Our research on strongly interacting Fermi gases takes place in these laboratories:
BEC1 studies strongly interacting fermionic superfluids of lithium6. Interactions between atoms can be tuned at will with the help of Feshbach resonances. This allows to study the crossover from a BoseEinstein condensate of tightly bound Li_{2} molecules to a BardeenCooperSchrieffer superfluid of longrange Cooper pairs. We are currently studying topological excitations in these superfluids such as solitons and vortices, which should carry Andreev bound states at their core. In the presence of spin imbalance, solitons should fill in with excess fermions and become stable, representing one limit of the longsought FuldeFerrellLarkinOvchinnikov state.
Fermi1 uses a mixture of ultracold bosonic sodium and fermionic potassium atoms to form quantum degenerate NaK molecules. In their ground state, these molecules are chemically stable and possess a large electric dipole moment. A degenerate Fermi gas of molecules with strong dipolar interactions would realize novel states of matter, such as topological superfluids, quantum crystals or supersolids. Strongly interacting dipolar molecules trapped in optical lattices should realize quantum magnetism at readily accessible temperatures. We have recently successfully created a Fermi gas of chemically stable ultracold molecules and find a long lifetime of several seconds, a great starting point for future studies.
In Fermi2, fermionic atoms are trapped in a single twodimensional plane of a 3D optical lattice. Fermions on each lattice site can be imaged simultaneously with singleatom resolution, allowing quantum simulation of lattice fermion models with siteresolved readout. The experiment is designed to realize the Fermi Hubbard model, believed to hold the key to our understanding of hightemperature superconductivity, but also topological states of fermionic matter, whose edge states should be directly detectable. We currently study spin transport in fermionic Mott insulators, something that cannot currently be done in solid state counterparts. We measure spin transport coefficients, such as the spin diffusivity and spin conductivity, which cannot at this point be calculated faithfully on a classical computer.
Fermi3 is our newest experiment, a mixture of sodium and lithium6, where we want to study twodimensional Fermi gases in the bulk, a paradigm of condensed matter physics. Surprises already abound…
Recent studies:
 Measuring total density correlations in a FermiHubbard gas via bilayer microscopy (3/25/2020)
Thomas Hartke, Botond Oreg, Ningyuan Jia, Martin Zwierlein
We report on the single atom and single siteresolved detection of the total density in a cold atom realization of the 2D FermiHubbard 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 fluctuationdissipation theorem. Siteresolved density correlations reveal the Pauli hole at low filling, and strong doublonhole correlations near half filling. These are shown to account for the difference between local and nonlocal density fluctuations in the Mott insulator. Our technique enables the study of atomresolved charge transport in the FermiHubbard model, the siteresolved observation of molecules, and the creation of bilayer FermiHubbard 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
We demonstrate microwave dressing on ultracold, fermionic 23Na40K groundstate molecules and observe resonant dipolar collisions with cross sections exceeding three times the swave 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 twostate picture based on the Condon approximation. Furthermore, we perform coupledchannels calculations that agree well with the experimentally observed collision rates. While collisions are observed here as laserinduced 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 noncommutative 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 noncommuting variables to dynamically create a BoseEinstein condensate in the lowest Landau level (LLL). We directly resolve the extent of the zeropoint 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.
 Universal Sound Diffusion in a Strongly Interacting Fermi Gas (9/5/2019)
Parth B. Patel, Zhenjie Yan, Biswaroop Mukherjee, Richard J. Fletcher, Julian Struck, Martin W. Zwierlein
Transport of strongly interacting fermions governs modern materials — from the highTc 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.
 Photoinduced twobody 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 threebody 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 twobody 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.
 Bose polarons near quantum criticality (4/13/2019)
Zoe Z. Yan, Yiqi Ni, Carsten Robens, Martin W. Zwierlein
Science, 2020 (to be published)
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 BoseEinstein condensate (BEC) with nearresonant interactions. Using locallyresolved radiofrequency spectroscopy, we probe the energy, spectral width, and shortrange correlations of the impurities as a function of temperature. Far below the superfluid critical temperature, the impurities form welldefined 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.
