A 2026 Nature Materials experiment (Baykusheva et al.) demonstrated coherent Floquet control of a strongly correlated excitation: an off-resonant midinfrared field dresses the Hubbard exciton of the one-dimensional Mott insulator Sr2CuO3, driving ultrafast uniaxial pi/2 rotations between its nearly degenerate bright and dark parity states, read out via resonant third-harmonic generation. The work leaves open directions stated in its own outlook: the extension from uniaxial rotations to full SU(2) control, i.e. azimuthal Bloch-sphere rotations of correlated few-state excitations with strong spin-orbit coupling; optimal control protocols combined with midinfrared pulse shaping for more complex Bloch-sphere trajectories; and the extension to resonant terahertz driving, programmable multi-pulse Ramsey-type sequences, and narrower-linewidth correlated-exciton platforms such as Rydberg excitons in Cu2O or magnetically coupled excitons in van der Waals materials for quantum sensing.

Background

A Mott insulator is a solid whose insulating character arises from electron-electron repulsion rather than band filling; the cuprate chain compound Sr2CuO3 is a paradigmatic one-dimensional Mott insulator described by an extended Hubbard model (onsite repulsion U, intersite repulsion V, hopping t). Photocreating an empty site (holon) and a doubly occupied site (doublon) on neighboring Cu sites produces a bound holon-doublon pair called a Hubbard exciton, the many-body analogue of a semiconductor exciton. Because the holon can sit to the left or right of the doublon, the two configurations form nearly degenerate even-parity (dark, optically forbidden) and odd-parity (bright, optically active) superpositions separated by at most ~16 meV, just below the ~1.8 eV charge gap above which lies a continuum of unbound holon-doublon excitations. The bright exciton has a lifetime of order 2 ps. Floquet engineering denotes the coherent dressing of quantum states by a strong periodic off-resonant field; driving on a timescale short compared with the exciton lifetime coherently rotates the many-body wavefunction within the parity doublet, which is why Baykusheva et al. (2026) could use intense nonresonant midinfrared (MIR) pulses to rotate the Hubbard-exciton state between bright and dark configurations, quantifying instantaneous rotation angles up to ~pi/2 through the resonant third-harmonic generation (THG) response. On the Bloch sphere spanned by the two exciton states, their protocol realizes rotations about a fixed uniaxial axis set by the drive dipole coupling the parity sectors. Full SU(2) controllability — the group of all rotations of a two-level system, including azimuthal-plane rotations about the polar axis — additionally requires a second, independently phased control handle, which the authors propose to obtain from sizable spin-orbit coupling (the relativistic coupling of electron spin and orbital motion) in correlated few-state excitations. With such handles, textbook few-level control schemes become available in a correlated solid: Ramsey interferometry (a two-pulse sequence measuring coherence), dynamical decoupling, and Waugh-Huber-Haeberlen recoupling sequences. Related but distinct frontiers exist on other platforms: Rydberg excitons (highly excited, hydrogen-like excitons in Cu2O with narrow linewidths) have been Floquet-dressed by microwave fields up to the ultrastrong regime, and magnon-coupled excitons in van der Waals magnets have been demonstrated, but neither involves coherent wavefunction rotations of a correlated excitation. The source notes more generally that the existence of Floquet-engineered states in strongly interacting systems featuring dissipation and decoherence remains an open question; in Sr2CuO3 the ratio of exciton lifetime to pump period and leakage into unbound holon-doublon states control sideband visibility and rotation amplitude, and the paper's supplementary analysis fixes a perturbative-expansion crossover at field strengths of 0.7-0.8 MV/cm. What the Sr2CuO3 experiment did not establish, and what the field has since left open, is (i) whether azimuthal SU(2) control can be implemented and sustained against decoherence in correlated few-state excitations with sizable spin-orbit coupling; and (ii) whether the technique extends to optimal control protocols with midinfrared pulse shaping, to resonant terahertz driving (lower required fields, but resonant absorption and heating), to programmable multi-pulse Ramsey-type sequences, and to narrower-linewidth excitonic systems.

Problem Statement

The demonstrated coherent Floquet control of Hubbard excitons covers one point in parameter space: a ~2 ps lifetime excitonic parity doublet in Sr2CuO3, driven off-resonantly in the midinfrared, with uniaxial pi/2-scale rotations read out by third-harmonic generation. The open problem, as posed by the source work's outlook, is to turn this single-point demonstration into a controlled and general capability for correlated excitations in solids, comprising two jointly serving components of one objective — programmable coherent control of correlated many-body excitations: (i) Achieve full SU(2) control of the correlated exciton qubit — azimuthal-plane rotations in addition to the demonstrated uniaxial rotations — by applying the demonstrated methods to correlated few-state excitations, in correlated materials with sizable spin-orbit coupling coupling the parity sectors, and characterize the decoherence limits of such azimuthal control in a correlated solid. (ii) Extend the technique along the outlook's programmable-control directions, demonstrating at least one of the following: optimal control protocols combined with midinfrared pulse shaping to tailor more complex Bloch-sphere trajectories and long-lived population transfer between quantum states; resonant terahertz driving, further reducing required pump field strengths; programmable multi-pulse sequences implementing Ramsey-type interferometry, dynamical decoupling, and Waugh-Huber-Haeberlen control schemes for the exciton state; and narrower-linewidth correlated-exciton platforms such as Rydberg excitons in Cu2O or magnon-coupled excitons in van der Waals magnets, where controlled wavefunction rotations could implement decoupling and recoupling sequences for exciton-based quantum sensing. A solution is a concrete, self-consistent coherent-control protocol for component (i) together with a quantitative feasibility and decoherence analysis (analytical or numerical) in a correlated solid, plus at least one of the directions enumerated under component (ii); an experimental demonstration is not required and is not expected, and a submission meeting this standard resolves the objective.

References

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