@article {3261, title = {Ever more optimized simulations of fermionic systems on a quantum computer}, year = {2023}, month = {3/6/2023}, abstract = {

Despite using a novel model of computation, quantum computers break down programs into elementary gates. Among such gates, entangling gates are the most expensive. In the context of fermionic simulations, we develop a suite of compilation and optimization techniques that massively reduce the entangling-gate counts. We exploit the well-studied non-quantum optimization algorithms to achieve up to 24\% savings over the state of the art for several small-molecule simulations, with no loss of accuracy or hidden costs. Our methodologies straightforwardly generalize to wider classes of near-term simulations of the ground state of a fermionic system or real-time simulations probing dynamical properties of a fermionic system.\ 

}, url = {https://arxiv.org/abs/2303.03460}, author = {Qingfeng Wang and Ze-Pei Cian and Ming Li and Igor L. Markov and Yunseong Nam} } @article {2569, title = {Resource-Optimized Fermionic Local-Hamiltonian Simulation on Quantum Computer for Quantum Chemistry}, journal = {Quantum}, volume = {5}, year = {2021}, month = {7/21/2021}, abstract = {

The ability to simulate a fermionic system on a quantum computer is expected to revolutionize chemical engineering, materials design, nuclear physics, to name a few. Thus, optimizing the simulation circuits is of significance in harnessing the power of quantum computers. Here, we address this problem in two aspects. In the fault-tolerant regime, we optimize the $\rzgate$ and $\tgate$ gate counts along with the ancilla qubit counts required, assuming the use of a product-formula algorithm for implementation. We obtain a savings ratio of two in the gate counts and a savings ratio of eleven in the number of ancilla qubits required over the state of the art. In the pre-fault tolerant regime, we optimize the two-qubit gate counts, assuming the use of the variational quantum eigensolver (VQE) approach. Specific to the latter, we present a framework that enables bootstrapping the VQE progression towards the convergence of the ground-state energy of the fermionic system. This framework, based on perturbation theory, is capable of improving the energy estimate at each cycle of the VQE progression, by about a factor of three closer to the known ground-state energy compared to the standard VQE approach in the test-bed, classically-accessible system of the water molecule. The improved energy estimate in turn results in a commensurate level of savings of quantum resources, such as the number of qubits and quantum gates, required to be within a pre-specified tolerance from the known ground-state energy. We also explore a suite of generalized transformations of fermion to qubit operators and show that resource-requirement savings of up to more than 20\% is possible.

}, doi = {https://doi.org/10.22331/q-2021-07-26-509}, url = {https://arxiv.org/abs/2004.04151}, author = {Qingfeng Wang and Ming Li and Christopher Monroe and Yunseong Nam} } @article {2535, title = {Feshbach resonances in p-wave three-body recombination within Fermi-Fermi mixtures of open-shell 6Li and closed-shell 173Yb atoms}, year = {2019}, month = {12/10/2019}, abstract = {

We report on observations and modeling of interspecies magnetic Feshbach resonances in dilute ultracold mixtures of open-shell alkali-metal 6Li and closed-shell 173Yb atoms with temperatures just above quantum degeneracy for both fermionic species. Resonances are located by detecting magnetic-field-dependent atom loss due to three-body recombination. We resolve closely-located resonances that originate from a weak separation-dependent hyperfine coupling between the electronic spin of 6Li and the nuclear spin of 173Yb, and confirm their magnetic field spacing by ab initio electronic-structure calculations. Through quantitative comparisons of theoretical atom-loss profiles and experimental data at various temperatures between 1 μK and 20 μK, we show that three-body recombination in fermionic mixtures has a p-wave Wigner threshold behavior leading to characteristic asymmetric loss profiles. Such resonances can be applied towards the formation of ultracold doublet ground-state molecules and quantum simulation of superfluid p-wave pairing.

