On the role of the optical phase and quantum coherence in high harmonic generation

In this work we analyze the role of the optical phase and coherence of the driving field in the process of high harmonic generation. We consider driving the process of high harmonic generation with incoherent classical and non-classical intense light fields, and show that harmonic radiation can be generated even in cases where the phase of the driving field is completely undetermined leading to vanishing mean electric field values. This implies that quantum optical coherence in the driving field is not necessary for generating high harmonic radiation, with the consequence that the emitted harmonic radiation in those cases do likewise not exhibit quantum optical coherence. We further show that the final quantum state of each harmonic is diagonal in the photon number basis, from which we conclude that the measurement of the high harmonic spectrum alone does not allow to infer on the coherence properties of the harmonic radiation.Comment: 5 page

Theory of entanglement and measurement in high harmonic generation

Quantum information science and intense laser matter interaction are two apparently unrelated fields. However, the recent developments of the quantum optical description of the intense laser driven process of high harmonic generation allow to conceive new light engineering protocols. Here, we introduce the notion of quantum information theory to intense laser driven processes by providing the quantum mechanical description of measurement protocols for high harmonic generation in atoms. We explicitly evaluate conditioning experiments on individual optical field modes, and provide the corresponding quantum operation for coherent states. The associated positive operator-valued measures are obtained, and give rise to the quantum theory of measurement for the generation of high dimensional entangled states, and coherent state superposition with controllable non-classical features on the attosecond timescale.Comment: 6 page

On the limitations of the semi-classical picture in high harmonic generation

The recent progress in the quantum optical formulation of the process of high harmonic generation has reached a point where the successful semi-classical model shows its limitations. Until recently the light source which drives the process was considered to be provided by a laser, in agreement with the classical picture. However, quantum optics allows to consider light fields beyond the classical realm, such as bright squeezed vacuum or Fock states. Both field states have vanishing electric field amplitudes, but can still lead to the generation of high harmonic radiation for sufficiently high intensities. This poses new questions about the range of validity of the semi-classical picture, which is the matter discussed here.Comment: 3 page

Metrological robustness of high photon number optical cat states

In the domain of quantum metrology cat states have demonstrated their utility despite their inherent fragility with respect to photon loss. Here, we introduce noise robust optical cat states which exhibit a metrological robustness for phase estimation in the regime of high photon numbers. These cat states are obtained from the intense laser driven process of high harmonic generation (HHG), and in the ideal case of vanishing losses, show almost twice the quantum Fisher information (QFI) compared to the even and odd cat states. However, and more importantly, these HHG-cat states are much more robust against noise such that the noisy HHG-cat outperforms the pure even/odd cat states even in the presence of more than $25\%$ losses in the regime of high photon numbers. Furthermore, in the regime of small losses, the HHG-cat remains almost pure while the even/odd cat state counterpart already decohere to the maximally mixed state. This demonstrates that high photon number optical cat states can indeed be used for metrological applications even in the presence of losses.Comment: 4 pages (3 figures

Molecular movie of ultrafast coherent rotational dynamics of OCS

Recording molecular movies on ultrafast timescales has been a longstanding goal for unravelling detailed information about molecular dynamics. Here we present the direct experimental recording of very-high-resolution and -fidelity molecular movies over more than one-and-a-half periods of the laser-induced rotational dynamics of carbonylsulfide (OCS) molecules. Utilising the combination of single quantum-state selection and an optimised two-pulse sequence to create a tailored rotational wavepacket, an unprecedented degree of field-free alignment, 〈cos2θ2D〉 = 0.96 (〈cos2θ〉 = 0.94) is achieved, exceeding the theoretical limit for single-pulse alignment. The very rich experimentally observed quantum dynamics is fully recovered by the angular probability distribution obtained from solutions of the time-dependent Schrödinger equation with parameters refined against the experiment. The populations and phases of rotational states in the retrieved time-dependent three-dimensional wavepacket rationalises the observed very high degree of alignment

Strong laser fields and their power to generate controllable high-photon-number coherent-state superpositions

Recently, intensely driven laser-matter interactions have been used to connect the fields of strong laser field physics with quantum optics by generating non-classical states of light. Here, we make a further key step and show the potential of strong laser fields for generating controllable high-photon-number coherent-state superpositions. This has been achieved by using two of the most prominent strong-laser induced processes: high-harmonic generation and above-threshold ionization. We show how the obtained coherent-state superpositions change from an optical Schr\"odinger "cat" state to a "kitten" state by changing the atomic density in the laser-atom interaction region, and we demonstrate the generation of a 9-photon shifted optical "cat" state which, to our knowledge, is the highest photon number optical "cat" state experimentally reported. Our findings anticipate the development of new methods that naturally lead to the creation of high-photon-number controllable coherent-state superpositions, advancing investigations in quantum technology.Comment: Revised version submitted to Physical Review

Analog simulation of high harmonic generation in atoms

The demanding experimental access to the ultrafast dynamics of materials challenges our understanding of their electronic response to applied strong laser fields. In this work, we show that trapped ultracold atoms with highly controllable potentials can become an enabling tool to describe phenomena in a scenario where some effects are more easily accessible and twelve orders of magnitude slower. For this purpose, we characterize the mapping between the attoscience platform and atomic simulators, and propose an experimental protocol to simulate the emission yield of High Harmonic Generation, a regime that has so far been elusive to cold atom simulation. As we illustrate, the benchmark offered by these simulators can provide new insights on the conversion efficiency of extended and short nuclear potentials, as well as the response to applied elliptical polarized fields or ultrashort few-cycle pulses

Entanglement and squeezing of the optical field modes in high harmonic generation

Squeezing of optical fields, used as a powerful resource for many applications, and the radiation properties in the process of high harmonic generation have thus far been considered separately. In this Letter, we want to clarify that the joint quantum state of all the optical field modes in the process of high harmonic generation is in general entangled and squeezed. We show that this is already the case in the simplest scenario of driving uncorrelated atoms by a classical laser light field. The previous observation of product coherent states after the high harmonic generation process is a consequence of the assumption that the ground state depletion can be neglected, which is related to vanishing dipole moment correlations. Furthermore, we analyze how the resulting quadrature squeezing in the fundamental laser mode after the interaction can be controlled and explicitly show that all field modes are entangled.Comment: 4 pages (2 figures

Entanglement and non-classical states of light in a strong-laser driven solid-state system

The development of sources delivering non-classical states of light is one of the main needs for applications of optical quantum information science. Here, we demonstrate the generation of non-classical states of light using strong-laser fields driving a solid-state system, by using the process of high-order harmonic generation, where an electron tunnels out of the parent site and, later on, recombines on it emitting high-order harmonic radiation, at the expense of affecting the driving laser field. Since in solid-state systems the recombination of the electron can be delocalized along the material, the final state of the electron determines how the electromagnetic field gets affected because of the laser-matter interaction, leading to the generation of entanglement between the electron and the field. These features can be enhanced by applying conditioning operations, i.e., quantum operations based on the measurement of high-harmonic radiation. We study non-classical features present in the final quantum optical state, and characterize the amount of entanglement between the light and the electrons in the solid. The work sets the foundation for the development of compact solid-state-based non-classical light sources using strong-field physics.Comment: We present a different formulation to that of the previous version, more in line with the approach followed in our previous works. 12 pages (8 main text + 4 Methods), 4 figures. Comments are welcom