Yb fermionic quantum simulator

Non-Hermitian topological matter : 2D Non-Hermitian Skin effect

Two-dimensional non-Hermitian skin effect in an ultracold Fermi gas  

arXiv:2311:07931 (2023)

The concept of non-Hermiticity has expanded the understanding of band topology leading to the emergence of counter-intuitive phenomena. One example is the non-Hermitian skin effect (NHSE), which involves the concentration of eigenstates at the boundary. However, despite the potential insights that can be gained from high-dimensional non-Hermitian quantum systems in areas like curved space, high-order topological phases, and black holes, the realization of this effect in high dimensions remains unexplored. Here, we create a two-dimensional (2D) non-Hermitian topological band for ultracold fermions in spin-orbit-coupled optical lattices with tunable dissipation, and experimentally examine the spectral topology in the complex eigenenergy plane. We experimentally demonstrate pronounced nonzero spectral winding numbers when the dissipation is added to the system, which establishes the existence of 2D skin effect. We also demonstrate that a pair of exceptional points (EPs) are created in the momentum space, connected by an open-ended bulk Fermi arc, in contrast to closed loops found in Hermitian systems. The associated EPs emerge and shift with increasing dissipation, leading to the formation of the Fermi arc. Our work sets the stage for further investigation into simulating non-Hermitian physics in high dimensions and paves the way for understanding the interplay of quantum statistics with NHSE.

Non-Hermitian topological matter: Exceptional point & Chiral spin transfer 

Chiral control of quantum states in non-Hermitian spin-orbit-coupled fermions

Nature Physics 18 385-389 (2022)

Spin–orbit coupling is an essential mechanism underlying quantum phenomena such as the spin Hall effect and topological insulators1. It has been widely studied in well-isolated Hermitian systems, but much less is known about the role dissipation plays in spin–orbit-coupled systems2. Here we implement dissipative spin–orbit-coupled bands filled with ultracold fermions, and observe parity-time symmetry breaking as a result of the competition between the spin–orbit coupling and dissipation. Tunable dissipation, introduced by state-selective atom loss, enables us to tune the energy gap and close it at the critical dissipation value, the so-called exceptional point3. In the vicinity of the critical point, the state evolution exhibits a chiral response, which enables us to tune the spin–orbit coupling and dissipation dynamically, revealing topologically robust chiral spin transfer when the quantum state encircles the exceptional point. This demonstrates that we can explore non-Hermitian topological states with spin–orbit coupling.

Machine Laerning-aided analysis of SU(N) fermions (Thermodynamics)

Heurisitic machinery for thermodynamic studies of SU(N) fermions with neural networks 

Nature Communications 12 2011 (2021)

The power of machine learning (ML) provides the possibility of analyzing experimental measurements with an unprecedented sensitivity. Here, we introduce a heuristic machinery by using machine learning analysis. We use our machinery to guide the thermodynamic studies in the density profile of ultracold fermions interacting within SU(N) spin symmetry prepared in a quantum simulator.  Guided by our machinery, we directly measure a thermodynamic compressibility from density fluctuations within the single image. Our machine learning framework shows a potential to validate theoretical descriptions of SU(N) Fermi liquids, and to identify less-pronounced effects even for highly complex quantum matter with minimal prior understanding.

SU(N) fermions - Bosonization

Evidence for bosonization in a three-dimensional gas of  SU(N) Fermions

Physical Review X  10 041052 (2020) [journal]

Whereas textbooks of quantum mechanics have introduced bosons and fermions as distinct particles with drastically different properties, modern developments in condensed-matter and high-energy physics have suggested that the boundary between bosons and fermions can be blurred. For example, a gas of interacting fermions, each identified by a different spin, is expected to behave like an ensemble of spinless bosons when the number of different spins in the system becomes very large. In this work, we use quantum simulation to verify such a “bosonization” phenomenon. 

SU(N) fermions - 2D

Collective excitations in two-dimensional SU(N) Fermi gases with tunable spin

Phys. Rev. Research Rapid Comm.  2 012028(R) (2020) [Journal]

We measure collective excitations of a harmonically trapped two-dimensional (2D) SU(N) Fermi gas of 173Yb confined to a stack of layers formed by a one-dimensional optical lattice. Quadrupole and breathing modes are excited and monitored with tunable spin. We observe that the quadrupole mode frequency decreases with increasing number of spin components due to the amplification of the interaction effect by N. Our result paves the way to investigate the collective property of 2D SU(N) Fermi liquid with enlarged spin.

Topological matter in optical lattices

Observation of nodal-line semimetal with ultracold fermions in an optical lattice

Nature Physics 15 911-916 (2019)  [Journal] 

The observation of topological phases beyond two dimensions, as widely reported in solid-state systems, has been an open challenge for ultracold atoms. Here, we for the first time realize a 3D spin–orbit coupled nodal-line semimetal in an optical Raman lattice filled with ultracold fermions, and observe the bulk line nodes in the band structure. Our results demonstrate an approach to effectively observe 3D band topology, and open the way to probe exotic topological physics for ultracold atoms in high dimensions.

Topological matter in optical lattices

Observation of symmetry-protected topological band with ultracold fermions

Science Advances 4, eaao4748 (2018) [Journal

Symmetry plays a fundamental role in understanding complex quantum matter, particularly in classifying topological quantum phases. An outstanding example is the time-reversal invariant topological insulator, a symmetry-protected topological (SPT) phase in the symplectic class of the Altland-Zirnbauer classification. We report the observation for ultracold atoms of a noninteracting SPT band in a one-dimensional optical lattice. This work opens the way to expanding the scope of SPT physics with ultracold atoms and studying nonequilibrium quantum dynamics in these exotic systems.

Spin-orbit coupling for ultracold fermions

Spin-orbit-coupled two-electron Fermi gases of ytterbium atoms

Phys. Rev. A Rapid Comm.  94, 061604(R) (2016) [Journal]

We demonstrate all-optical implementation of spin-orbit coupling (SOC) in a two-electron Fermi gas of 173Yb atoms by coupling two hyperfine ground states with a narrow optical transition. Due to the SU(N) symmetry of the 1S0 ground-state manifold which is insensitive to external magnetic fields, an optical ac Stark effect is applied to split the ground spin states. The realization of all-optical SOC for ytterbium fermions should offer a route to a long-lived spin-orbit-coupled Fermi gas and greatly expand our capability of studying spin-orbit physics with alkaline-earth-metal-like atoms.