Research

Our lab's objective is to address questions in condensed matter physics, quantum dynamics, and physical chemistry by achieving full quantum control over all degrees of freedom in a molecular gas.  In particular, we will explore novel states of matter arising from molecular many-body systems.  This will lead to the emergence of topological superfluids that will show extraordinary characteristics like frictionless flow, the presence of exotic particles, and resistance to disorder.

Quantum control over molecules represents one of the most exciting opportunities in physics.   Recently, interacting molecular gases have been prepared at ~10 nK temperatures where molecules behave like waves rather than classical particles. Crucially, interactions between ultracold molecules are long-range, well understood, and can be precisely tuned, making this platform ideal to study emergent quantum phenomena.

The Yan Lab will work with a never-before-tamed species: the potassium - silver (KAg) molecule, which has a special, exaggerated property- the distribution of its electrons is highly lopsided, forming an "electric dipole moment."  Unlike atoms, "polar" molecules can meet and interact over long distances, much like how opposite poles of magnets will attract. Compared to existing ultracold molecules, KAg's electric dipolar interaction is stronger by over an order of magnitude, so many-body effects arising from interactions are easier to engineer and detect.  

Scientific program

Our lab will produce ultracold KAg by first preparing ultracold potassium and silver atoms, using workhorse techniques such as laser cooling, evaporative cooling, optical dipole traps, and optical tweezers. Then, we will associate the atoms into a bound dimer via a "Feshbach resonance," and transfer the dimer to the absolute ground energy state. 

Starting from a gas of interacting molecules, we will lower the ensemble's temperature and increase its density until the molecules stop behaving like classical billiard balls but evolve into a strongly interacting quantum soup: a topological p+ip superfluid. Here, bound pairs of molecules can move past each other without resistance, similar to how in superconducting metals, electrons conduct current without dissipation by forming Cooper pairs.  

In a complementary thrust, we will load individual molecules in an optical tweezer array in order to simulate lattice spin models.  These "textbook" condensed matter models offer insight into how magnetic behavior arises in quantum materials.

A programmable tweezer array of fermionic atoms

We developed a programmable quantum simulator of electronic lattice systems, using strongly interacting ultracold atoms in software-defined optical tweezer arrays. Unlike in most existing tweezer array platforms, here, atoms coherently tunneled between sites, giving access to the physics of itinerant systems of relevance to high-temperature superconductivity.  We realized the Fermi-Hubbard model, creating up to eight sites of tunnel-coupled tweezers of lithium-6 atoms.

A quantum gas microscope of polar molecules

We pioneered the detection of polar NaRb molecules trapped in a two-dimensional optical lattice with single-site resolution. For the first time, we were able to see hundreds of ground state molecules in situ, giving access to correlation functions that were previously inaccessible. We studied the tendency of indistinguishable bosonic molecules to bunch together, in an analogy to the pioneering work by Hanbury Brown and Twiss on photon number fluctuations, and we detected two-particle correlations with our shot-noise-limited microscope. 

Pivoting to the simulation of quantum magnetism, we probed site-resolved correlations in the quantum dynamics of various lattice spin models. Using two rotational states of the molecules to code a spin-1/2 system, we induced dipolar resonant spin-exchange interactions, enabling the study of XY and XXZ spin models. We witnessed two-molecule entangling dynamics leading to oscillations in the spin correlation functions at short times, an important prerequisite to using molecules as qubits for quantum computation.

Strongly interacting Bose-Fermi mixtures

Using ultracold gases of sodium and potassium atoms, we realized a concept predicted by Landau in 1933: strong interactions between an electron in an ionic lattice will form a "polaron," or a composite particle that has characteristics of both the original electron and the lattice, but is distinct from both.  In our experiment, one atomic species played the role of the electron while another species played the role of the surrounding lattice. By varying the interspecies interaction strength via Feshbach resonances, we continuously tuned this quantum material to the strong-coupling regime, providing a controlled realization of the famous Landau polaron.  Our work was the first exploration of the strong-coupling polaron in the quantum critical regime and could shed light on other complex systems with quantum critical behavior such as high temperature superconductors and heavy fermions.

See Yan et al, Science (2020)