The post Thermography of the superfluid transition in a strongly interacting Fermi gas first appeared on Ultracold Quantum Gases Group.

]]>preprint version https://arxiv.org/abs/2212.13752

Heat transport can serve as a fingerprint identifying different states of matter. In a normal liquid, a hotspot diffuses, whereas in a superfluid, heat propagates as a wave called “second sound.” Direct imaging of heat transport is challenging, and one usually resorts to detecting secondary effects. In this study, we establish thermography of a strongly interacting atomic Fermi gas, whose radio-frequency spectrum provides spatially resolved thermometry with subnanokelvin resolution. The superfluid phase transition was directly observed as the sudden change from thermal diffusion to second-sound propagation and is accompanied by a peak in the second-sound diffusivity. This method yields the full heat and density response of the strongly interacting Fermi gas and therefore all defining properties of Landau’s two-fluid hydrodynamics.

The post Thermography of the superfluid transition in a strongly interacting Fermi gas first appeared on Ultracold Quantum Gases Group.

]]>The post Geometric squeezing of rotating quantum gases into the lowest Landau level first appeared on Ultracold Quantum Gases Group.

]]>The simulation of quantum Hall physics with rotating quantum gases is witnessing a revival due to recent experimental advances that enabled the observation of a Bose-Einstein condensate entirely contained in its lowest kinetic energy state, i.e. the lowest Landau level. We theoretically describe this experimental result, and show that it can be interpreted as a squeezing of the geometric degree of freedom of the problem, the guiding center metric. This “geometric squeezing” offers an unprecedented experimental control over the quantum geometry in Landau-level analogues, and at the same time opens a realistic path towards achieving correlated quantum phases akin to quantum Hall states with neutral atoms.

The post Geometric squeezing of rotating quantum gases into the lowest Landau level first appeared on Ultracold Quantum Gases Group.

]]>The post Observation of chiral edge transport in a rapidly-rotating quantum gas first appeared on Ultracold Quantum Gases Group.

]]>Ruixiao Yao, Sungjae Chi, Biswaroop Mukherjee, Airlia Shaffer, Martin Zwierlein, Richard J. Fletcher

The frictionless, directional propagation of particles at the boundary of topological materials is one of the most striking phenomena in transport. These chiral edge modes lie at the heart of the integer and fractional quantum Hall effects, and their extraordinary robustness against noise and disorder reflects the quantization of Hall conductivity in these systems. Despite their central importance, controllable injection of edge modes, and direct imaging of their propagation, structure, and dynamics, is challenging. Here, we demonstrate the distillation of individual chiral edge states in a rapidly-rotating bosonic superfluid confined by an optical boundary. Tuning the wall sharpness, we reveal the smooth crossover between soft wall behaviour in which the propagation speed is proportional to wall steepness, and the hard wall regime exhibiting chiral free particles. From the skipping motion of atoms along the boundary, we spectroscopically infer the energy gap between the ground and first excited edge bands, and reveal its evolution from the bulk Landau level splitting for a soft boundary, to the hard wall limit.

The post Observation of chiral edge transport in a rapidly-rotating quantum gas first appeared on Ultracold Quantum Gases Group.

]]>The post Dissipationless flow in a Bose-Fermi mixture first appeared on Ultracold Quantum Gases Group.

]]>Interacting mixtures of bosons and fermions are ubiquitous in nature. They form the backbone of the standard model of physics, provide a framework for understanding quantum materials such as unconventional superconductors and two-dimensional electronic systems, and are of technological importance in 3He/4He dilution refrigerators. Bose-Fermi mixtures are predicted to exhibit an intricate phase diagram featuring coexisting liquids, supersolids, composite fermions, coupled superfluids, and quantum phase transitions in between. However, their coupled thermodynamics and collective behavior challenge our understanding, in particular for strong boson-fermion interactions. Clean realizations of fully controllable systems are scarce. Ultracold atomic gases offer an ideal platform to experimentally investigate Bose-Fermi mixtures, as the species concentration and interaction strengths can be freely tuned. Here, we study the collective oscillations of a spin-polarized Fermi gas immersed in a Bose-Einstein condensate (BEC) as a function of the boson-fermion interaction strength and temperature. Remarkably, for strong interspecies interactions the fermionic collective excitations evolve to perfectly mimic the bosonic superfluid collective modes, and fermion flow becomes dissipationless. With increasing number of thermal excitations in the Bose gas, the fermions’ dynamics exhibit a crossover from the collisionless to the hydrodynamic regime, reminiscent of the emergence of hydrodynamics in two-dimensional electron fluids. Our findings open the door towards understanding non-equilibrium dynamics of strongly interacting Bose-Fermi mixtures.

