Our group studies ultracold gases near Absolute Zero temperature. At temperatures a million times colder than interstellar space, and at densities a million times thinner than air, quantum mechanics takes center stage: Atoms behave as waves, they interfere like laser light, and form novel states of matter, such as Bose-Einstein condensates and fermionic superfluids. In ultracold Fermi gases, atoms team up in pairs that can flow without friction. This is directly related to superconductivity in metals, where electron pairs transport current without resistance. In contrast to bulk materials, we can freely tune the interaction between atoms and, for example, explore the crossover from a Bose-Einstein condensate of tightly bound molecules to a Bardeen-Cooper-Schrieffer superfluid of long-range fermion pairs. Our goal is to use these gases as model systems for strongly interacting quantum matter, from High-T_{c} superconductors to Neutron Stars.

- Universal Sound Diffusion in a Strongly Interacting Fermi Gas (12/4/2020)
Parth B. Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard J. Fletcher, Julian Struck, Martin W. Zwierlein

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.

- Bose polarons near quantum criticality (4/10/2020)
Zoe Z. Yan, Yiqi Ni, Carsten Robens, Martin W. Zwierlein

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.

- Congratulations to Dr. Richard Fletcher on his Assistant Professorship at MIT (3/29/2020)
We are all excited for Rich to be starting his own research group at MIT!

- Measuring total density correlations in a Fermi-Hubbard gas via bilayer microscopy (3/25/2020)
Thomas Hartke, Botond Oreg, Ningyuan Jia, Martin Zwierlein

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.

- Resonant dipolar collisions of ultracold molecules induced by microwave dressing (3/5/2020)
Zoe Z. Yan, Jee Woo Park, Yiqi Ni, Huanqian Loh, Sebastian Will, Tijs Karman, Martin Zwierlein

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.

- Geometric squeezing into the lowest Landau level (11/21/2019)
Richard J. Fletcher, Airlia Shaffer, Cedric C. Wilson, Parth B. Patel, Zhenjie Yan, Valentin Crépel, Biswaroop Mukherjee, Martin W. Zwierlein

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.

- Photo-induced two-body loss of ultracold molecules (5/16/2019)
Arthur Christianen, Martin W. Zwierlein, Gerrit C. Groenenboom, Tijs Karman

Phys. Rev. Lett. 123, 123402 (2019)

The lifetime of nonreactive ultracold bialkali gases was conjectured to be limited by sticky collisions amplifying three-body loss. We show that the sticking times were previously overestimated and do not support this hypothesis. We find that electronic excitation of NaK+NaK collision complexes by the trapping laser leads to the experimentally observed two-body loss. We calculate the excitation rate with a quasiclassical, statistical model employing ab initio potentials and transition dipole moments. Using longer laser wavelengths or repulsive box potentials may suppress the losses.

- Congratulations to Dr. Lawrence Cheuk on his Assistant Professorship at Princeton University (4/12/2019)
We wish Lawrence success and wonderful discoveries with his new research group!

Check out his homepage: https://sites.google.com/view/cheuk-lab/home

- Spectral response and contact of the unitary Fermi gas (2/26/2019)
Biswaroop Mukherjee, Parth B. Patel, Zhenjie Yan, Richard J. Fletcher, Julian Struck, Martin W. Zwierlein

Spectral response and contact of the unitary Fermi gas

We measure radiofrequency (rf) spectra of the homogeneous unitary Fermi gas at temperatures ranging from the Boltzmann regime through quantum degeneracy and across the superfluid transition. For all temperatures, a single spectral peak is observed. Its position smoothly evolves from the bare atomic resonance in the Boltzmann regime to a frequency corresponding to nearly one Fermi energy at the lowest temperatures. At high temperatures, the peak width reflects the scattering rate of the atoms, while at low temperatures, the width is set by the size of fermion pairs. Above the superfluid transition, and approaching the quantum critical regime, the width increases linearly with temperature, indicating non-Fermi-liquid behavior. From the wings of the rf spectra, we obtain the contact, quantifying the strength of short-range pair correlations. We find that the contact rapidly increases as the gas is cooled below the superfluid transition.