 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 nonFermiliquid behavior. From the wings of the rf spectra, we obtain the contact, quantifying the strength of shortrange 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 JeanPhilippe 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 halffilled FermiHubbard model. For strong interactions, spin diffusion is driven by superexchange and doublonholeassisted 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 spinimbalanced, homogeneous Fermi gas with unitarity limited interactions, from a Fermi liquid of polarons at low temperatures to a classical Boltzmann gas at high temperatures. Radiofrequency spectroscopy gives access to the energy, lifetime and the shortrange 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.
 TwoPhoton 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, 47394745 (2018)
We employ twophoton 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, 47464751 (2018)
We perform photoassociation spectroscopy in an ultracold 23Na6Li 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 neardissociation 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.
 SecondScale 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 subμK 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 longrange interactions between dipolar molecules can further enable twoqubit gates, allowing quantum storage and processing in the same physical system. Within the observed coherence time, 10^{4} one and twoqubit gate operations will be feasible.
 LongLived 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 magnetoassociation across a narrow Feshbach resonance followed by a twophoton STIRAP transfer to the triplet ground state, we produce 3×10^4 ground state molecules in a spinpolarized state. We observe a lifetime of 4.6s in an isolated molecular sample, approaching the pwave 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
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 spinpolarized 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 spinbalanced 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 spinbalanced Fermi gas, saturation in a fully polarized Fermi gas, and strong attraction in the polaronic regime of a partially polarized Fermi gas. 
Two and Threebody Contacts in the Unitary Bose Gas
Richard J. Fletcher, Raphael Lopes, Jay Man, Nir Navon, Robert P. Smith, Martin W. Zwierlein, Zoran Hadzibabic
Science 355, 377380 (2017), arXiv:1608.04377 In manybody systems governed by pairwise contact interactions, a wide range of observables is linked by a single parameter, the twobody contact, which quantifies twoparticle 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 threebody correlations arising from Efimov physics, and provide a direct measurement of the associated threebody contact. 
Spatial Charge and Spin Correlations in the 2D FermiHubbard Model
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, 12601264 (2016), arXiv:1606.04089 Strong electron correlations lie at the origin of transformative phenomena such as colossal magnetoresistance and hightemperature superconductivity. Already near room temperature, doped copper oxide materials display remarkable features such as a pseudogap and a “strange metal” phase with unusual transport properties. The essence of this physics is believed to be captured by the FermiHubbard model of repulsively interacting, itinerant fermions on a lattice. Here we report on the siteresolved observation of charge and spin correlations in the twodimensional (2D) FermiHubbard model realized with ultracold atoms. Antiferromagnetic spin correlations are maximal at halffilling 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 linkedcluster expansion (NLCE) and determinantal quantum Monte Carlo (DQMC) calculations is found. Positive nonlocal moment correlations directly imply potential energy fluctuations due to doublonhole pairs, which should play an important role for transport in the FermiHubbard model.

Coherent Microwave Control of Ultracold NaK Molecules
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. Longlived ensembles and full quantum state control are prerequisites for the use of ultracold molecules in quantum simulation, precision measurements and quantum information processing. 
Observation of 2D Fermionic Mott Insulators
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 siteresolved observation of characteristic states of the twodimensional repulsive FermiHubbard model, using ultracold 40K atoms in an optical lattice. By varying the tunneling, interaction strength, and external confinement, we realize metallic, Mottinsulating, and bandinsulating states. We directly measure the local moment, which quantifies the degree of onsite magnetization, as a function of temperature and chemical potential. Entropies per particle as low as 0.99(6)kB indicate that nearestneighbor antiferromagnetic correlations should be detectable using spinsensitive imaging. 
Cascade of Solitonic Excitations in a Superfluid Fermi Gas
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 Φsoliton, 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
Illustration: JoseLuis Olivares/MIT 
Jee Woo Park, Sebastian A. Will, and Martin W. Zwierlein Phys. Rev. Lett. 114, 205302 (2015) Coverage in Scientific American, ProPhysik.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 twophoton 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 twobody 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 manybody physics with strongly dipolar Fermi gases of NaK molecules is within experimental reach. 