The post Dissipationless flow in a Bose-Fermi mixture first appeared on Ultracold Quantum Gases Group.

]]>The post Direct observation of non-local fermion pairing in an attractive Fermi-Hubbard gas first appeared on Ultracold Quantum Gases Group.

]]>Pairing of fermions lies at the heart of superconductivity, the hierarchy of nuclear binding energies and superfluidity of neutron stars. The Hubbard model of attractively interacting fermions provides a paradigmatic setting for fermion pairing, featuring a crossover between Bose-Einstein condensation (BEC) of tightly bound pairs and Bardeen-Cooper-Schrieffer (BCS) superfluidity of long-range Cooper pairs, and a “pseudo-gap” region where pairs form already above the superfluid critical temperature. We here directly observe the non-local nature of fermion pairing in a Hubbard lattice gas, employing spin- and density-resolved imaging of ∼1000 fermionic 40K atoms under a bilayer microscope. Complete fermion pairing is revealed by the vanishing of global spin fluctuations with increasing attraction. In the strongly correlated regime, the fermion pair size is found to be on the order of the average interparticle spacing. We resolve polaronic correlations around individual spins, resulting from the interplay of non-local pair fluctuations and charge-density-wave order. Our techniques open the door toward in-situ observation of fermionic superfluids in a Hubbard lattice gas.

The post Direct observation of non-local fermion pairing in an attractive Fermi-Hubbard gas first appeared on Ultracold Quantum Gases Group.

]]>The post Quantum Register of Fermion Pairs first appeared on Ultracold Quantum Gases Group.

]]>Nature **601**, 537–541 (2022). download

featured in MIT News

Fermions are the building blocks of matter, forming atoms and nuclei, complex materials and neutron stars. Our understanding of many-fermion systems is however limited, as classical computers are often insufficient to handle the intricate interplay of the Pauli principle with strong interactions. Quantum simulators based on ultracold fermionic atoms instead directly realize paradigmatic Fermi systems, albeit in “analog” fashion, without coherent control of individual fermions. Digital qubit-based quantum computation of fermion models, on the other hand, faces significant challenges in implementing fermionic anti-symmetrization, calling for an architecture that natively employs fermions as the fundamental unit. Here we demonstrate a robust quantum register composed of hundreds of fermionic atom pairs trapped in an optical lattice. With each fermion pair forming a spin-singlet, the qubit is realized as a set of near-degenerate, symmetry-protected two-particle wavefunctions describing common and relative motion. Degeneracy is lifted by the atomic recoil energy, only dependent on mass and lattice wavelength, thereby rendering two-fermion motional qubits insensitive against noise of the confining potential. We observe quantum coherence beyond ten seconds. Universal control is provided by modulating interactions between the atoms. Via state-dependent, coherent conversion of free atom pairs into tightly bound molecules, we tune the speed of motional entanglement over three orders of magnitude, yielding 10^4 Ramsey oscillations within the coherence time. For site-resolved motional state readout, fermion pairs are coherently split into a double well, creating entangled Bell pairs. The methods presented here open the door towards fully programmable quantum simulation and digital quantum computation based on fermions.

The post Quantum Register of Fermion Pairs first appeared on Ultracold Quantum Gases Group.

]]>The post Crystallization of Bosonic Quantum Hall States first appeared on Ultracold Quantum Gases Group.

