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. Here we study resonantly interacting Fermi gases connected by a tunable, ballistic quantum point contact, finding a non-linear 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. PACS numbers:The effect of strong interactions between the constituents of a quantum many-body system is at the origin of several challenging questions in physics. Whilst the ground states of strongly interacting systems are increasingly better understood [1], the properties out of equilibrium and at finite temperature often remain puzzling, as these are determined by the excitations above the ground state. In laboratory experiments, strongly interacting systems are found in certain materials, as well as in quantum fluids and gases [1]. In solid-state systems, a conceptually simple and clean approach to probe non-equilibrium physics is provided by transport measurements through the well-defined geometry of a quantum point contact (QPC) [2][3][4]. Yet, the technical hurdles to realise a controlled QPC between strongly correlated materials pose a big challenge. Ultra-cold atomic Fermi gases in the vicinity of a Feshbach resonance, the so-called unitary regime, provide an alternative route to study correlated systems [5]. Superfluidity has been established at low temperature [6], but the finite-temperature properties are only partially understood [7][8][9], a situation similar to the field of strongly correlated materials.Recent progresses in the manipulation of cold atomic gases have allowed to create a mesoscopic device featuring quantised conductance between two reservoirs in the non-interacting regime [16]. We use this technique to create a QPC in a strongly interacting Fermi gas consisting of 1.7(2) × 10 5 6 Li atoms in each of the two lowest hyperfine states, in a magnetic field of 832 G, where the interaction strength diverges due to a broad Feshbach resonance. The atoms form a strongly correlated superfluid, with a pairing gap larger than the chemical potential [5]. Typical temperatures in the cloud are T = 100(4) nK at a chemical potential of µ = 360 nK · k B . The setup is presented in Figure 1A [11]. The QPC is characterised by transverse trapping frequencies of ν x = 10.0(4) and ν z = 10(3) kHz in x-and z-direction. An optical attractive "gate" potential is used to tune the chemical potenti...

We develop Bogoliubov theory of spin-1 and spin-2 Bose-Einstein condensates (BECs) in the presence of a quadratic Zeeman effect, and derive the Lee-Huang-Yang (LHY) corrections to the ground-state energy, pressure, sound velocity, and quantum depletion. We investigate all the phases of spin-1 and spin-2 BECs that can be realized experimentally. We also examine the stability of each phase against quantum fluctuations and the quadratic Zeeman effect. Furthermore, we discuss a relationship between the number of symmetry generators that are spontaneously broken and that of Nambu-Goldstone (NG) modes. It is found that in the spin-2 nematic phase there are special Bogoliubov modes that have gapless linear dispersion relations but do not belong to the NG modes.

An extremely polarized mixture of an ultracold Fermi gas is expected to reduce to a Fermi polaron system, which consists of a single impurity immersed in the Fermi sea of majority atoms. By developing a many-body T-matrix theory, we investigate spectral properties of the polarized mixture in experimentally relevant regimes in which the system of finite impurity concentration at nonzero temperature is concerned. We explicitly demonstrate presence of polaron physics in the polarized limit and discuss effects of many polarons in an intermediate regime in a selfconsistent manner. By analyzing the spectral function at finite impurity concentration, we extract the attractive and repulsive polaron energies. We find that a renormalization of majority atoms via an interaction with minority atoms and a thermal depletion of the impurity chemical potential are of significance to depict the many-polaron regime.

We consider a two-leg Bose-Hubbard ladder in the presence of a magnetic flux. We make use of Gross-Pitaevskii, Bogoliubov, bosonization, and renormalization group approaches to reveal a structure of ground-state phase diagrams in a weak-coupling regime relevant to cold atom experiments. It is found that except for a certain flux φ = π, the system shows different properties as changing hoppings, which also leads to a quantum phase transition similar to the ferromagnetic XXZ model. This implies that population-imbalance instability occurs for certain parameter regimes. On the other hand, for φ = π, it is shown that an umklapp process caused by commensurability of a magnetic flux stabilizes a superfluid with chirality and the system does not experience such a phase transition.

We show that quasi-Nambu-Goldstone (NG) modes, which play prominent roles in high energy physics but have been elusive experimentally, can be realized with atomic Bose-Einstein condensates. The quasi-NG modes emerge when the symmetry of a ground state is larger than that of the Hamiltonian. When they appear, the conventional vacuum manifold should be enlarged. Consequently, topological defects that are stable within the conventional vacuum manifold become unstable and decay by emitting the quasi-NG modes. Contrary to conventional wisdom, however, we show that the topological defects are stabilized by quantum fluctuations that make the quasi-NG modes massive, thereby suppressing their emission.