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.

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.

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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.

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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:

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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.

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Science 370, 1222-1226 (2020)

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

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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.

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Science, 368, 190-194 (2020)

The emergence of quasiparticles in strongly interacting matter represents one of the cornerstones of modern physics. However, when different phases of matter compete near a quantum critical point, the very existence of quasiparticles comes under question. Here we create Bose polarons near quantum criticality by immersing atomic impurities in a Bose-Einstein condensate (BEC) with near-resonant interactions. Using locally-resolved radiofrequency spectroscopy, we probe the energy, spectral width, and short-range correlations of the impurities as a function of temperature. Far below the superfluid critical temperature, the impurities form well-defined quasiparticles. However, their inverse lifetime, given by their spectral width, is observed to increase linearly with temperature, a hallmark of quantum critical behavior. Close to the BEC critical temperature, the spectral width exceeds the binding energy of the impurities, signaling a breakdown of the quasiparticle picture near quantum criticality.

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Phys. Rev. Lett. 125, 113601 (2020)

DOI:10.1103/PhysRevLett.125.113601

We report on the single atom and single site-resolved detection of the total density in a cold atom realization of the 2D Fermi-Hubbard model. Fluorescence imaging of doublons is achieved by splitting each lattice site into a double well, thereby separating atom pairs. Full density readout yields a direct measurement of the equation of state, including direct thermometry via the fluctuation-dissipation theorem. Site-resolved density correlations reveal the Pauli hole at low filling, and strong doublon-hole correlations near half filling. These are shown to account for the difference between local and non-local density fluctuations in the Mott insulator. Our technique enables the study of atom-resolved charge transport in the Fermi-Hubbard model, the site-resolved observation of molecules, and the creation of bilayer Fermi-Hubbard systems.

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Phys. Rev. Lett. 125, 063401 (2020)

10.1103/PhysRevLett.125.063401

We demonstrate microwave dressing on ultracold, fermionic 23Na40K ground-state molecules and observe resonant dipolar collisions with cross sections exceeding three times the s-wave unitarity limit. The origin of these collisions is the resonant alignment of the approaching molecules’ dipoles along the intermolecular axis, which leads to strong attraction. We explain our observations with a conceptually simple two-state picture based on the Condon approximation. Furthermore, we perform coupled-channels calculations that agree well with the experimentally observed collision rates. While collisions are observed here as laser-induced loss, microwave dressing on chemically stable molecules trapped in box potentials may enable the creation of strongly interacting dipolar gases of molecules.