In a mesoscopic conductor, electric resistance is detected even if the device is defect-free. We engineered and studied a cold-atom analog of a mesoscopic conductor. It consists of a narrow channel connecting two macroscopic reservoirs of fermions that can be switched from ballistic to diffusive. We induced a current through the channel and found ohmic conduction, even when the channel is ballistic. We measured in situ the density variations resulting from the presence of a current and observed that density remains uniform and constant inside the ballistic channel. In contrast, for the diffusive case with disorder, we observed a density gradient extending through the channel. Our approach opens the way toward quantum simulation of mesoscopic devices with quantum gases.

Thermoelectric effects, such as the generation of a particle current by a temperature gradient, have their origin in a reversible coupling between heat and particle flows. These effects are fundamental probes for materials and have applications to cooling and power generation. Here we demonstrate thermoelectricity in a fermionic cold atoms channel, ballistic or diffusive, connected to two reservoirs. We show that the magnitude of the effect and the efficiency of energy conversion can be optimized by controlling the geometry or disorder strength. Our observations are in quantitative agreement with a theoretical model based on the Landauer-Büttiker formalism. Our device provides a controllable model-system to explore mechanisms of energy conversion and realizes a cold atom based heat engine. PACS numbers:In general, heat and particle transport are coupled processes [1]. This coupling leads to thermoelectric effects, such as a Seebeck voltage drop in a conductor subject to a thermal gradient. These effects are important for probing elementary excitations in materials, for example, giving access to the sign of charge carriers [2]. Moreover, they have practical applications to refrigeration, and power generation from waste-heat recovery [3,4]. Recently, there has also been interest in thermoelectric effects in nano-and molecular-scale electronic devices [5,6]. The progress in modeling solid-state physics with cold atoms [7,8] raises the question whether thermoelectricity can be observed in such a controlled setting [9,10], where set-ups analogous to mesoscopic devices were realized [11][12][13]. Whilst the thermodynamic interplay between thermal and density collective modes has been seen in a second sound experiment [14], thermoelectric effects have so far not been investigated.Here, we demonstrate a cold atoms device in which a particle current is generated by a temperature bias. We prepare a mesoscopic channel connecting two atomic reservoirs having equal particle numbers. Heating one of the reservoirs establishes a temperature bias and the compressible cloud forming the hot reservoir expands. Hence, one naively expects an initial particle flow from the cold denser side to the hot. In contrast, we observe the opposite effect: a net particle current initially directed from the hot to the cold side. This is a direct manifestation of the intrinsic thermoelectric power of the channel. The temperature bias leads to a current of highenergy particles from hot to cold and a current of lowenergy particles from cold to hot. In our channel, particles are transported at a rate which increases with energy, leading to an asymmetry between the high-energy and low-energy particle currents. This results in a total current from hot to cold, which overcomes the thermodynamic effect of the reservoirs. Hence, work is performed by carrying atoms from lower to higher chemical potential, and our system can be regarded as a cold-atoms based heat engine.A schematic view of the experimental setup is shown in figure 1A. It is based on our pre...

Purpose: The aim of the 2016 quantitative susceptibility mapping (QSM) reconstruction challenge was to test the ability of various QSM algorithms to recover the underlying susceptibility from phase data faithfully. Methods: Gradient-echo images of a healthy volunteer acquired at 3T in a single orientation with 1.06 mm isotropic resolution. A reference susceptibility map was provided, which was computed using the susceptibility tensor imaging algorithm on data acquired at 12 head orientations. Susceptibility maps calculated from the single orientation data were compared against the reference susceptibility map. Deviations were quantified using the following metrics: root mean squared error (RMSE), structure similarity index (SSIM), highfrequency error norm (HFEN), and the error in selected white and gray matter regions. Results: Twenty-seven submissions were evaluated. Most of the best scoring approaches estimated the spatial frequency content in the ill-conditioned domain of the dipole kernel using compressed sensing strategies. The top 10 maps in each category had similar error metrics but substantially different visual appearance. Conclusion: Because QSM algorithms were optimized to minimize error metrics, the resulting susceptibility maps suffered from over-smoothing and conspicuity loss in fine features such as vessels. As such, the challenge highlighted the need for better numerical image quality criteria.