}, url = {https://arxiv.org/abs/1912.04874}, author = {Alaina Green and Hui Li and Jun Hui See Toh and Xinxin Tang and Katherine McCormick and Ming Li and Eite Tiesinga and Svetlana Kotochigova and Subhadeep Gupta} } @article {2304, title = {Fractal Universality in Near-Threshold Magnetic Lanthanide Dimers}, journal = {Science Advances}, volume = {4}, year = {2018}, month = {2018/02/16}, pages = {eaap8308}, abstract = {

Ergodic quantum systems are often quite alike, whereas nonergodic, fractal systems are unique and display characteristic properties. We explore one of these fractal systems, weakly bound dysprosium lanthanide molecules, in an external magnetic field. As recently shown, colliding ultracold magnetic dysprosium atoms display a soft chaotic behavior with a small degree of disorder. We broaden this classification by investigating the generalized inverse participation ratio and fractal dimensions for large sets of molecular wave functions. Our exact close-coupling simulations reveal a dynamic phase transition from partially localized states to totally delocalized states and universality in its distribution by increasing the magnetic field strength to only a hundred Gauss (or 10 mT). Finally, we prove the existence of nonergodic delocalized phase in the system and explain the violation of ergodicity by strong coupling between near-threshold molecular states and the nearby continuum.

}, doi = {https://doi.org/10.1126/sciadv.aap8308}, url = {https://arxiv.org/abs/1802.09586}, author = {Constantinos Makrides and Ming Li and Eite Tiesinga and Svetlana Kotochigova} } @article {2255, title = {Orbital quantum magnetism in spin dynamics of strongly interacting magnetic lanthanide atoms}, year = {2018}, abstract = {

Laser cooled lanthanide atoms are ideal candidates with which to study strong and unconventional quantum magnetism with exotic phases. Here, we use state-of-the-art closed-coupling simulations to model quantum magnetism for pairs of ultracold spin-6 erbium lanthanide atoms placed in a deep optical lattice. In contrast to the widely used single-channel Hubbard model description of atoms and molecules in an optical lattice, we focus on the single-site multi-channel spin evolution due to spin-dependent contact, anisotropic van der Waals, and dipolar forces. This has allowed us to identify the leading mechanism, orbital anisotropy, that governs molecular spin dynamics among erbium atoms. The large magnetic moment and combined orbital angular momentum of the 4f-shell electrons are responsible for these strong anisotropic interactions and unconventional quantum magnetism. Multi-channel simulations of magnetic Cr atoms under similar trapping conditions show that their spin-evolution is controlled by spin-dependent contact interactions that are distinct in nature from the orbital anisotropy in Er. The role of an external magnetic field and the aspect ratio of the lattice site on spin dynamics is also investigated.

}, url = {https://arxiv.org/abs/1804.10102}, author = {Ming Li and Eite Tiesinga and Svetlana Kotochigova} } @article {1958, title = {Pendular trapping conditions for ultracold polar molecules enforced by external electric fields}, journal = {Physical Review A}, volume = {95}, year = {2017}, month = {2017/06/26}, pages = {063422}, abstract = {

We theoretically investigate trapping conditions for ultracold polar molecules in optical lattices, when external magnetic and electric fields are simultaneously applied. Our results are based on an accurate electronic-structure calculation of the polar\ 23Na40K polar molecule in its absolute ground state combined with a calculation of its rovibrational-hyperfine motion. We find that an electric field strength of\ 5.26(15)\ kV/cm and an angle of\ 54.7o\ between this field and the polarization of the optical laser lead to a trapping design for\ 23Na40K molecules where decoherences due laser-intensity fluctuations and fluctuations in the direction of its polarization are kept to a minimum. One standard deviation systematic and statistical uncertainties are given in parenthesis. Under such conditions pairs of hyperfine-rotational states of\ v=0\ molecules, used to induce tunable dipole-dipole interactions between them, experience ultrastable, matching trapping forces.

}, doi = {10.1103/PhysRevA.95.063422}, url = {https://arxiv.org/abs/1703.03839}, author = {Ming Li and Alexander Petrov and Constantinos Makrides and Eite Tiesinga and Svetlanta Kotochigova} }