TwoPhoton Pathway to Ultracold Ground State Molecules
Jee Woo Park, Sebastian A. Will, and Martin W. Zwierlein New J. Phys. 17, 075016 (2015) in Focus on New Frontiers of Cold Molecules Research In the quest for the creation of a Fermi gas of chemically stable, ultracold molecules, we were able to show that NaK is a highly promising candidate, featuring chemical stability, broad Feshbach resonances at easily accessible magnetic fields, a strong permanent electric dipole moment of 2.7 Debye. However, to convert predominantly triplet Feshbach molecules into the singlet rovibrational ground state via a twophoton process requires an intermediate state of mixed singlettriplet character. Spinorbit coupling in NaK is weak, and efficient twophoton coupling therefore requires an accidental degeneracy between singlet and triplet excited states. We have identified two such possible “bridges” for twophoton transfer of NaK, and demonstrated coherent twophoton coupling between the Feshbach and the absolute rovibrational ground state. The binding energy of NaK is measured to be 5212.0447(1) cm1, a thousandfold improvement in accuracy compared to previous determinations.

March 9th, 2015: A Quantum Gas Microscope for Fermionic Atoms
Lawrence W. Cheuk, Matthew A. Nichols, Melih Okan, Thomas Gersdorf, Vinay V. Ramasesh, Waseem S. Bakr, Thomas Lompe, Martin W. Zwierlein Phys. Rev. Lett. 114, 193001 (2015) Selected as one of the Physics Breakthroughs in 2015 by IOP’s Physics World Coverage in Physics World, Optics & Photonics, Tech Times, Laser Focus World, and others We realize a quantumgas microscope for fermionic atoms trapped in an optical lattice, which allows one to probe strongly correlated fermions at the singleatom level. We combine 3D Raman sideband cooling with highresolution optics to simultaneously cool and image individual atoms with singlelatticesite resolution at a detection fidelity above 95%. The imaging process leaves the atoms predominantly in the 3D motional ground state of their respective lattice sites, inviting the implementation of a Maxwell’s demon to assemble lowentropy manybody states. Singlesiteresolved imaging of fermions enables the direct observation of magnetic order, timeresolved measurements of the spread of particle correlations, and the detection of manyfermion entanglement. 
Motion of a Solitonic Vortex in the BECBCS Crossover
Mark J.H. Ku, Wenjie Ji, Biswaroop Mukherjee, Elmer GuardadoSanchez, Lawrence W. Cheuk, Tarik Yefsah, Martin W. Zwierlein Physics Viewpoint “Solitons with a Twist” by Frederic Chevy We observe a longlived solitary wave in a superfluid Fermi gas of Li atoms after phaseimprinting. Tomographic imaging reveals the excitation to be a solitonic vortex, oriented transverse to the long axis of the cigarshaped atom cloud. The precessional motion of the vortex is directly observed, and its period is measured as a function of the chemical potential in the BECBCS crossover. The long period and the correspondingly large ratio of the inertial to the bare mass of the vortex are in good agreement with estimates based on superfluid hydrodynamics that we derive here using the known equation of state in the BECBCS crossover. 
Heavy Solitons in a Fermionic Superfluid

Tarik Yefsah, Ariel T. Sommer, Mark J.H. Ku, Lawrence W. Cheuk, Wenjie Ji, Waseem S. Bakr, and Martin W. Zwierlein
Topological excitations are found throughout nature, in proteins and DNA, as dislocations in crystals, as vortices and solitons in superfluids and superconductors, and generally in the wake of symmetrybreaking phase transitions. In fermionic systems, topological defects may provide bound states for fermions that often play a crucial role for the system’s transport properties. Famous examples are Andreev bound states inside vortex cores, fractionally charged solitons in relativistic quantum field theory, and the spinless charged solitons responsible for the high conductivity of polymers. However, the free motion of topological defects in electronic systems is hindered by pinning at impurities. Here we create longlived solitons in a strongly interacting fermionic superfluid by imprinting a phase step into the superfluid wavefunction, and directly observe their oscillatory motion in the trapped superfluid. As the interactions are tuned from the regime of BoseEinstein condensation (BEC) of tightly bound molecules towards the BardeenCooperSchrieffer (BCS) limit of longrange Cooper pairs, the effective mass of the solitons increases dramatically to more than 200 times their bare mass. This signals their filling with Andreev states and strong quantum fluctuations. For the unitary Fermi gas, the mass enhancement is more than fifty times larger than expectations from meanfield Bogoliubovde Gennes theory. Our work paves the way towards the experimental study and control of Andreev bound states in ultracold atomic gases. In the presence of spin imbalance, the solitons created here represent one limit of the long soughtafter FuldeFerrellLarkinOvchinnikov (FFLO) state of mobile Cooper pairs. 
