Ab initio approaches to molecular quantum matter and quantum information science

at 9:00for 40min

In this talk, I will present theoretical and computational approaches to describe excited-states in quantum matter, and predicting emergent states created by external drives. Understanding the role of such light- matter interactions in the regime of correlated electronic systems is of paramount importance to fields of study across condensed matter physics, quantum optics, and ultrafast dynamics 1. The simultaneous contribution of processes that occur on many time and length-scales have remained elusive for state-of- the-art calculations and model Hamiltonian approaches alike, necessitating the development of new methods in computational chemistry. I will discuss our work at the intersection of ab initio cavity quantum-electrodynamics and electronic structure methods to treat electrons, photons and phonons on the same quantized footing, accessing new observables in strong light-matter coupling. Current approximations in the field almost exclusively focus on electronic excitations, neglecting electron-photon effects, for example, thereby limiting the applicability of conventional methods in the study of polaritonic systems, which requires understanding the coupled dynamics of electronic spins, nuclei, phonons and photons. With our approach we can access correlated electron-photon and photon-phonon dynamics 2–6 , essential to our latest work on driving quantum materials far out-of-equilibrium to control the coupled electronic and vibrational degrees-of-freedom 7–13 . In the second part of my talk, I will demonstrate how the same approach can be generalized in the context of control of molecular quantum matter and molecular quantum transduction. As a first example, I will discuss a cavity-mediated approach to break the inversion symmetry allowing for highly tunable even-order harmonic generation (e.g. second- and fourth-harmonic generation) naturally forbidden in such systems. This relies on a quantized treatment of the coupled light-matter system, similar to the driven case, where the molecular matter is confined within an electromagnetic environment and the incident (pump) field is treated as a quantized field in a coherent state (with just a few photons). When the light-molecule system is strongly coupled, it leads to two important features: (i) a controllable strong-coupling-induced symmetry breaking, and (ii) a tunable and highly efficient nonlinear conversion efficiency of the harmonic generation processes 14–17 . Both of these have implications for molecular quantum architectures. Being able to control molecules at a quantum level gives us access to degrees of freedom such as the vibrational or rotational degrees to the internal state structure. Finally, I will give an outlook on connecting ideas in cavity control of molecules with quantum information science.

[1] Head-Marsden, K., Flick, J., Ciccarino, C. J. & Narang, P. Quantum Information and Algorithms for Correlated Quantum Matter. Chem. Rev. 121, 3061–3120 (2021).

[2] Rivera, N., Flick, J. & Narang, P. Variational Theory of Nonrelativistic Quantum Electrodynamics. Phys. Rev. Lett. 122, 193603 (2019).

[3] Flick, J., Rivera, N. & Narang, P. Strong light-matter coupling in quantum chemistry and quantum photonics. Nanophotonics 7, 1479–1501 (2018).

[4] Flick, J. & Narang, P. Cavity-Correlated Electron-Nuclear Dynamics from First Principles. Physical Review Letters vol. 121 Preprint at https://doi.org/10.1103/physrevlett.121.113002 (2018).

[5] Schäfer, C., Flick, J., Ronca, E., Narang, P. & Rubio, A. Shining Light on the Microscopic Resonant Mechanism Responsible for Cavity-Mediated Chemical Reactivity. arXiv [quant-ph] (2021).

[6] Wang, D. S., Neuman, T., Flick, J. & Narang, P. Light-matter interaction of a molecule in a dissipative cavity from first principles. J. Chem. Phys. 154, 104109 (2021).

[7] Juraschek, D. M., Meier, Q. N. & Narang, P. Parametric Excitation of an Optically Silent Goldstone- Like Phonon Mode. Physical Review Letters vol. 124 Preprint at https://doi.org/10.1103/physrevlett.124.117401 (2020).

[8] Juraschek, D. M., Narang, P. & Spaldin, N. A. Phono-magnetic analogs to opto-magnetic effects. Phys. Rev. Research 2, 043035 (2020).

[9] Juraschek, D. M., Neuman, T. & Narang, P. Giant effective magnetic fields from optically driven chiral phonons in 4f paramagnets. Phys. Rev. Research 4, 013129 (2022).

[10] Juraschek, D. M. & Narang, P. Magnetic control in the terahertz. Science vol. 374 1555–1556 (2021).

[11] Wang, Y. et al. Axial Higgs mode detected by quantum pathway interference in RTe3. Nature 606, 896–901 (2022).

[12] Disa, A. S. et al. Optical stabilization of fluctuating high temperature ferromagnetism in YTiO 3 . arXiv [cond-mat.str-el] (2021).

[13] Zhang, Z. et al. Nonlinear coupled magnonics: Terahertz field-driven magnon upconversion. arXiv [cond-mat.mtrl-sci] (2022).

[14] Welakuh, D. M. & Narang, P. Transition from Lorentz to Fano Spectral Line Shapes in Nonrelativistic Quantum Electrodynamics. ACS Photonics 9, 2946–2955 (2022).

[15] Welakuh, D. M. & Narang, P. Nonlinear optical processes in centrosymmetric systems by strong- coupling-induced symmetry breaking. arXiv [physics.optics] (2022).

[16] Welakuh, D. M. & Narang, P. Tunable Nonlinearity and Efficient Harmonic Generation from a Strongly Coupled Light-Matter System. arXiv [physics.optics] (2022).

[17] Philbin, J. P. et al. Molecular van der Waals fluids in cavity quantum electrodynamics. arXiv [physics.chem-ph] (2022).