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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 such a Fermi gas, atoms team up in pairs that can flow without friction. This has analogies to electron pairs in a superconductor that 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.
News and HighlightS
March 6, 2012: Mark Ku wins Harvey Fellowship
Congratulations to Mark for winning a Harvey Fellowship!
January 12, 2012: Revealing the Superfluid Lambda Transition in a Unitary Fermi Gas
 
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Mark J. H. Ku, Ariel T. Sommer, Lawrence W. Cheuk, Martin W. Zwierlein, Science Perspective by Wilhelm Zwerger We have observed the superfluid phase transition in a strongly interacting
Fermi gas via high-precision measurements of the local compressibility, density
and pressure down to near-zero 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 lambda-like feature at
the critical temperature of Tc/TF = 0.167(13). This is the first clear
thermodynamic signature of the superfluid transition in a spin-balanced atomic
Fermi gas. Our measurements provide a benchmark for many-body theories on
strongly interacting fermions, relevant for problems ranging from high-temperature superconductivity to the equation of state of neutron stars. |
October 20, 2011: Bose-Fermi mixture of Na-K with widely tunable interactions
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Jee Woo Park, Cheng-Hsun Wu, Ibon Santiago, Tobias G. Tiecke, Peyman Ahmadi, Martin W. Zwierlein, Phys. Rev. A Rapid Comm., to be published, We have created a quantum degenerate Bose-Fermi mixture of 23Na and 40K with widely tunable interactions via broad interspecies Feshbach resonances. Twenty Feshbach resonances between 23Na and 40K 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 bound-state 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 23Na-40K where strong, long-range dipolar interactions will set the dominant energy scale.
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October 17, 2011: Feynman diagrams versus Feynman quantum emulator
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K. Van Houcke, F. Werner, E. Kozik, N. Prokofev, 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 cross-validation between a new theoretical approach, Bold Diagrammatic Monte Carlo (BDMC), and precision experiments on ultra-cold atoms. Specifically, we compute and measure with unprecedented accuracy the normal-state 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 non-perturbative regime using BDMC. This opens the door to the solution of some of the most challenging problems across many areas of physics. |
October 13, 2011: Evolution of Fermion Pairing from Three to Two Dimensions
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Ariel T. Sommer, Lawrence W. Cheuk, Mark Jen-Hao 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 two-dimensional layers formed by a one-dimensional optical
lattice. Decreasing the dimensionality leads to the opening of a gap in
radiofrequency spectra, even on the BCS-side of a Feshbach resonance. With
increasing lattice depth, the measured binding energy EB of fermion pairs
increases in surprising agreement with mean-field theory for the BEC-BCS
crossover in two dimensions. |
September 8, 2011: Ariel Sommer wins MIT's Martin Deutsch Prize
Congratulations to Ariel for winning MIT's Deutsch Prize for Excellence in Experimental Physics!
March 23, 2011: Strongly Interacting Isotopic Bose-Fermi Mixture Immersed in a Fermi Sea
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Cheng-Hsun Wu, Ibon Santiago, Jee Woo Park, Peyman Ahmadi, Martin W. Zwierlein, 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 s-wave resonance for the combination 41K-40K, opening up studies of strongly interacting isotopic Bose-Fermi mixtures. For large imbalance, we enter the polaronic regime of dressed impurities immersed in a bosonic or fermionic bath. |
March 11, 2011: Spin Transport in Polaronic and Superfluid Fermi Gases
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Ariel Sommer, Mark Ku, and Martin W. Zwierlein, 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 Lithium-6 in a mixture of two spin states at a Feshbach resonance. In particular, we study the spin-dipole 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 spin-dipole excitation and we observe strong, unitarity-limited damping of the spin-dipole 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. |
January 4, 2011: Universal Spin Transport in a Strongly Interacting Fermi Gas
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Ariel Sommer, Mark Ku, Giacomo Roati, and Martin W. Zwierlein, Nature 472, 201-204 (2011) Preprint: arXiv:1101.0780 (2011) Strongly interacting Fermi gases are ubiquitous in nature, be it electrons in high-temperature superconductors, nuclear matter or quarks. In our recent Nature letter we study transport in gases of fermionic atoms that interact as strongly as quantum mechanics allows. We show that interactions are strong enough to reverse spin currents (shown in the figure to the left), and that the speed of diffusion is set by a fundamental quantum limit given by constants of nature. These results have implications for other fields of physics where transport of fermions plays a major role, such as spintronics and the study of the Early Universe. Read also: |
November 5, 2010: Zwierlein recipient of Presidential Early Career Award (PECASE)
