Prof. Martin Zwierlein

Martin Zwierlein’s research focuses on ultracold quantum gases of atoms and molecules. Just a few billionths of a degree above Absolute Zero and a million times thinner than air, these gases provide ideal model systems for many-body physics in a clean and controllable environment.

After the realization of Bose-Einstein condensation (BEC) in dilute bosonic gases in 1995, the observation of superfluidity in Fermi gases had been a long-standing goal in the field of ultracold atoms. Together with his colleagues at MIT, Zwierlein observed BEC of pairs of fermionic lithium atoms in 2003. With the help of Feshbach resonances, interactions between fermions could be tuned at will. This enabled Zwierlein to access the crossover from a BEC of molecules to a Bardeen-Cooper-Schrieffer (BCS) state of long-range pairs. Superfluidity was demonstrated in 2005 by setting the strongly interacting Fermi gas in rotation and observing an ordered lattice of quantized vortices. Scaled to the density of electrons in a metal, this form of superfluidity would occur already far above room temperature.

Zwierlein and colleagues moved on to address an old question on the ground state of imbalanced fermionic mixtures, wherein not every “spin up” fermion can find a “spin down” partner. At a critical spin imbalance, the Clogston-Chandrasekhar limit observed by Zwierlein, the superfluid state is destroyed and a strongly interacting Fermi mixture remains.

Most recently, Zwierlein worked on an experiment on fermions and bosons in optical lattices at the University of Mainz. Fermi mixtures with repulsive interactions, confined to an optical lattice, might enable the simulation of an important model in the context of high-temperature superconductors, the fermionic Hubbard model.

Professor Zwierlein will investigate ultracold mixtures of different fermionic species. An equal mixture of fermionic lithium-6 and potassium-40 atoms would constitute a fermionic superfluid in which the pairing partners are not related to each other by time-reversal symmetry. In their vibrational ground state, heteronuclear LiK molecules would possess a large electric dipole moment, opening up possibilities to study quantum gases with anisotropic long-range interactions. Fermi mixtures involving more than one spin state per atomic species can serve as a rudimentary model system of exotic matter, such as quark (“color”) superfluids in the core of neutron stars.