentum distributions for bosonic mixtures in state-dependent honeycomb lattices due to scattering processes within the first milliseconds of the expansion time. These findings are of fundamental importance in a broad range of systems, including state-dependent lattices and superlattices, where the lattice symmetry does not cancel the influence of the scattering processes on the interference pattern. Beyond that, the interactions during a free expansion can be used as an interferometric probe to reveal novel quantum phases, such as supersolids.   

Emergence of a Turbulent Cascade in a Quantum Gas

The recent realisation of Bose-Einstein condensates in uniform traps has opened interesting possibilities to study far-from-equilibrium phenomena with textbook systems. In this talk, we will present a study where we drive a homogeneous Bose--Einstein condensate (BEC) out of equilibrium with an oscillating force that pumps energy into the system at the largest lengthscale. In the limit of weak drives, the BEC's response is linear, well captured by its lowest-lying excitations. For stronger drives, a nonlinear response is apparent and we observe a gradual development of a cascade characterised by an isotropic power-law distribution in momentum space. Our conclusions are well supported by comparison with numerical simulations of the Gross--Pitaevskii equation.   

Shock Waves in the BEC to BCS Crossover of a Fermi Gas

We observe shock waves in a Fermi gas near a Feshbach resonance, using a micro-mirror array to create a spatially controlled, blue-detuned, repulsive optical potential. We separate an optically-trapped gas of $^{\mathrm{6}}$Li into two clouds with steep density profiles. When the repulsive potential beam is extinguished, the two halves of the cloud collide in the optical trap, producing shock waves. Using in-situ imaging, we find that the steep density gradients associated with shockwaves are most pronounced near resonance and become less pronounced as the magnetic field is tuned above resonance to create a weakly interacting Fermi gas or below resonance to create a weakly interacting Bose gas of dimers. Using this method, we study the crossover from dispersive to dissipative non-linear hydrodynamics as a function of interaction strength and temperature.   

Exact mapping between different dynamics of isotropically trapped quantum gases

Experiments on trapped quantum gases can probe challenging regimes of quantum many-body dynamics, where strong interactions or non-equilibrium states prevent exact theoretical treatment. In this talk, we present a class of exact mappings between all the observables of different experiments, under the experimentally attainable conditions that the gas particles interact via a homogeneously scaling two-body potential which is in general time-dependent, and are confined in an isotropic harmonic trap. We express our result through an identity relating second-quantized field operators in the Heisenberg picture of quantum mechanics which makes it general. It applies to arbitrary measurements on possibly multi-component Bose or Fermi gases in arbitrary initial quantum states, no matter how highly excited or far from equilibrium. We use an example to show how the results of two different and currently feasible experiments can be mapped onto each other by our spacetime transformation. DAMOP sorting category: 6.11 Nonlinear dynamics and out-of-equilibrium trapped gases