- Spin Transport in a Mott Insulator of Ultracold Fermions (12/6/2018)
Matthew A. Nichols, Lawrence W. Cheuk, Melih Okan, Thomas R. Hartke, Enrique Mendez, T. Senthil, Ehsan Khatami, Hao Zhang, Martin W. Zwierlein

Science,**363**, 383 (2019)Science Perspective by Jean-Philippe Brantut, EPFL Lausanne

MIT News Article by Helen Knight

Strongly correlated materials are expected to feature unconventional transport properties, where charge, spin, and heat conduction are potentially independent probes of the dynamics. In contrast to charge transport, the measurement of spin transport in such materials is highly challenging. Here we observe spin conduction and diffusion in a system of ultracold fermionic atoms that realizes the half-filled Fermi-Hubbard model. For strong interactions, spin diffusion is driven by super-exchange and doublon-hole-assisted tunneling, and strongly violates the quantum limit of charge diffusion. The technique developed in this work can be extended to finite doping, which can shed light on the complex interplay between spin and charge in the Hubbard model.

- Boiling a Unitary Fermi Liquid (11/1/2018)
Zhenjie Yan, Parth B. Patel, Biswaroop Mukherjee, Richard J. Fletcher, Julian Struck, Martin W. Zwierlein

Phys. Rev. Lett.

**122**, 093401 (2019)See Viewpoint by Pietro Massignan: From Quantum Quasiparticles to a Classical Gas

We study the thermal evolution of a highly spin-imbalanced, homogeneous Fermi gas with unitarity limited interactions, from a Fermi liquid of polarons at low temperatures to a classical Boltzmann gas at high temperatures. Radio-frequency spectroscopy gives access to the energy, lifetime and the short-range correlations of Fermi polarons at low temperatures T. In this regime we observe a characteristic $\propto T^2$ dependence of the spectral width, corresponding to the quasiparticle decay rate expected for a Fermi liquid. At high T the spectral width decreases again towards the scattering rate of the classical, unitary Boltzmann gas, $\propto T^{1/2}$. In the transition region between the quantum degenerate and classical regime, the spectral width attains its maximum, on the scale of the Fermi energy, indicating the breakdown of a quasiparticle description. Density measurements in a harmonic trap directly reveal the majority dressing cloud surrounding the minority spins, and yield the compressibility along with the effective mass of Fermi polarons.

- Two-Photon Spectroscopy of the NaLi Triplet Ground State (12/19/2017)
Timur M. Rvachov, Hyungmok Son, Juliana J. Park, Sepehr Ebadi, Martin W. Zwierlein, Wolfgang Ketterle, Alan O. Jamison

Phys. Chem. Chem. Phys., 20, 4739-4745 (2018)

We employ two-photon spectroscopy to study the vibrational states of the triplet ground state potential (a3Σ+) of the 23Na6Li molecule. Pairs of Na and Li atoms in an ultracold mixture are photoassociated into an excited triplet molecular state, which in turn is coupled to vibrational states of the triplet ground potential. Vibrational state binding energies, line strengths, and potential fitting parameters for the triplet ground a3Σ+potential are reported. We also observe rotational splitting in the lowest vibrational state.

- Photoassociation of Ultracold NaLi (12/19/2017)
Timur M. Rvachov, Hyungmok Son, Juliana J. Park, Pascal M. Notz, Tout T. Wang, Martin W. Zwierlein, Wolfgang Ketterle, Alan O. Jamison

Phys. Chem. Chem. Phys., 2018,20, 4746-4751 (2018)

We perform photoassociation spectroscopy in an ultracold 23Na-6Li mixture to study the c3Σ+ excited triplet molecular potential. We observe 50 vibrational states and their substructure to an accuracy of 20 MHz, and provide line strength data from photoassociation loss measurements. An analysis of the vibrational line positions using near-dissociation expansions and a full potential fit is presented. This is the first observation of the c3Σ+ potential, as well as photoassociation in the NaLi system.

- Congratulations to Dr. Julian Struck for receiving the CNRS Junior Research Chair at ENS Paris (8/1/2017)
All the best of luck to Julian!

- Second-Scale Nuclear Spin Coherence Time of Ultracold NaK Molecules (7/28/2017)
Jee Woo Park, Zoe Z. Yan, Huanqian Loh, Sebastian A. Will, Martin W. Zwierlein Coherence, the stability of the relative phase between quantum states, lies at the heart of quantum mechanics. Applications such as precision measurement, interferometry, and quantum computation are enabled by physical systems that have quantum states with robust coherence. With the creation of molecular ensembles at sub-μK temperatures, diatomic molecules have become a novel system under full quantum control. Here, we report on the observation of stable coherence between a pair of nuclear spin states of ultracold fermionic NaK molecules in the singlet rovibrational ground state. Employing microwave fields, we perform Ramsey spectroscopy and observe coherence times on the scale of one second. This work opens the door for the exploration of single molecules as a versatile quantum memory. Switchable long-range interactions between dipolar molecules can further enable two-qubit gates, allowing quantum storage and processing in the same physical system. Within the observed coherence time, 10

