High Harmonic Generation and X-ray Quantum Optics (Invited)

The process of high-harmonic generation is a mature and feasible plan to build coherent quantum light sources. It is widely used in the emerging X-ray quantum optical field and is expected for its deep application. X-ray is theoretically proved better than the currently widely used microwave and visible light band due to its high precision, high diffraction limit, robustness, penetration and focusing ability. Developing quantum optical models, quantum information units and quantum optical components in X-ray bands, could help to their promote precision and miniaturize development. Recent research combines quantum optics with X-ray, producing new types of light sources such as X-ray free electronic lasers, quantum systems, single photon, pulse, and high harmonic generation of coherent quantum light resource under the X-ray regime. The cross between X-ray optics and quantum optics is attracting more and more attention from researchers, becoming a research field with significant breakthroughs and application prospects. The atomic system used in X-ray frequency band quantum optical experiments is the Fe Mossbauer nuclear ensemble, because it has narrow nuclear resonance width narrow(4.7 neV) and long coherence time due to its abundance of cooperative effects. In recent years, researchers have achieved the proof of spontaneous generation coherence under the ensemble, as well as the multi-layer nuclear ensemble model scheme with strong coupling properties. On the other hand, researchers have achieved multiple paths of maintaining the transition in X-ray nuclei such as electromagnetic induced transparency, acoustic induced transparency, and optomechanically induced transparency. In the past five years, the methods above have been optimized, filling the gap in the regulation of X-ray quantum system. X-ray quantum optics requires high-quality X-ray light sources. At present, the scheme of super-fine magnetic field regulation single photon storage has achieved energy and phase storage. In addition, the waveform modulation of X-ray single photon mainly adopts the equivalent scheme of using the mechanical movement of the resonance absorber to phase modulate the pulse, and has pioneered the realization of strong controllability and high precision desktop single photon pulse source. The current XFEL light source generally has coherence problems, however, high-harmonic generation have inherent coherence, which is expected to produce a new generation of attosecond X-ray sources. The current research on high harmonic generating X-ray pulses is mature in the water window energy level. At the same time, various optimization schemes of intensity, photon energy, and phase matching have been proposed. On the other hand, the strong coherence of high-order harmonic X-ray sources makes them firece competitors in ultrafast spectroscopy and ultrafast dynamics detection that can replace XFEL and other X-ray sources. Recently, the theory of high-order harmonics has been advanced to the full quantum stage, and it has been proven that the spectral characteristics of high-order harmonics are strongly correlated with the quantum characteristics of the driving light. It is pointed out that HHG can be used to generate highly coherent, attosecond pulses with properties such as beam gathering and entanglement. In summary, looking back at the past 30 years, X-ray quantum optics has produced many achievements with stage significance in the construction of atomic systems, light sources and photons, pulse regulation etc., marking that the physical regime at the X-ray level is gradually entering the exploration of non-classical effects and the design and regulation scheme at the quantum optical level. With the breakthrough of high harmonic in light sources, theory and applications, we believe that X-ray attosecond light source based on HHG process has a broader prospect in X-ray quantum optics because of its coherence and ultrafast advantages.