The quantum-optical nature of high harmonic generation

Area of Science:

• Quantum optics
• Extreme nonlinear optics
• Attosecond science

Background:

• High harmonic generation (HHG) is a nonlinear process producing attosecond pulses.
• Classical models of HHG do not capture its quantum-optical nature.
• Conditions for significant quantum effects in HHG remain largely unexplored.

Purpose of the Study:

• To develop a fully quantum theory for extreme nonlinear optics.
• To investigate quantum effects altering the spectrum and photon statistics of HHG.
• To identify conditions where HHG properties depart from classical predictions.

Main Methods:

• Development of a fully quantum theoretical framework for nonlinear optics.
• Analysis of spectral and statistical properties of high harmonic generation under quantum conditions.
• Investigation of the breakdown of the dipole approximation in quantum HHG.

Main Results:

• Prediction of quantum effects altering HHG spectrum and photon statistics.
• Observation of shifted frequency combs in HHG spectra.
• Identification of spectral features due to the breakdown of the dipole approximation.
• Demonstration that HHG frequency components can be bunched and squeezed.
• Characterization of emitted photons as superpositions of all spectral frequencies (photons as combs).

Conclusions:

• The fully quantum theory predicts novel quantum phenomena in HHG.
• These findings suggest new avenues for generating attosecond light with quantum properties like squeezing and entanglement.
• The developed approach is broadly applicable to other extreme nonlinear optical processes.