The ability of particles to flow with very low resistance is characteristic of superfluid and superconducting states, leading to their discovery in the past century. Although measuring the particle flow in liquid helium or superconducting materials is essential to identify superfluidity or superconductivity, no analogous measurement has been performed for superfluids based on ultracold Fermi gases. Here we report direct measurements of the conduction properties of strongly interacting fermions, observing the well-known drop in resistance that is associated with the onset of superfluidity. By varying the depth of the trapping potential in a narrow channel connecting two atomic reservoirs, we observed variations of the atomic current over several orders of magnitude. We related the intrinsic conduction properties to the thermodynamic functions in a model-independent way, by making use of high-resolution in situ imaging in combination with current measurements. Our results show that, as in solid-state systems, current and resistance measurements in quantum gases provide a sensitive probe with which to explore many-body physics. Our method is closely analogous to the operation of a solid-state field-effect transistor and could be applied as a probe for optical lattices and disordered systems, paving the way for modelling complex superconducting devices.

Local density fluctuations and density profiles of a Fermi gas are measured in situ and analyzed. In the quantum degenerate regime, the weakly interacting 6 Li gas shows a suppression of the density fluctuations compared to the nondegenerate case, where atomic shot noise is observed. This manifestation of antibunching is a direct result of the Pauli principle and constitutes a local probe of quantum degeneracy. We analyze our data using the predictions of the fluctuation-dissipation theorem and the local density approximation, demonstrating a fluctuation-based temperature measurement. A finite-size system in thermodynamic equilibrium with its surrounding shows characteristic fluctuations, which carry important information about the correlation properties of the system. In a classical gas, fluctuations of the number of atoms contained in a small sub-volume yield a Poisson distribution, reflecting the uncorrelated nature of the gas. An intriguing situation arises when the thermal de Broglie wavelength approaches the interparticle separation and the specific quantum statistics of the constituent particles becomes detectable. For bosons, positive density correlations build up, until Bose-Einstein condensation occurs, as measured in Hanbury Brown-Twiss (HBT) experiments [1][2][3][4][5][6][7]. The effect of bunching also manifests itself in enhanced density fluctuations in real space [8]. In contrast, fermions obey the Pauli principle. This gives rise to anticorrelations, which have been observed in HBT experiments [9][10][11][12][13], and are expected to squeeze density fluctuations below the classical shot-noise limit [14]. Moreover, for trapped fermions, the reduction of fluctuations varies in space, reaching a maximum in the dense center of the cloud, which should be accessible to a local measurement.In this Letter we report on high-resolution in situ measurements of density fluctuations in an ultracold Fermi gas of weakly interacting 6 Li atoms. We extract the mean and the variance of the density profile from a number of absorption images recorded under the same experimental conditions. Our measurements show that the density fluctuations in the center of the trap are suppressed for a quantum degenerate gas as compared to a nondegenerate gas. We analyze our data using the fluctuation-dissipation theorem, which relates the density fluctuations of the gas to its isothermal compressibility. This allows us to extract the temperature of the system [8,15,16].We first describe the experimental procedure to obtain a quantum degenerate gas of about 6 Â 10 4 6 Li atoms equally populating the two lowest hyperfine states. Following the method described in [17], the atoms are loaded into an optical dipole trap created by a far offresonant laser with a wavelength of 1064 nm, focused to a 1=e 2 radius of ð22 AE 1Þ m [18]. The cloud is then optically moved [19] into a glass cell that provides high optical access; see Fig. 1(a). In the glass cell, forced evaporation is performed by reducing the trap power from initially 2 W to 4.7...