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.
News and HighlightS
February 19, 2013: 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 symmetry-breaking 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 long-lived 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. Our work paves the way towards the experimental study and control of Andreev bound states in ultracold atomic gases.
February 9, 2013: New Scientist article: "The quantum Sims"
Work in our group is featured in the NewScientist article by Michael Brooks,
"The quantum Sims - Matter's deepest mysteries recreated"
January 25, 2013: Ariel Sommer Thesis Defense
Congratulations to Ariel for defending his Ph.D. thesis on "Strongly Interacting Fermi Gases:
Non-Equilibrium Dynamics and Dimensional Crossover" with a brilliant talk.
January 24, 2013: Cheng-Hsun Wu Thesis Defense
Congratulations to Cheng for a fantastic Ph.D. thesis on "Strongly Interacting Quantum Mixtures of
Ultracold Atoms" and a great presentation! Cheers!
January 16, 2013: Waseem Bakr wins MIT's 2013 Infinite Kilometer (K) award
Congratulations to Waseem on winning MIT's School of Science's 2013 Infinite Kilometer (K) award!
November 12, 2012: Collective Modes in a Unitary Fermi Gas
Meng Khoon Tey, Leonid A. Sidorenkov, Edmundo R. Sánchez Guajardo, Rudolf Grimm, Mark J. H. Ku, Martin W. Zwierlein, Yan-Hua Hou, Lev Pitaevskii, Sandro Stringari
In this joint theoretical and experimental work with Rudi Grimm's experimental group at the University of Innsbruck and the theoretical group of Sandro Stringari and Lev Pitaevskii at the University of Trento we investigate first sound in a strongly interacting Fermi gas.
The frequency of higher-order collective oscillations of first sound nature is strongly dependent on temperature and thus provides a stringent test of our previously measured equation of state. The experimental results from Innsbruck agree with high accuracy with the predictions of theory based on our experimental equation of state measurements and provide the first observation of the temperature dependence of collective frequencies near the superfluid phase transition.
September 6, 2012: Martin Zwierlein receives William W. Buechner Teaching Prize
August 27, 2012: APS Viewpoint: Spin-Orbit Coupling Comes in From the Cold
Our work on spin-orbit coupling was covered as an
APS Viewpoint in Physics 5, 96 (2012):
by Erich J. Mueller
MIT News also wrote an article about this experiment:
Article in IOP Physics World
July 1, 2012: Martin Zwierlein promoted to Associate Professor with tenure
June 21, 2012: Ultracold Fermionic Feshbach Molecules of 23Na40K
Cheng-Hsun Wu, Jee Woo Park, Peyman Ahmadi, Sebastian Will, Martin W. Zwierlein,
We report on the formation of ultracold fermionic Feshbach molecules of 23Na40K, 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 open-channel 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.
June 6, 2012: Dr. Waseem Bakr wins the APS DAMOP Thesis Prize 2012
Congratulations to Waseem for receiving the APS DAMOP Thesis Prize 2012 for his doctoral thesis "Microscopic Studies of Quantum Phase Transitions in Optical Lattices" with Prof. Markus Greiner
June 5, 2012: Vinay Ramasesh wins Malcolm Cotton Brown Award from MIT physics
Congratulations to Vinay for receiving the Malcolm Cotton Brown Award "presented to a senior of high academic standing who plans to pursue graduate study in experimental physics".
May 15, 2012: Spin-Injection Spectroscopy of a Spin-Orbit Coupled Fermi Gas
Lawrence W. Cheuk, Ariel T. Sommer, Zoran Hadzibabic, Tarik Yefsah, Waseem S. Bakr, Martin W. Zwierlein,
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 spin-orbit coupling in an atomic Fermi gas, a highly controllable form of quantum degenerate matter. We reveal the spin-orbit gap via spin-injection spectroscopy, which characterizes the energy-momentum dispersion and spin composition of the quantum states. For energies within the spin-orbit gap, the system acts as a spin diode. To fully inhibit transport, we open an additional spin gap, thereby creating a spin-orbit coupled lattice whose spinful band structure we probe. In the presence of s-wave interactions, such systems should display induced p-wave pairing, topological superfluidity, and Majorana edge states.
See Physics Viewpoint by Erich Mueller, Physics 5, 96 (2012)
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
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.
December 7, 2011: Mark Ku wins DARPA OLE best paper award
Congratulations to Mark for winning the DARPA Optical Lattice Emulator (OLE) program best paper award for "Revealing the Superfluid Lambda Transition in the Universal Thermodynamics of a Unitary Fermi Gas"!
October 20, 2011: Bose-Fermi mixture of Na-K with widely tunable interactions
Jee Woo Park, Cheng-Hsun Wu, Ibon Santiago, Tobias G. Tiecke, Peyman Ahmadi, Martin W. Zwierlein,
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.
October 17, 2011: Feynman diagrams versus Feynman quantum emulator
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
Ariel T. Sommer, Lawrence W. Cheuk, Mark Jen-Hao Ku, Waseem S. Bakr, Martin W. Zwierlein,
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
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
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
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.
November 5, 2010: Zwierlein recipient of Presidential Early Career Award (PECASE)
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
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
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
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
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
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
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
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
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 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.
DARPA - OLE/-YFA
with funds from ARO