Floquet Engineering Achieves Control of Hubbard Excitons in Sr CuO

Scientists are tackling the formidable challenge of controlling the many-body wavefunction in materials, potentially unlocking precise manipulation of emergent phenomena. D. R. Baykusheva, D. P. Carmichael, and C. S. Weber, alongside I-T. Lu, F. Glerean, and T. Meng, demonstrate a significant step forward by utilising Floquet engineering , a technique employing periodic light to coherently alter quantum states , to control a strongly correlated Hubbard exciton within the one-dimensional Mott insulator SrCuO₂. While previous applications of Floquet engineering have largely focused on simpler systems, this research pioneers the manipulation of complex, many-body wavefunctions, quantifying ultrafast exciton rotations using resonant third-harmonic generation and paving the way for programmable control of correlated states and exciton-based technologies.

Floquet control of Hubbard excitons in Sr2CuO3

Researchers employed Floquet engineering, a technique utilising coherent dressing of quantum states with periodic non-resonant optical fields, to achieve this breakthrough, moving beyond previous applications focused on weakly interacting systems. This innovative approach leverages the unique properties of Hubbard excitons, bound holon-doublon pairs stabilised by strong Coulomb interactions, offering a natural two-level system for quantum control. The study unveils a dynamical renormalization of the harmonic response as the exciton rotates into a dark state, confirming the coherent manipulation of the many-body wavefunction. Scanning the THG resonance further revealed the presence of Floquet sidebands, a definitive signature of coherent periodic driving and a key indicator of successful Floquet engineering.
By establishing this level of control, the research establishes a pathway towards manipulating complex quantum systems with unprecedented precision. The work opens new possibilities for quantum control of solids, enabling the design of programmable pulse sequences to tailor material properties. Researchers utilized ultrafast, nonresonant midinfrared pulses to dress the excitonic wavefunction, generating coherent rotations between nearly-degenerate states of opposite parity in Sr₂CuO₃. This precise control, achieving arbitrary angles up to π/2, is consistent with all-optical U(1) control of the many-body wavefunction, a remarkable feat in strongly correlated materials.

The team’s approach leverages the large optical nonlinearities exhibited by Hubbard excitons in one-dimensional Mott insulators, making them ideally suited for optically probing and controlling their quantum states. Furthermore, the research establishes a foundation for exciton-based quantum sensing, potentially leading to novel devices and applications. Sr₂CuO₃, a half-filled Mott insulator composed of chains of corner-sharing CuO₄ units, exhibits extremely weak interchain coupling, reinforcing its one-dimensional character and facilitating spin-charge separation. The material’s low-energy behaviour is well described by an extended Hubbard model, supporting the formation of these Hubbard excitons with near-degenerate even and odd parity states separated by at most 16 meV. Despite a substantial on-chain exchange coupling of J = 2800 K, magnetic ordering only occurred below TN ≈5 K, confirming extremely weak interchain coupling and reinforcing the material’s one-dimensional character. Broadband infrared spectroscopy was then used to extract the frequency-dependent reduced optical conductivity ωσ₁(ω) along the b axis, revealing excitonic states just below the 1.8 eV charge-transfer gap. An extended Hubbard model in the large Mott gap limit was fitted to the experimental data, allowing extraction of key Hubbard parameters.

Researchers then leveraged the large optical nonlinearities exhibited by Hubbard excitons in one-dimensional Mott insulators to optically probe and control their quantum states. Centrosymmetric Sr₂CuO₃ exhibited a colossal enhancement of the third-order susceptibility, reaching χ(3)(−3ω; ω, ω, ω) ≈1.4 · 10⁷ pm²/V² at the 3ω resonance with the Hubbard exciton. This enhancement, driven by strong dipole coupling between odd- and even-parity states, enabled the generation of third-harmonic photons at the exciton energy upon in-gap excitation. Crucially, THG optical measurements probed the Hubbard exciton symmetry with a sensitivity exceeding that of linear absorption.

To implement quantum control, the team utilized intense midinfrared (MIR) pulses to coherently manipulate the Hubbard exciton. The two excitonic states were treated as a many-body analogue of a quantum-optical two-level system, described by an interaction Hamiltonian Hint(t) = μug· EMIR |u⟩⟨g|+h. c., where EMIR(t) = E₀ MIR cos(Ωt) represents the pump field. Time-resolved resonant THG was then employed to probe the evolving state, revealing coherent rotations of the wavefunction on a Bloch sphere, a direct consequence of the MIR field driving Rabi oscillations between the |u⟩ and |g⟩ states. This innovative approach enabled the redistribution of spectral weight into Floquet sidebands at Eexc ±2nΩ, demonstrating the creation of Floquet-Hubbard excitons.

Hubbard exciton control via Floquet engineering offers new

Experiments revealed that the exciton state undergoes controllable rotations by arbitrary angles, reaching and exceeding π/2 as a function of electric field strength. This establishes a novel Floquet engineering regime where wavefunction mixing dominates, realising a Rabi problem at ultrastrong coupling. The team measured the third harmonic yield decreasing as the quantum state rotated, vanishing at a rotation angle of π, confirming the manipulation of the exciton’s wavefunction. Data shows a 322.2(2.2) fs Gaussian profile for the time-resolved differential third-harmonic intensity, providing precise temporal resolution of the exciton dynamics.

Results demonstrate the coherent manipulation of an addressable many-body excitation in a strongly-correlated electron system, establishing a platform for ultrafast quantum control in solids. The combination of optimal control protocols and midinfrared pulse shaping offers a route to tailoring complex Bloch-sphere trajectories and driving long-lived population transfer between quantum states. Analysis of Floquet sidebands revealed the emergence of a below-gap sideband in the differential third-harmonic intensity at a MIR field of 0.8 MV/cm, confirming the generation of Floquet replicas spaced by ħΩ. Furthermore, the study highlights potential applications in quantum sensing, as the exciton energy and lineshape are highly sensitive to electronic interactions and chemical potentials. Controlled wavefunction rotations, particularly in systems with narrower excitonic linewidths, could enhance sensitivity and functionalise these systems as quantum sensors. The differential third-order nonlinearity, p |∆I₃ω/I₃ω| ∝∆χ⁽³⁾/χ⁽³⁾, was measured at ħω = 0.59 eV as a function of MIR field and instantaneous rotation angle, providing insights into the nonlinear optical response of the material.

Exciton Wavefunction Control via Floquet Engineering offers new

Scientists have demonstrated quantum control of Hubbard excitons within the Mott insulator strontium copper oxide (Sr₂CuO₃). This work establishes a novel regime of Floquet engineering where wavefunction mixing is dominant, effectively creating a Rabi problem under ultrastrong coupling conditions. The team observed controllable rotations of the exciton state by angles reaching and exceeding π/2, demonstrating a high degree of tunability and establishing a platform for ultrafast quantum control in solids. The authors acknowledge that the achieved rotation angles slightly undershoot theoretical predictions, likely due to interactions with unbound holon-doublon states, a limitation inherent to the system’s complexity.

Future research could involve employing optimal control protocols and midinfrared pulse shaping to create more intricate Bloch-sphere trajectories and facilitate long-lived population transfer between quantum states. Extending these techniques to the terahertz regime may reduce pump field requirements and enable resonant coupling between near-degenerate quantum states. Furthermore, the development of full SU(2) control through azimuthal plane rotations could unlock new possibilities for designing novel photoinduced states of matter and enhancing quantum sensing capabilities, potentially functionalising materials like Rydberg excitons in Cu₂O as sensitive quantum sensors.