Sebastian Krinner

Conduction of Ultracold Fermions Through a Mesoscopic Channel

Pretend Wires Cold atomic gases have been successfully used to simulate solid-state phenomena such as quantum criticality. However, simulating mesoscopic electronic transport like that realized in quantum wires is challenging. Brantut et al. (p. 1069, published online 2 August) connected two reservoirs of fermionic 6Li atoms (simulating electrons) with a narrow channel (simulating a wire), created a nonequilibrium situation by applying a magnetic field gradient, and observed the flow through the channel. When the mean-free path of the atoms exceeded the length of the channel, the atomic density in the channel was constant in the central region and only changed at the ends, indicating the presence of contact resistance. The opposite diffusive regime created by imposing a disordered laser potential produced a uniformly varying density inside the channel. Lithium atoms are used to simulate electronic transport. 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.

A Thermoelectric Heat Engine with Ultracold Atoms

Cold Thermoelectrics Thermoelectric effects—such as the creation of a voltage drop in response to a thermal gradient (known as the Seebeck effect)—can be used for a number of applications, including converting wasted heat into power. However, especially in solids that exhibit electronic interactions, this type of behavior is not well understood. Brantut et al. (p. 713, published online 24 October; see the Perspective by Heikkilä) studied the Seebeck effect in the very controllable setting of cold atomic gases. Two initially identical reservoirs of 6Li atoms were connected using a quasi–two-dimensional channel, and the particle current after heating one of the reservoirs was measured. The atoms moved from the warmer to the cooler reservoir, the extent of which fit with theoretical predictions as the disorder in the channel and its geometry were varied. A flow of particles in response to a thermal gradient is observed in a channel connecting two reservoirs of 6Li atoms. [Also see Perspective by Heikkilä] 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 in the ballistic and diffusive regimes, 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.

Observation of quantized conductance in neutral matter.

In transport experiments, the quantum nature of matter becomes directly evident when changes in conductance occur only in discrete steps, with a size determined solely by Planck’s constant h. Observations of quantized steps in electrical conductance have provided important insights into the physics of mesoscopic systems and have allowed the development of quantum electronic devices. Even though quantized conductance should not rely on the presence of electric charges, it has never been observed for neutral, massive particles. In its most fundamental form, it requires a quantum-degenerate Fermi gas, a ballistic and adiabatic transport channel, and a constriction with dimensions comparable to the Fermi wavelength. Here we report the observation of quantized conductance in the transport of neutral atoms driven by a chemical potential bias. The atoms are in an ultraballistic regime, where their mean free path exceeds not only the size of the transport channel, but also the size of the entire system, including the atom reservoirs. We use high-resolution lithography to shape light potentials that realize either a quantum point contact or a quantum wire for atoms. These constrictions are imprinted on a quasi-two-dimensional ballistic channel connecting the reservoirs. By varying either a gate potential or the transverse confinement of the constrictions, we observe distinct plateaux in the atom conductance. The conductance in the first plateau is found to be equal to the universal conductance quantum, 1/h. We use Landauer’s formula to model our results and find good agreement for low gate potentials, with all parameters determined a priori. Our experiment lets us investigate quantum conductors with wide control not only over the channel geometry, but also over the reservoir properties, such as interaction strength, size and thermalization rate.

Observing the drop of resistance in the flow of a superfluid Fermi gas

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.

Connecting strongly correlated superfluids by a quantum point contact

Simulating electronic transport with atoms Two superconductors connected by a bridge made out of nonsuperconducting material form a so-called Josephson junction (see the Perspective by Belzig). Valtolina et al. replaced the superconductors with two reservoirs of a superfluid Fermi gas and connected them by a weak link to allow atoms to move from one side to the other. Then they made one reservoir more populated than the other and studied the ensuing dynamics as a function of interaction strength between the atoms. In a related experiment, Husmann et al. kept the interaction strength at its maximum, but varied the temperature and the properties of the link. As temperature increased, the superfluid disappeared and thermal transport took over. Science, this issue p. 1498, p. 1505; see also p. 1470 Two reservoirs of strongly interacting fermionic 6Li atoms are used to simulate a mesoscopic electronic device. [Also see Perspective by Belzig] 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 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.

