Energy conservation in quantum optical high harmonic generation

Abstract: The use of energy conservation arguments is ubiquitous in understanding the process of high harmonic generation, yet a complete quantum optical description of photon exchange remained elusive. Here, we solve this gap in description by introducing the energy conserving subspace in high harmonic generation in which many photons of the driving field are absorbed to generate a single photon of higher energy. The presented solution to energy conservation in quantum optical high harmonic generation naturally results in highly entangled states of light with non-classical properties in their marginals. This further allows to explain recent experimental results for quantum state engineering, for which we provide analytical bounds on the fidelity with a high photon number optical cat state.