Ultracold Fermionic Feshbach Molecules of ^{23}Na^{40}K
ChengHsun Wu, Jee Woo Park, Peyman Ahmadi, Sebastian Will, Martin W. Zwierlein, Phys. Rev. Lett. 109, 085301 (2012) We report on the formation of ultracold fermionic Feshbach molecules of ^{23}Na^{40}K, the first fermionic molecule that is chemically stable in its ground state. The lifetime of the nearly degenerate molecular gas exceeds 100 ms in the vicinity of the Feshbach resonance. The measured dependence of the molecular binding energy on the magnetic field demonstrates the openchannel character of the molecules over a wide field range and implies significant singlet admixture. This will enable efficient transfer into the singlet vibrational ground state, resulting in a stable molecular Fermi gas with strong dipolar interactions. 
SpinInjection Spectroscopy of a SpinOrbit Coupled Fermi Gas
Lawrence W. Cheuk, Ariel T. Sommer, Zoran Hadzibabic, Tarik Yefsah, Waseem S. Bakr, Martin W. Zwierlein, Phys. Rev. Lett. 109, 095302 (2012), The coupling of the spin of electrons to their motional state lies at the heart of recently discovered topological phases of matter. Here we create and detect spinorbit coupling in an atomic Fermi gas, a highly controllable form of quantum degenerate matter. We reveal the spinorbit gap via spininjection spectroscopy, which characterizes the energymomentum dispersion and spin composition of the quantum states. For energies within the spinorbit gap, the system acts as a spin diode. To fully inhibit transport, we open an additional spin gap, thereby creating a spinorbit coupled lattice whose spinful band structure we probe. In the presence of swave interactions, such systems should display induced pwave pairing, topological superfluidity, and Majorana edge states. See Physics Viewpoint by Erich Mueller, Physics 5, 96 (2012) Article in IOP Physics World 
Revealing the Superfluid Lambda Transition in a Unitary Fermi Gas
Mark J. H. Ku, Ariel T. Sommer, Lawrence W. Cheuk, Martin W. Zwierlein
Science 335, 563 (2012), published online on Science Express Jan 12th, 2012, 10.1126/science.1214987, arXiv:1110.3309
Science Perspective by Wilhelm Zwerger
We have observed the superfluid phase transition in a strongly interacting Fermi gas via highprecision measurements of the local compressibility, density and pressure down to nearzero entropy. Our data completely determine the universal thermodynamics of strongly interacting fermions without any fit or external thermometer. The onset of superfluidity is observed in the compressibility, the chemical potential, the entropy, and the heat capacity. In particular, the heat capacity displays a characteristic lambdalike feature at the critical temperature of T_{c}/T_{F} = 0.167(13). This is the first clear thermodynamic signature of the superfluid transition in a spinbalanced atomic Fermi gas. Our measurements provide a benchmark for manybody theories on strongly interacting fermions, relevant for problems ranging from hightemperature superconductivity to the equation of state of neutron stars. Data files for figures: Fig. 1: Normalized compressibility vs normalized pressure: fig1.dat Fig. 2: Compressibility, specific heat, condensate fraction vs temperature: fig2.dat Fig. 3: Chemical potential, energy, free energy, entropy: fig3.dat Fig. 4: Density and pressure versus fugacity: fig4.dat

Quantum degenerate BoseFermi mixture of chemically different atomic species with widely tunable interactions
Jee Woo Park, ChengHsun Wu, Ibon Santiago, Tobias G. Tiecke, Peyman Ahmadi, Martin W. Zwierlein, Phys. Rev. A 85, 051602(R) (2012), arXiv:1110.4552 (2011)
We have created a quantum degenerate BoseFermi mixture of ^{23}Na and ^{40}K with widely tunable interactions via broad interspecies Feshbach resonances. Twenty Feshbach resonances between ^{23}Na and ^{40}K were identified. The large and negative triplet background scattering length between the two speices causes a sharp enhancement of the fermion density in the presence of a Bose condensate. As explained via the asymptotic boundstate model (ABM), this strong background scattering leads to a series of wide Feshbach resonances observed at low magnetic fields. Our work opens up the prospect to create chemically stable, fermionic ground state molecules of ^{23}Na^{40}K where strong, longrange dipolar interactions will set the dominant energy scale.