Two-terminal transport measurements with cold atoms

In recent years, the ability of cold atom experiments to explore condensed-matter-related questions has dramatically progressed. Transport experiments, in particular, have expanded to the point in which conductance and other transport coefficients can now be measured in a way that is directly analogous to solid-state physics, extending cold-atom-based quantum simulations into the domain of quantum electronic devices. In this topical review, we describe the transport experiments performed with cold gases in the two-terminal configuration, with an emphasis on the specific features of cold atomic gases compared to solid-state physics. We present the experimental techniques and the main experimental findings, focusing on-but not restricted to-the recent experiments performed by our group. We finally discuss the perspectives opened up by this approach, the main technical and conceptual challenges for future developments, and potential applications in quantum simulation for transport phenomena and mesoscopic physics problems.

Mapping out spin and particle conductances in a quantum point contact

Significance Predicting the transport properties of interacting particles in the quantum regime is challenging. A conceptually simple situation is realized by connecting two reservoirs through a quantum point contact. For noninteracting fermions, the conductance is quantized in units of the inverse of Planck’s constant, reflecting the contribution to transport of an individual quantum state. We use a cold atomic Fermi gas to map out the spin and particle conductance in a quantum point contact for increasing attractive interactions and observe remarkable effects of interactions on both spin and particle transport. Our work provides maps of conductance over a wide range of parameters and in the presence of many-body correlations, yielding new insights into the nature of strongly attractive Fermi gases. We study particle and spin transport in a single-mode quantum point contact, using a charge neutral, quantum degenerate Fermi gas with tunable, attractive interactions. This yields the spin and particle conductance of the point contact as a function of chemical potential or confinement. The measurements cover a regime from weak attraction, where quantized conductance is observed, to the resonantly interacting superfluid. Spin conductance exhibits a broad maximum when varying the chemical potential at moderate interactions, which signals the emergence of Cooper pairing. In contrast, the particle conductance is unexpectedly enhanced even before the gas is expected to turn into a superfluid, continuously rising from the plateau at 1/h for weak interactions to plateau-like features at nonuniversal values as high as 4/h for intermediate interactions. For strong interactions, the particle conductance plateaus disappear and the spin conductance gets suppressed, confirming the spin-insulating character of a superfluid. Our observations document the breakdown of universal conductance quantization as many-body correlations appear. The observed anomalous quantization challenges a Fermi liquid description of the normal phase, shedding new light on the nature of the strongly attractive Fermi gas.

Observation of a Fragmented, Strongly Interacting Fermi Gas.

Describing the behaviour of strongly interacting particles in the presence of disorder is among the most challenging problems in quantum many-body physics. The controlled setting of cold atom experiments provides a new avenue to address these challenges [1], complementing studies in solid state physics, where a number of puzzling findings have emerged in experiments using superconducting thin films [2,3]. Here we investigate a strongly interacting thin film of an atomic Fermi gas subject to a random potential. We use high-resolution in-situ imaging [4-7] to resolve the atomic density at the length scale of a single impurity, which would require scanning probe techniques in solid state physics [8]. This allows us to directly observe the fragmentation of the density profile and to extract its percolation properties. Transport measurements in a two-terminal configuration indicate that the fragmentation process is accompanied by a breakdown of superfluidity. Our results suggest that percolation of paired atoms is responsible for the loss of superfluidity, and that disorder is able to increase the binding energy of pairs.

Low-Latency Digital Signal Processing for Feedback and Feedforward in Quantum Computing and Communication

Feedback is a main component of many algorithms for quantum computing and communication. A key requirement for any quantum feedback scheme is that the

Superfluidity with disorder in a thin film of quantum gas.

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