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Connecting strongly correlated superfluids by a quantum point contact

Work of physicists at the University of Geneva (UNIGE) and the Swiss Federal Institute of Technology in Zurich (ETHZ), in which they connected two materials with unusual quantum-mechanical properties through a quantum constriction, could open up a novel path towards both a deeper understanding of physics and future electronic devices.


By Thierry Giamarchi, Department of Quantum Matter Physics, UNIGE -Jean-Philippe Brantut, Institute for Quantum Electronics, Quantum optics group, ETH Zürich – Tilman Esslinger, Institute for Quantum Electronics, Quantum optics group, ETH Zürich

Based on article published in Science

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.

Such a contact consists in a narrow constriction separating two large reservoirs. The reservoirs can be metallic contacts, two-dimensional electron gases in a semiconductor structure or even clouds of cold atoms. When the transverse size of the contact is comparable with the de Broglie wavelength of the particles, the conduction of particles can be controlled at the level of the quantum wave function. There are only few materials in which such quantum point contacts can be engineered in a controlled way without introducing disturbance in the transport. Most of such contacts were realized in weakly interacting materials. Pushing the realization to the realm of strongly correlated systems has proven to be a considerable challenge.

Artist view of the quantum point contact between two cold atom clouds. © Dominik Husmann ETHZ

In a recent publication in Science, such a contact has been studied in a strongly interacting superfluid made of cold Fermionic atoms. The experiments were performed in the quantum optics group at ETH-Zürich, and compared with theoretical calculations performed at the university of Geneva. The experiment consists in a contact imprinted onto a gas of cold Lithium atoms cooled down to nano-Kelvin temperatures. The interactions between the atoms is  rendered strongly attractive through the use of a scattering resonance, and the gas is then superfluid at the lowest temperatures.

Such a system realizes a strongly correlated superconductor for which the critical temperature is comparable to the Fermi energy.

The quality of the junction is such that the physics of superfluid transport in the contact involves multiple Andreev reflections,   processes by which an excitation is created in the contact and the corresponding energy serves to transport a possibly large number of pairs from one reservoir to the other. For highly transparent junctions, such as the quantum point contact in the cold Fermi gas, this process is very efficient and leads to a distinctive fingerprint in the current-bias relation. This relation was measured in the experiment and quantitatively compared with the calculations.

Strongly correlated systems such the Fermi gas close to a scattering resonance have very intricate properties at finite temperature, some of these are actually not fully understood. By measuring transport through the quantum point contact, a new window opens to study these properties. Transport in the contact was measured for various temperatures and densities, showing a rich set of properties. In particular, the large conductance at high temperature where superfluidity is expected to disappear provides a new piece of the puzzle of the nature of the normal phase of the strongly interacting Fermi gas.

This work also provides a first handle on Andreev states in strongly interacting systems. Further manipulation and measurements could give a more complete picture of these states, and evidence the distinctive features of strong interactions. Furthermore, such states are one example of emergent physics that appears when a material with many-body correlations is structured into a device. More complex devices could be manufactured based on cold atomic gases in the future, following the same principle. The emergent state at interfaces that will appear in these devices could have fascinating properties, and provide a unique probe to understand the long standing questions on the nature of strongly interacting gases.

Press release in French  in English in German

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