In this paper we examine some effects of quantum interference on high harmonic generation. We demonstrate in particular that preparing the initial state in a coherent superposition of bound states leads to a harmonic spectrum with distinct plateaus with different conversion efficiencies. We show how this scheme may provide a way of controlling the coherent output that is produced in an experiment. \textcopyright{} 1996 The American Physical Society.
In this paper we examine some effects of quantum interference on high harmonic generation. We demonstrate in particular that preparing the initial state in a coherent superposition of bound states leads to a harmonic spectrum with distinct plateaus with different conversion efficiencies. We show how this scheme may provide a way of controlling the coherent output that is produced in an experiment. \textcopyright{} 1996 The American Physical Society.
For decades, most research of high harmonic generation (HHG) considered matter as quantum but light as classical. Recently, HHG driven by quantum states of light such as bright squeezed vacuum was predicted to reach beyond the classical HHG cutoff. Moreover, in squeezed coherent illumination, it was shown that the underlying dynamics are significantly modified by the photon statistics effective force. Here we show that HHG driven by quantum light results in quantum high harmonics. We derive a formula for the quantum state of the high harmonics, when driven by arbitrary quantum light states, and then explore specific cases of experimental relevance. Specifically, for a moderately squeezed pump, HHG driven by squeezed coherent light results in squeezed high harmonics. Harmonic squeezing is optimized by syncing ionization times with the pump's squeezing phase. Beyond this regime, as pump squeezing is increased, the harmonics initially acquire squeezed thermal photon statistics, and then occupy an intricate quantum state which strongly depends on the semiclassical nonlinear response function of the interacting system. Our results pave the way for generation of squeezed extreme-ultraviolet ultrashort pulses, and more generally, quantum frequency conversion into previously inaccessible spectral ranges, which may enable ultrasensitive attosecond metrology. Published by the American Physical Society 2024
We investigate high harmonic generation (HHG) of an atom in the presence of squeezed vacuum of a single harmonic mode. Based on a fully quantum time-dependent Schrödinger equation, we derive an analytical formula for the harmonic amplitude. Our simulations of the HHG spectrum with this formula show that the harmonic amplitude of the corresponding squeezed mode can undergo significant changes with different parameters of the squeezed vacuum. Using the time-frequency analysis method, the physics underlying the effects of the vacuum quantum fluctuation (VQF) on the harmonic generation is revealed, which is found to be consistent with the explanation of Fermi's golden rule. Our work establishes a profound connection between harmonic generation and VQF, and may provide an unconventional approach to manipulating harmonic emission. Published by the American Physical Society 2024
High-harmonic generation is a light up-conversion process occurring in a strong laser field, leading to coherent bursts of extreme ultrashort broadband radiation [Lewenstein , Phys. Rev. A , 2117 (1994)]. As a new perspective, we propose that ultrafast strong-field electronic or photonic processes such as high-harmonic generation can potentially generate nonclassical states of light well before the decoherence of the system occurs [Gorlach , Nat. Commun. , 4598 (2020); Stammer ., Phys. Rev. Lett. , 123603 (2022)]. This could address fundamental challenges in quantum technology such as scalability, decoherence, or the generation of massively entangled states [Lewenstein , Luca Argenti Michael Chini, 27 (2024)]. Here, we report experimental evidence of the nonclassical nature of the harmonic emission in several semiconductors excited by a femtosecond infrared laser. By investigating single- and double-beam intensity cross-correlation [Loudon, Rep. Prog. Phys. , 913 (1980)], we measure characteristic nonclassical features in the single-photon statistics. 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 [Wasak, Phys. Rev. A , 033616 (2014)]. This result is supported by the theory of multimodal detection and the Hamiltonian from which the effective squeezing modes of the harmonics can be derived [Gonoskov , Phys. Rev. B , 125110 (2024); Christ New J. Phys. , 033027 (2011)]. With this work, we show experimentally that high-harmonic generation is a new quantum bosonic platform that intrinsically produces nonclassical 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 up new routes for the quantum industry, such as optical quantum computing, communication, and imaging. Published by the American Physical Society 2024
High-harmonic generation is one of the most fundamental processes in strong laser-field physics that has led to countless achievements in atomic physics and beyond. However, a rigorous quantum electrodynamical picture of the process has never been reported. Here, we prove rigorously and demonstrate experimentally that the quantum state of the driving laser field, as well as that of harmonics, is coherent. Projecting this state on its part corresponding to harmonic generation, it becomes a superposition of a state, amplitude-shifted due to the quantum nature of light, and the initial state of the laser. This superposition interpolates between a Schrodinger kitten, and a genuine Schrodinger cat state. This work opens new paths for ground-breaking investigations in strong laser-field physics and quantum technology. We dedicate the work to the memory of Roy J. Glauber, the inventor of coherent states.
Coherent control is at the core of contemporary quantum physics. Unclassical states of matter, known as "superposition states" are the main building block of modern day experiments. They are at the basis of quantum information, known as Q-bits, they directly regulate the conversion efficiency in nonlinear harmonic waves in the field of nonlinear optics, they improve the precision of atomic clocks etc. The practical need of good and stable control over these states of matter dictate the need of novel generation protocols, which are robust towards unwanted interactions with the environment and imperfections in the control parameters. We investigate the simplest effects of the system-environment interaction and also provide novel robust superposition generation protocols. An emblematic sub-field on nonlinear optics is the nonlinear microscopy field. This type of microscopy has a tremendous success where standard linear microscopy fails, that is in multilayer samples with similar refractive indecies. We investigate contrast improving techniques for nonlinear microscopy based on suppression of the background signal and enhancement of the signal from the sample.