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-Tc superconductors to Neutron Stars.

  • 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.

  • Second-Scale Nuclear Spin Coherence Time of Ultracold NaK Molecules (6/14/2016)

    Jee Woo Park, Zoe Z. Yan, Huanqian Loh, Sebastian A. Will, Martin W. Zwierlein

    preprint arXiv:1606.04184

    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, 104 one- and two-qubit gate operations will be feasible.

  • 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)


    Physics Synopsis

    MIT News

    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.