]]>Nature 601, 58-62 (2022) download

Featured in MIT News, Smithsonian Magazine, Popular Science, and on the Apple News feed

The dominance of interactions over kinetic energy lies at the heart of strongly correlated quantum matter, from fractional quantum Hall liquids, to atoms in optical lattices and twisted bilayer graphene. Crystalline phases often compete with correlated quantum liquids, and transitions between them occur when the energy cost of forming a density wave approaches zero. A prime example occurs for electrons in high magnetic fields, where the instability of quantum Hall liquids towards a Wigner crystal is heralded by a roton-like softening of density modulations at the magnetic length. Remarkably, interacting bosons in a gauge field are also expected to form analogous liquid and crystalline states. However, combining interactions with strong synthetic magnetic fields has been a challenge for experiments on bosonic quantum gases. Here, we study the purely interaction-driven dynamics of a Landau gauge Bose-Einstein condensate in and near the lowest Landau level (LLL). We observe a spontaneous crystallization driven by condensation of magneto-rotons, excitations visible as density modulations at the magnetic length. Increasing the cloud density smoothly connects this behaviour to a quantum version of the Kelvin-Helmholtz hydrodynamic instability, driven by the sheared internal flow profile of the rapidly rotating condensate. At long times the condensate self-organizes into a persistent array of droplets, separated by vortex streets, which are stabilized by a balance of interactions and effective magnetic forces.

The post Crystallization of Bosonic Quantum Hall States first appeared on Ultracold Quantum Gases Group.

]]>The post BEC Prize 2021 first appeared on Ultracold Quantum Gases Group.

]]>https://bec2021.org/bec-award/

Tilman Esslinger and Rudolf Grimm received the BEC Senior Award, and Tin-Lun (Jason) Ho the BEC Lifetime Award. The happy tetramer:

The post BEC Prize 2021 first appeared on Ultracold Quantum Gases Group.

]]>The post Geometric squeezing into the lowest Landau level first appeared on Ultracold Quantum Gases Group.

]]>Science 18 Jun 2021:

Vol. 372, Issue 6548, pp. 1318-1322

The physics of rotation plays a fundamental role across all physical arenas, from nuclear matter, to weather patterns, star formation, and black holes. The behaviour of neutral objects in a rotating frame is equivalent to that of charged particles in a magnetic field, which exhibit intriguing transport phenomena such as the integer and fractional quantum Hall effects. An intrinsic feature of both these systems is that translations along different directions do not commute, implying a Heisenberg uncertainty relation between spatial coordinates. This underlying non-commutative geometry plays a crucial role in quantum Hall systems, but its effect on the dynamics of individual wavefunctions has not been observed. Here, we exploit the ability to squeeze non-commuting variables to dynamically create a Bose-Einstein condensate in the lowest Landau level (LLL). We directly resolve the extent of the zero-point cyclotron orbits, and demonstrate geometric squeezing of the orbits’ guiding centres by more than 7 dB below the standard quantum limit. The condensate attains an aspect ratio exceeding 100 and an angular momentum of more than 1000ℏ per particle. This protocol naturally prepares a condensate in which all atoms occupy a single Landau gauge wavefunction in the LLL, with an interparticle distance approaching the size of the cyclotron orbits, offering a new route towards strongly correlated fluids and bosonic quantum Hall states.

The post Geometric squeezing into the lowest Landau level first appeared on Ultracold Quantum Gases Group.

]]>The post Universal Sound Diffusion in a Strongly Interacting Fermi Gas first appeared on Ultracold Quantum Gases Group.

]]>Science 370, 1222-1226 (2020)

MIT News: Physicists capture the sound of a “perfect” fluid

BBC Radio 4:

Radio podcasts: WGBH Radio

New Scientist, Popular Mechanics

Transport of strongly interacting fermions governs modern materials — from the high-Tc cuprates to bilayer graphene –, but also nuclear fission, the merging of neutron stars and the expansion of the early universe. Here we observe a universal quantum limit of diffusivity in a homogeneous, strongly interacting Fermi gas of atoms by studying sound propagation and its attenuation via the coupled transport of momentum and heat. In the normal state, the sound diffusivity D monotonically decreases upon lowering the temperature T, in contrast to the diverging behavior of weakly interacting Fermi liquids. As the superfluid transition temperature is crossed, D attains a universal value set by the ratio of Planck’s constant h and the particle mass m. This finding of quantum limited sound diffusivity informs theories of fermion transport, with relevance for hydrodynamic flow of electrons, neutrons and quarks.

The post Universal Sound Diffusion in a Strongly Interacting Fermi Gas first appeared on Ultracold Quantum Gases Group.

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