^{4}one- and two-qubit gate operations will be feasible. - Long-Lived Ultracold Molecules with Electric and Magnetic Dipole Moments (7/12/2017)
Timur M. Rvachov, Hyungmok Son, Ariel T. Sommer, Sepehr Ebadi, Juliana J. Park, Martin W. Zwierlein, Wolfgang Ketterle, Alan O. Jamison

Phys. Rev. Lett. 119, 143001 (2017)

We create fermionic dipolar 23Na6Li molecules in their triplet ground state from an ultracold mixture of 23Na and 6Li. Using magneto-association across a narrow Feshbach resonance followed by a two-photon STIRAP transfer to the triplet ground state, we produce 3×10^4 ground state molecules in a spin-polarized state. We observe a lifetime of 4.6s in an isolated molecular sample, approaching the p-wave universal rate limit. Electron spin resonance spectroscopy of the triplet state was used to determine the hyperfine structure of this previously unobserved molecular state.

- Homogeneous Atomic Fermi Gases (10/31/2016)
Biswaroop Mukherjee, Zhenjie Yan, Parth B. Patel, Zoran Hadzibabic, Tarik Yefsah, Julian Struck, Martin W. Zwierlein Phys. Rev. Lett. 118, 123401 (2017), arXiv:1610.10100

We report on the creation of homogeneous Fermi gases of ultracold atoms in a uniform potential. In the momentum distribution of a spin-polarized gas, we observe the emergence of the Fermi surface and the saturated occupation of one particle per momentum state. This directly confirms Pauli blocking in momentum space. For the spin-balanced unitary Fermi gas, we observe spatially uniform pair condensates. For thermodynamic measurements, we introduce a hybrid potential that is harmonic in one dimension and uniform in the other two. The spatially resolved compressibility reveals the superfluid transition in a spin-balanced Fermi gas, saturation in a fully polarized Fermi gas, and strong attraction in the polaronic regime of a partially polarized Fermi gas.

- Martin Zwierlein receives I.I. Rabi Prize of the American Physical Society (10/12/2016)
- Two and Three-body Contacts in the Unitary Bose Gas (8/15/2016)
Richard J. Fletcher, Raphael Lopes, Jay Man, Nir Navon, Robert P. Smith, Martin W. Zwierlein, Zoran Hadzibabic Science 355, 377-380 (2017), arXiv:1608.04377

In many-body systems governed by pairwise contact interactions, a wide range of observables is linked by a single parameter, the two-body contact, which quantifies two-particle correlations. This profound insight has transformed our understanding of strongly interacting Fermi gases. Here, using Ramsey interferometry, we study coherent evolution of the resonantly interacting Bose gas, and show that it cannot be explained by only pairwise correlations. Our experiments reveal the crucial role of three-body correlations arising from Efimov physics, and provide a direct measurement of the associated three-body contact.

- Congratulations to Dr. Tarik Yefsah for starting his position as CNRS Permanent Researcher (8/1/2016)
All the best to Tarik for a continued string of wonderful discoveries.

Check out his latest news: http://www.lkb.upmc.fr/ultracoldfermigases/yefsah/

- Congratulations to Dr. Sebastian Will for his Assistant Professorship at Columbia University (8/1/2016)
All the best of luck and success to Sebastian with his new research group at Columbia.

Check out their webpage: https://www.will-lab.com/ - Spatial Charge and Spin Correlations in the 2D Fermi-Hubbard Model (6/13/2016)
Lawrence W. Cheuk, Matthew A. Nichols, Katherine R. Lawrence, Melih Okan, Hao Zhang, Ehsan Khatami, Nandini Trivedi, Thereza Paiva, Marcos Rigol, Martin W. Zwierlein Science 353, 1260-1264 (2016), arXiv:1606.04089

Strong electron correlations lie at the origin of transformative phenomena such as colossal magneto-resistance and high-temperature superconductivity. Already near room temperature, doped copper oxide materials display remarkable features such as a pseudo-gap and a “strange metal” phase with unusual transport properties. The essence of this physics is believed to be captured by the Fermi-Hubbard model of repulsively interacting, itinerant fermions on a lattice. Here we report on the site-resolved observation of charge and spin correlations in the two-dimensional (2D) Fermi-Hubbard model realized with ultracold atoms. Antiferromagnetic spin correlations are maximal at half-filling and weaken monotonically upon doping. Correlations between singly charged sites are negative at large doping, revealing the Pauli and correlation hole\textemdash a suppressed probability of finding two fermions near each other. However, as the doping is reduced below a critical value, correlations between such local magnetic moments become positive, signaling strong bunching of doublons and holes. Excellent agreement with numerical linked-cluster expansion (NLCE) and determinantal quantum Monte Carlo (DQMC) calculations is found. Positive non-local moment correlations directly imply potential energy fluctuations due to doublon-hole pairs, which should play an important role for transport in the Fermi-Hubbard model.

