Skip to main content

Mesoscopic Fermi Gases

Final Report Summary - MESOFERMI (Mesoscopic Fermi Gases)

The ERC project MesoFermi brought together the fields of ultracold two-dimensional (2D) Fermi gases and of mesoscopic systems. In recent years, ultracold Fermi gases have emerged as a versatile platform to study superfluidity. In these systems, clouds of thousands of fermionic atoms are cooled to nano-Kelvin temperatures. The ensuing ultracold quantum matter is excellently suited to explore hard questions of many-body quantum physics such as the nature and stability of superfluidity and superconductivity. The systems are especially appealing for quantum simulation due to their unique tunability. For instance, the form and strength of the trapping potential, the dimensionality and the interaction strength can be controlled essentially at will. To reduce the dimensionality to 2D, the ultracold cloud is pressed between two light sheets, freezing out all motion in the vertical direction.

One of the key achievements of the project was the creation of the first homogeneous 2D Fermi gases. Two-dimensional structures are present in almost all known superconductors with high critical temperatures, but the role of the reduced dimensionality is still under debate. The realized homogeneous systems are ideally suited to to probe local as well as nonlocal properties of strongly interacting many-body systems and hence open new exciting opportunities for research.

Using these systems we have pioneered the investigation of fermionic superfluidity in 2D. We have split a homogeneous 2D pair condensate into two separate reservoirs by imprinting a narrow potential structure onto the atoms and realized a Josephson junction in 2D quantum gases for the first time. This achievement has enabled us to unambiguously show phase coherence and provide strong evidence for superfluidity. We probed the current-phase relation of the junction and for the first time observed the famous Josephson relation I(ϕ)=Ic*sin(ϕ) using ultracold atoms. Additionally, we measured the critical current as a function of interaction strength and showed that Josephson oscillations, and hence our signature of superfluidity, persist across the 2D BEC-BCS crossover. This measurement of the critical current constitutes a novel probe for phase coherence in a 2D superfluid, which enabled us to perform a quantitative measurement of the Berezinskii-Kosterlitz-Thouless scaling exponent assuming algebraic long range order.

When trying to disturb the system with a moving periodic potential, we observe that the system is protected against heating below a critical velocity, providing the first unambiguous proof for superfluidity in 2D Fermi gases. Measurements of the speed of sound in 2D allow us to compare results for the equation of state from static and dynamic measurements. Furthermore, the damping of the sound modes gives access to the diffusivity, which we observe to be quantum limited.

We have loaded individual fermionic atoms into individual optical tweezers and mesoscopic 2x2 arrays. We have managed to image them with single atom, single site resolution in order to investigate mesoscopic Hubbard models.