Observation of a displaced squeezed state in high-harmonic generation

High-harmonic generation (HHG) and quantum optics are historically two distant fields of science. HHG as a central technique to generate attosecond pulses via atoms, molecules, or solids has been mostly treated as a classical source of light. Entanglement and squeezing of HHG light would open fascinating perspectives in quantum technology, with the emergence of a field of basic and applied research: attosecond quantum optics. Here, we measure entanglement in a tripartite solid-state HHG system through the violation of a Cauchy-Schwarz inequality. Displaced squeezing is experimentally observed and supported by numerical simulations with excellent agreement to the data. The Schmidt number is estimated, which indicates a low-dimensional multimode structure for each harmonic, an important property for most quantum technologies. At the applied level, the semiconductor HHG source is scalable, integrable, operates at room temperature with compact lasers, and thus represents a useful and exploitable resource in quantum photonics.