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Dynamical and dissipative control of ultra-cold atomic Fermi gases

Michael Köhl & Corinna Kollath


In this collaborative project we plan to experimentally and theoretically investigate low-dimensional spin-1/2 Fermi gases in optical lattices coupled to dissipative and driven environments. The aim is to use the coupling to tailored environments in order to evolve an interacting many-body system into a desired quantum state, which otherwise is unstable or unreachable and to characterize its stability, and spectral and dynamic properties. The targeted state is the so-called η-pair state which is a highly energetically excited pairing state of the repulsively interacting Fermi-Hubbard model. This state is formed from on-site pairs (doubly occupied sites), which have a π-phase shift between neighboring lattice sites. The η-pair condensate has even been shown by C.N. Yang to be an exact eigenstate of the Hubbard model protected by an additional SU(2) symmetry. However, the η-pair state has yet escaped experimental observation owing to the difficulty of addressing this highly excited state individually. Being energetically very far from the ground state of the Hubbard model, the η-pair state cannot be accessed in thermal equilibrium, but the fermions have to couple to an external reservoir in order to reach and/or stabilize the state.

The project has three scientific objectives, each of which relates to the core questions addressed in the CRC:
(1) The dissipative creation of η-pair correlations. We study experimentally and theoretically the formation of η-pair correlations induced by the interplay of dissipation when coupling a Fermi gas to a near-resonant optical excitation and the intrinsic tunneling dynamics. This is a prime example of a quantum state prepared by a tailored environment, where the target state is created by the dissipative dynamics.
(2) The dynamical creation of η-pairs in a Fermi gas in an optical lattice coupled to the variation of an additional superlattice. This controlled coupling to the additional superlattice potential can bring the atoms into the highly excited η-pair condensate. The time-varying superlattice effectively couples the Fermi gas to an environment from which a controllable amount of energy can be deposited into the quantum gas.
(3) Spectroscopy of the η-pair state. We will theoretically and experimentally study the excitation spectra and correlation functions of the dissipatively and dynamically created quantum states and determine their stability and protection against perturbations.



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