- Observation of 2D Fermionic Mott Insulators (4/1/2016)
Lawrence W. Cheuk, Matthew A. Nichols, Katherine R. Lawrence, Melih Okan, Hao Zhang, Martin W. Zwierlein Phys. Rev. Lett. 116, 235301 (2016); arXiv:1604.00096 (2016)

We report on the site-resolved observation of characteristic states of the two-dimensional repulsive Fermi-Hubbard model, using ultracold 40K atoms in an optical lattice. By varying the tunneling, interaction strength, and external confinement, we realize metallic, Mott-insulating, and band-insulating states. We directly measure the local moment, which quantifies the degree of on-site magnetization, as a function of temperature and chemical potential. Entropies per particle as low as 0.99(6)kB indicate that nearest-neighbor antiferromagnetic correlations should be detectable using spin-sensitive imaging.

- Coherent Microwave Control of Ultracold NaK Molecules (4/1/2016)
Sebastian A. Will, Jee Woo Park, Zoe Z. Yan, Huanqian Loh, Martin W. Zwierlein Phys. Rev. Lett. 116, 225306 (2016), arXiv:1604.00120

We demonstrate coherent microwave control of rotational and hyperfine states of trapped, ultracold, and chemically stable NaK molecules. Starting with all molecules in the absolute rovibrational and hyperfine ground state, we study rotational transitions in combined magnetic and electric fields and explain the rich hyperfine structure. Following the transfer of the entire molecular ensemble into a single hyperfine level of the first rotationally excited state, J=1, we observe collisional lifetimes of more than 3s, comparable to those in the rovibrational ground state, J=0. Long-lived ensembles and full quantum state control are prerequisites for the use of ultracold molecules in quantum simulation, precision measurements and quantum information processing.

- Cascade of Solitonic Excitations in a Superfluid Fermi Gas (1/27/2016)
Mark J.-H. Ku, Biswaroop Mukherjee, Tarik Yefsah, and Martin W. Zwierlein Phys. Rev. Lett. 116, 045304 (2016), arXiv:1507.01047

We follow the time evolution of a superfluid Fermi gas of resonantly interacting 6Li atoms after a phase imprint. Via tomographic imaging, we observe the formation of a planar dark soliton, its subsequent snaking, and its decay into a vortex ring, which in turn breaks to finally leave behind a single solitonic vortex. In intermediate stages we find evidence for an exotic structure resembling the Φ-soliton, a combination of a vortex ring and a vortex line. Direct imaging of the nodal surface reveals its undulation dynamics and its decay via the puncture of the initial soliton plane. The observed evolution of the nodal surface represents dynamics beyond superfluid hydrodynamics, calling for a microscopic description of unitary fermionic superfluids out of equilibrium.

- Ultracold Dipolar Gas of Fermionic NaK Molecules (5/15/2015)
Illustration: Jose-Luis Olivares/MIT

Jee Woo Park, Sebastian A. Will, and Martin W. Zwierlein

Phys. Rev. Lett.

**114**, 205302 (2015)Coverage in Scientific American, Pro-Physik.de, Huffington Post, Live Science, and others

We report on the creation of an ultracold (500 Nanokelvin) dipolar gas of fermionic NaK molecules in their absolute rovibrational and hyperfine ground state. The molecular gas is formed from a mixture of ultracold gases of sodium and potassium atoms, which are first associated into a very loosely bound (Feshbach) molecule. These highly vibrationally excited molecules are then coherently transferred into the absolute rovibrational ground state. The two-photon process bridges an energy gap worth 7500 Kelvin, without the injection of heat. The nearly quantum degenerate molecular gas displays a lifetime longer than 2.5 seconds, highlighting NaK’s stability against two-body chemical reactions. A homogeneous electric field is applied to induce a dipole moment of up to 0.8 Debye. With these advances, the exploration of many-body physics with strongly dipolar Fermi gases of NaK molecules is within experimental reach.