From classical to quantum nature of high harmonic generation (Prof. Hamed Merdji)

Part 1 – Classical HHG : I will first present classical high-harmonic generation in semiconductor as a light up-conversion process occurring in a strong laser field, leading to coherent attosecond bursts of extreme broadband radiation. I will show how a strong laser polarization dependence that can be used to gate isolated attosecond pulses. I will show how nano-structuration of semiconductors can be used to generate structured light such as beam carrying orbital angular momentum or to produce a dramatic enhancement of the HHG emission. Our all semiconductor HHG nano-emitters can work sustainability over days and can be used as ultrafast petahertz optoelectronic devices.
Part 2 – Quantum HHG : We propose that attosecond electronic or photonic processes such as high-harmonic generation can potentially generate non-classical states of light well before the decoherence of the system occurs. This could address fundamental limitations in quantum technology such as scalability, decoherence or the generation of massively entangled states. We recently reported experimental evidence of the non-classical nature of the harmonic emission in several semiconductors excited by a femtosecond mid-infrared laser (Theidel et al, submitted to Nature, in review). By investigating single- and double beam intensity cross-correlation, we observe two-mode squeezing in the generated harmonic radiation, which depends on the laser intensity that governs the transition from Super-Poissonian to Poissonian photon statistics. The measured violation of the Cauchy-Schwarz inequality realizes a direct test of multipartite entanglement in high-harmonic generation.
In conclusion, semiconductor HHG is a new platform that can classically produce tailored structured light on nanometer and ultrafast scales. Furthermore, its inherent nature exhibits non-classical states of light with unique features such as multipartite broadband entanglement or multimode squeezing. The source operates at room temperature using standard semiconductors and a standard commercial fiber laser, opening new routes for the quantum industry. The attosecond control of light states open the vision of quantum processing on unprecedented timescales, an obvious perspective for future quantum optical computers.