Feynman diagrams versus Feynman quantum emulator
K. Van Houcke, F. Werner, E. Kozik, N. Prokofev,
B. Svistunov, M. Ku, A. Sommer, L. W. Cheuk, A. Schirotzek, M. W. Zwierlein
Nature Physics 10.1038/nphys2273
published online March 18 2012, arXiv:1110.3747 (2011)
Precise understanding of strongly interacting fermions, from electrons in modern materials to nuclear matter, presents a major goal in modern physics. However, the theoretical description of interacting Fermi systems is usually plagued by the intricate quantum statistics at play. Here we present a crossvalidation between a new theoretical approach, Bold Diagrammatic Monte Carlo (BDMC), and precision experiments on ultracold atoms. Specifically, we compute and measure with unprecedented accuracy the normalstate equation of state of the unitary gas, a prototypical example of a strongly correlated fermionic system. Excellent agreement demonstrates that a series of Feynman diagrams can be controllably resummed in a nonperturbative regime using BDMC. This opens the door to the solution of some of the most challenging problems across many areas of physics. 
Evolution of Fermion Pairing from Three to Two Dimensions
Ariel T. Sommer, Lawrence W. Cheuk, Mark JenHao Ku, Waseem S. Bakr, Martin W. Zwierlein
Phys. Rev. Lett. 108, 045302 (2012), arXiv:1110.3058
Viewpoint in Physics 5, 10 (2012) by Mohit Randeria
We follow the evolution of fermion pairing in the dimensional crossover from 3D to 2D as a strongly interacting Fermi gas of 6Li atoms becomes confined to a stack of twodimensional layers formed by a onedimensional optical lattice. Decreasing the dimensionality leads to the opening of a gap in radiofrequency spectra, even on the BCSside of a Feshbach resonance. With increasing lattice depth, the measured binding energy E_{B} of fermion pairs increases in surprising agreement with meanfield theory for the BECBCS crossover in two dimensions. 
Strongly Interacting Isotopic BoseFermi Mixture Immersed in a Fermi Sea
ChengHsun Wu, Ibon Santiago, Jee Woo Park, Peyman Ahmadi, Martin W. Zwierlein,
PRA 84, 011601(R) (2011), arXiv:1103.4630
We have created a triply quantum degenerate mixture of bosonic 41K and two fermionic species, 40K and 6Li. The boson is shown to be an efficient coolant for the two fermions, spurring hopes for the observation of fermionic superfluids with imbalanced masses. We observe multiple heteronuclear Feshbach resonances, in particular a wide swave resonance for the combination 41K40K, opening up studies of strongly interacting isotopic BoseFermi mixtures. For large imbalance, we enter the polaronic regime of dressed impurities immersed in a bosonic or fermionic bath. 
Spin Transport in Polaronic and Superfluid Fermi Gases
Ariel Sommer, Mark Ku, and Martin W. Zwierlein,
New Journal of Physics 13, 055009 (2011)
Focus on Strongly Correlated Quantum Fluids: From Ultracold Quantum Gases to QCD Plasmas
Preprint: arXiv:1103.2337 (2011)
We present measurements of spin transport in ultracold gases of fermionic Lithium6 in a mixture of two spin states at a Feshbach resonance. In particular, we study the spindipole mode, where the two spin components are displaced from each other against a harmonic restoring force. We prepare a highly imbalanced, or polaronic, spin mixture with a spindipole excitation and we observe strong, unitaritylimited damping of the spindipole mode. In gases with small spin imbalance, below the Pauli limit for superfluidity, we observe strongly damped spin flow even in the presence of a superfluid core. This indicates strong mutual friction between superfluid and polarized normal spins, possibly involving Andreev reflection at the superfluid–normal interface. 
Universal Spin Transport in a Strongly Interacting Fermi Gas
Ariel Sommer, Mark Ku, Giacomo Roati, and Martin W. Zwierlein
Preprint arXiv:1101.0780 (2011)
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Nature News&Views by John Thomas
Collision of two spin states of an ultracold Fermi gas. Although each spin cloud is a million times thinner than air, the two spin states essentially completely repel each other. The interactions between unlike spins are as strong as quantum mechanics allows. A spin down atom scatters with spin up atoms at every encounter, i.e. the mean free path for collisions is just one interparticle spacing – the shortest possible in a gas. This leads to the minimum diffusivity and the smallest spin conductivity ever possible. This leads to the interesting fact that an almost perfect fluid, i.e. the best conductor of mass, is the worst conductor for spin.

