Center for Quantum Phenomena Seminars

Quantum many-body systems are significantly more challenging to simulate than their classical counterparts. This is primarily due to the quantum sign problem, which prevents a reformulation in terms of stochastically sampling classical configurations. While many “sign problem-free” models have been discovered, which admit high-accuracy numerical simulations, many believe that certain phases of matter are intrinsically sign-problematic. In this talk, I present counterexamples to a number of no-go theorems by constructing sign problem-free models for phases of matter believed to be sign-problematic, and develop a revised perspective for how one should think about intrinsic sign problems.

In this seminar, I will discuss coherent ferron dynamics and terahertz (THz) photophysics in van der Waals (vdW) ferroelectrics, with a particular focus on the ferroelectric semiconductors NbOX2 (X = I, Br, Cl), which exhibit robust in-plane spontaneous polarization at room temperature. I will first introduce the fundamental structural, electronic, and optical properties of NbOX2 and briefly discuss their remarkable nonlinear optical responses in the visible range. I will then describe highly efficient, broadband THz emission generated through giant optical rectification, demonstrating the potential of NbOI2 as an ultrathin THz source. The main focus of the talk will be the emergence of ferron excitations in the thin-film limit. In addition to broadband THz emission, we observe an extremely narrowband THz response, which we associate with collective excitations of the ferroelectric polarization, i.e., ferrons. Femtosecond optical excitation launches coherent ferron wavepackets that propagate over macroscopic distances while maintaining coherence for more than 200 ps, reminiscent of coherent magnons. These observations highlight the potential of ferrons as room-temperature information carriers operating at THz frequencies. Time permitting, I will also discuss our current experimental and theoretical understanding of these excitations, together with several important questions that remain unresolved.

The ultrafast demagnetization is a surprising phenomenon discovered in 1996 in which a femtosecond laser pulse (fsLP) pumping of a ferromagnet leads to its demagnetization, as nonclassical dynamics where magnetization vector is shrinking in length while not rotating. Despite being one of the primary experimental observables, THz electromagnetic radiation from either a single magnetic layer, or much more enhanced from ferromagnet/spin-orbit-material bilayers [1], has rarely been calculated. This has forced experimentalists to develop intuitive pictures [1], such as of interlayer spin current generated by “spin voltage” that is converted into charge currents via spin-orbit coupling induced mechanisms. This talk will discuss the recently developed framework [2], combining time-dependent density functional theory (TDDFT) and Jefimenko formulas for time-retarded electric and magnetic fields as proper solutions of the Maxwell equations, that make it possible to compute emitted THz radiation from first principles. Inspection of time-dependent charge currents, as the only source of the electric field in the far-field region within Jefimenko formulas, then reveals that a major contribution to THz radiation arises from overlooked intralayer charge current within the ferromagnetic metallic layer that is pumped by demagnetization dynamics. Interlayer spin current is also pumped by ultrafast demagnetization dynamics [3], flowing toward spin-orbit-material layer where it is converted into charge current as often assumed. Its origin can be explained [4] by the same time-dependent quantum transport calculations which account for conventional spin pumping by rotating magnetization of fixed length when ferromagnets are driven by microwave photons of much lower energy than in ultrafast demagnetization phenomena. Finally, in the case of fsLP-driven antiferromagnetic Mott insulators, as motivated by experiments [5] on NiO/Pt bilayers, standard TDDFT is not applicable due to intricacies of handling time-dependent Hubbard U. We replace it with tensor network and/or exact diagonalization calculations for multi-orbital Hubbard-Hund-Heisenberg model which includes proximity spin-orbit coupling due to adjacent heavy metal Pt layer. This reveals [6] nonclassical dynamics of the Néel vector and nonequilibrium magnetization, which leads to both THz radiation and peculiar high harmonic generation, including the noninteger ones, while not requiring any interlayer spin transport as in the case of ferromagents as also confirmed experimentally [5].

References
[1] T. S. Seifert, D. Go, H. Hayashi, R. Rouzegar, F. Freimuth, K. Ando, Y. Mokrousov, and T. Kampfrath, Nat. Nanotechnol. 18, 1132 (2023).
[2] A. Kefayati and B. K. Nikolić, Phys. Rev. Lett. 133, 136704 (2024).
[3] A. Kefayati, Y. Ren, M. B. Jungfleisch, L. Gundlach, J. Q. Xiao, and B. K. Nikolić, Phys. Rev. B 111, L140415 (2025).
[4] J. Varela-Manjarres, A. Kefayati, M. B. Jungfleisch, J. Q. Xiao, and B. K. Nikolić, Phys. Rev. B 110, L060410 (2024) (accompanied by popular highlight at https://physics.aps.org/articles/v17/s97).
[5] T.W. J. Metzger et al., Phys. Rev. Lett. 135, 076702 (2025).
[6] F. Garcia-Gaitan, A. E. Feiguin, and B. K. Nikolić, Phys. Rev. Lett. 135, 086704 (2025).

I introduce Quantum Printing- a concept for controlling quantum matter by directly imprinting the quantum numbers carried by structured light (spin angular momentum, orbital angular momentum, chirality, and entanglement) onto coherent electron and magnetic states. Structured light acts as a quantum “ink” that writes, manipulates, and erases macroscopic quantum orders in both charged fluids (superconductors and quantum Hall systems) and neutral fluids (magnets), going beyond conventional thermal or magnetic control. I will also introduce extensions of these ideas to reversible optical writing and erasing of topological skyrmions in Heisenberg magnets with THz structured beams, enabling error-corrected magnetic memory, sub-diffraction patterning, and calligraphy of continuous textures [2].

Work supported by DOE DE-SC-0025580, ERC-2018-SYG 810451 HERO, KAW 2019.0068.
[1] G. Aeppli, et al , Quantum Printing, arXiv:2509.16792, (2025).
[2] Y. Liu, et.al, Quantum Printing: Laguerre-Gaussian Beam Induced Topological Magnetic Textures (2026) in preparation.

We develop a bootstrap approach to deriving lower bounds on the spectral gap of the first Haldane pseudopotential Hamiltonian on the sphere with NΦ flux quanta for arbitrary NΦ, including in the thermodynamic limit NΦ→∞. Our approach can in principle be generalized to pseudopotentials describing other model wavefunctions such as the Moore-Read state or Read-Rezayi sequence. Our approach is based on Knabe's observation that if H is a Hamiltonian with ground state energy 0 then the spectral gap is at least ϵ>0 if and only if H2⪰ϵH. To show this inequality, we split H2 into two parts, one that can be analytically bounded and another that can be bounded using a finite semidefinite program (SDP). Crucially, the size of this program is determined only by cutoff parameters, and the program depends on NΦ only parametrically, allowing us to obtain numerically exact (up to solver precision) lower bounds on the gap. In order to turn these numerical bounds into mathematically rigorous bounds, we introduce an algorithm that can produce nearly-optimal exact feasible points for large rational SDPs. We additionally provide an algorithm that establishes the rank stability of the semidefinite program in the limit NΦ→∞ which is needed to certify the bound in the thermodynamic limit. Beyond establishing the Laughlin gap and revealing a potential method for deriving spectral gap lower bounds for other pseudopotentials, our work provides a general method for turning numerical semidefinite programming bounds into rigorous mathematical proofs, which may have wide applicability.