A research team proposes an interferometric resonant inelastic X-ray scattering (RIXS) protocol that, rather than merely probing elementary excitations in quantum matter, creates entanglement between spatially separated regions of a solid. An incident X-ray photon is coherently split into two arms by a beam splitter, each illuminating an opposite end of the sample. When the scattered photons are recombined at a second beam splitter and an energy-selected photon is detected, the material is projected into a coherent superposition of excitations at both ends.

The scheme is the X-ray, solid-state analogue of the Duan-Lukin-Cirac-Zoller (DLCZ) heralding protocol, widely used in quantum optics with atoms and cavities. Using a Kramers-Heisenberg formalism with quantized photon modes, the authors show that orthogonal and degenerate end excitations yield maximally entangled states. The proposal is illustrated in three theoretical models: two decoupled sites producing a heralded Bell state, an open tight-binding chain with reflection-parity-dependent interference —a useful benchmark for the interferometer's coherence— and a spin-1 Haldane (AKLT) chain, from which an entangled pair of fractionalized edge spins emerges.

The idea opens a route to generating quantum entanglement on demand in condensed-matter systems using the infrastructure of state-of-the-art coherent X-ray sources, without the need to trap individual atoms or operate at dilution temperatures. The work also discusses practical considerations and challenges for realizing the protocol in X-ray experiments, where the temporal and spectral coherence of the beams and detection efficiency play a critical role.

If these barriers are overcome, the technique could allow direct study of many-body entangled states in solids, including topological edge modes, and serve as a testbed for quantum information based on collective excitations of matter. Next steps include quantifying realistic heralding rates at existing beamlines and identifying compounds with well-isolated edge excitations that maximize the fidelity of the generated state.