Transport of fermions is central in many fields of physics. Electron transport runs modern technology, defining states of matter such as superconductors and insulators. Transport of electron spin, rather than of charge, is being explored as a new way to carry information. Neutrino transport energizes supernova explosions following the collapse of a dying star, and hydrodynamic transport of the quarkgluon plasma governed the expansion of the early Universe. However, our understanding of nonequilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold gases of fermionic atoms realize a pristine model for such systems and can be studied in real time with the precision of atomic physics. It has been established that even above the superfluid transition such gases flow as an almost perfect fluid with very low viscosity when interactions are tuned to a scattering resonance. However, in this work we show that spin currents, as opposed to mass currents, are maximally damped, and that interactions can be strong enough to reverse spin currents, with opposite spin components reflecting off each other. We determine the spin drag coefficient, the spin diffusivity, and the spin susceptibility, as a function of temperature on resonance and show that they obey universal laws at high temperatures. At low temperatures, the spin diffusivity approaches a minimum value set by h/m, the quantum limit of diffusion, where h is Planck’s constant and m the atomic mass. For repulsive interactions, our measurements appear to exclude a metastable ferromagnetic state.
Observation of Fermi Polarons
The fate of a single particle interacting with its environment is one of the grand themes of physics. A wellknown example is that of the electron moving through the crystal lattice of ions in a solid. The electron attracts positive ions, repels negative ones and thereby distorts the lattice. In other words, it polarizes its surroundings. The electron and the surrounding lattice distortions is best described as a new particle, the lattice polaron. It is a socalled quasiparticle with an energy and mass that differ from that of the bare electron. Polarons are crucial for the understanding of colossal magnetoresistance materials and they are responsible for conduction in fullerenes and polymers. Another famous impurity problem is the Kondo effect: Here, a magnetic impurity interacts with a Fermi sea of electrons, hindering their transport and leading to an increase in the metal’s resistance below a certain temperature.
In the present work, we have observed Fermi polarons, dressed “spin down” impurity atoms immersed in a Fermi sea of “spin up” atoms. The interactions between the impurity and the environment can be freely tuned by means of a Feshbach resonance. This allows us to determine the polaron energy as function of interaction strength.
a) For weak interactions, the impurity (blue) can propagate freely through the environment (red), a Fermi sea of atoms. b) As the interaction is increased, the impurity starts to attract its surroundings, “dressing” itself with a cloud of environment atoms. This is the Fermi Polaron. c) For strong attraction, the spin down atom will bind exactly one spin up partner, forming a molecule. The transition from polarons to molecules occurs as soon as the binding can overcome Pauli blocking of the environment.
Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms
Andre Schirotzek, ChengHsun Wu, Ariel Sommer, and Martin W. Zwierlein
Phys. Rev. Lett. 102, 230402 (2009). paper download 
See accompanying Viewpoint commentary Physics 2, 48 (2009) 
HighTemperature Superfluidity
Vortices in gas clouds Shown at the right are lattices of vortices (minitornadoes) in an ultracold gas of sodium atoms (green ball), in a gas of lithium molecules, made out of a “red” and a “blue” lithium atom, and in a strongly interacting Fermi gas, where the lithium atom pairs are only held together by the stabilizing presence of all the other particles in the gas. Those vortices are the direct proof of superfluidity in these systems. The background shows hurricane Isabel in the summer of 2003, NASA image ISS007E14887.
Vortices and Superfluidity in a Strongly Interacting Fermi Gas NatureLink  condmat archive 
Fermionic Superfluidity with Imbalanced Spin Populations
Whether it occurs in superconductors, helium3 or inside a neutron star, fermionic superfluidity requires pairing of fermions, particles with halfinteger spin. For an equal mixture of two states of fermions (“spin up” and “spin down”), pairing can be complete and the entire system will become superfluid. When the two populations of fermions are unequal, not every particle can find a partner. Will the system nevertheless stay superfluid?
Fermionic Superfluidity with Imbalanced Spin Populations ScienceLink  condmat archive 
A condensate emerges in an imbalanced Fermi mixture Image 1  Image 2  
BoseEinstein Condensation of Molecules  Condensation of Fermion Pairs Close to a Feshbach Resonance ISI Fast breaking comment 

Formation Time of a Fermion Pair Condensate 