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On Friday, Nov. 5, President Barack Obama named Martin Zwierlein, Assistant Professor of Physics, and six other researchers from MIT as recipients of the Presidential Early Career Awards for Scientists and Engineers (PECASE), the highest honor bestowed by the U.S. government on science and engineering professionals in the early stages of their independent research careers. |
October 28, 2010: A localized magnetic impurity in a fermionic superfluid
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Eric Vernier, David Pekker, Martin W. Zwierlein, Eugene Demler Preprint arXiv:1010.6085 (2010) We consider a localized impurity atom that interacts with a cloud of fermions in the paired state. We develop an effective scattering length description of the interaction between an impurity and a fermionic atom using their vacuum scattering length. Treating the pairing of fermions at the mean-field level, we show that the impurity atom acts like a magnetic impurity in the condensed matter context, and leads to the formation of a pair of Shiba bound states inside the superconducting gap. In addition, the impurity atom can lead to the formation of deeply bound states below the Fermi sea. |
October 15, 2010: Zwierlein wins Packard Fellowship
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MIT physicist Martin Zwierlein has won a 2010 David and Lucille Packard Fellowship. Zwierlein, an assistant professor of physics, is one of 17 recipients of this year’s awards, which are among nation's largest nongovernmental fellowship prizes. |
August 7, 2010: Zwierlein wins DARPA Young Faculty Award
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The Defense Advanced Research Projects Agency (DARPA) of the U.S. Department of Defense (DoD) has named Martin Zwierlein, an MIT assistant professor of physics, as one of 33 winners nationally of its annual Young Faculty Award (YFA) competition. |
June 17, 2010: Zwierlein recipient of 2010 Jonathan Allen Junior Faculty Award
June 4, 2010: Sara Campbell wins MIT Orloff award & NSF Graduate Fellowship
Congratulations to Sara for winning the Department of Physics' Joel Matthew Orloff Award for Service to the Community and an NSF graduate fellowship.
May 13, 2010: Competition between pairing and ferromagnetic instabilities
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David Pekker, Mehrtash Babadi, Rajdeep Sensarma, Nikolaj Zinner, Lode Pollet, Martin W. Zwierlein, Eugene Demler Preprint arXiv:1005.2366 (2010) We study the quench dynamics of a two-component ultracold Fermi gas from the weak into the strong interaction regime, where the short time dynamics are governed by the exponential growth rate of unstable collective modes. We obtain an effective interaction that takes into account both Pauli blocking and the energy dependence of the scattering amplitude near a Feshbach resonance. Using this interaction we analyze the competing instabilities towards Stoner ferromagnetism and pairing. |
April 5, 2010: Zwierlein named ONR Young Investigator
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The Navy's Office of Naval Research (ONR) has named Martin Zwierlein as one of its 17 new Young Investigators. Zwierlein’s work under the program will be on "Strongly Interacting Fermi Gases in Two Dimensions." |
January 28th, 2010: Zwierlein selected for AFOSR Young Investigators Research Program
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Martin Zwierlein has been selected for an AFOSR Young Investigators Research Program (YIP) award to conduct basic research on “Quantum Engineering of Strongly Correlated Matter with Ultracold Fermi Gases”. |
January 22, 2010: André Schirotzek Thesis Defense
André defended his thesis - with vast success, of course. Congratulations!
October 13, 2009: Bose-Einstein Condensation of 41-K in Fermi I
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The first degenerate gas in our new multi-species apparatus "Fermi I" has seen the (laser-)light of day: A Bose-Einstein Condensate of 41-K atoms. The apparatus allows to cool all potassium species, lithium and sodium. |
September 10, 2009: André Schirotzek wins MIT's Martin Deutsch Prize
Congratulations to André for winning MIT's Deutsch Prize for Excellence in Experimental Physics!
June 8, 2009: Observation of Fermi Polarons
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We have observed Fermi Polarons, dressed spin down impurities swimming in a Fermi sea of spin up atoms. These polarons constitute the quasiparticles in the Fermi liquid of strongly interacting, imbalanced Fermi mixtures. Remarkably, despite resonant interactions between the bare particles, the interactions between polarons are found to be weak. Read more in our PRL-article and in the Viewpoint Commentary by F. Chevy in Physics. |
Review
Review on Ultracold Fermi Gases:
Wolfgang Ketterle and Martin W. Zwierlein
Making, probing and understanding ultracold Fermi gases
in Ultracold Fermi Gases, Proceedings of the International School of Physics “Enrico Fermi”, Course CLXIV,
eds. M. Inguscio, W. Ketterle, and C. Salomon (Amsterdam, IOS Press, 2008), e-print: arXiv: 0801.2500.
FUNDING
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DARPA - OLE/-YFA with funds from ARO |
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The group of Prof. Martin Zwierlein is part of the Center for Ultracold Atoms, the Research Laboratory of Electronics and the Department of Physics, at the Massachusetts Institute of Technology.
