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Quantum Spectroscopy: exploring new states of matter out of equilibrium

Periodic Reporting for period 3 - QSpec-NewMat (Quantum Spectroscopy: exploring new states of matter out of equilibrium)

Reporting period: 2019-10-01 to 2021-03-31

The objective of this project is to discover the general principles that describe and control quantum systems far from equilibrium and orchestrate the behavior of many electrons and atoms to create new phenomena/states of matter. We are investigating electronic excited states, and all phenomena that stem from them - from molecules to solids. The long-term goal is to develop, implement and provide a novel theoretical and computational toolbox within the in-house theoretical framework of quantum electrodynamical density functional theory (QEDFT) applicable to complex materials. In short, we want

1. To establish, develop and implement the “first principles non-equilibrium QEDFT toolbox”.
2. To demonstrate new light induced “non-equilibrium states of matter” that have no equilibrium counterpart and identify the spectroscopic fingerprints of those new states.
3. To investigate atoms and molecules with quantum optical fields. Which novel states arise in the strong light-matter coupling regime?
4. Investigate how to imprint photon correlations into matter. What are the implications for photoactive devices for energy conversion/storage?
5. To investigate a new class of polariton condensates in materials and the demonstration of lasing of entangled photons.
6. To establish a new coherent quantum-control scheme within QEDFT: control of molecular chemistry (e.g. nuclear dynamics, entanglement between reactants, intersystem crossings).
7. Describe quantum coherent time-resolved spectroscopy. We will explore how to generate a condensate of correlated photons (e.g. laser of entangled photons).
8. Develop a framework for the simulation of non-adiabatic dynamics of molecules driven far from equilibrium by external electromagnetic fields.
9. To explore the possibility to construct more accurate xc-approximations for conventional DFT by directly use the fact that photons mediate the interaction between particles in QED.

In order to fulfill the ambitious goals of the project we need an enormous investment in advancing the available and in developing new methodologies. These methodologies can be divided in two types, one on the fundamentals of QEDFT and closely related theoretical frameworks and the second on the development of a novel framework to describe non-adiabatic dynamics of systems driven far from equilibrium by external field. Examples of the processes we want to address include exciton-exciton interaction, quantum coherence-assisted energy and charge transport, photo-chemistry and non-adiabatic dynamics, novel dynamical-topological phases of interacting with many body systems, new non-equilibrium states of matter, etc. Those works will set-up the ground for the development of two new fields of research QED-Chemistry and QED-materials that will have implications in the development of new and more efficient materials and novel technologies for quantum information processing and energy-materials of relevance to our society. In here, we will provide new tools that allow us to gather a new understanding of materials out of equilibrium for technologically relevant applications in light harvesting and light-driven phenomena.

The comprehensive understanding of this phenomena will enable its application on everyday processes, just to name a few:
• the manipulation of chemical properties via photons, namely the improvement in efficiency of standard chemical reactions by strong coupling to cavity photons
• ultimate control of the properties of materials and molecules at the nanoscale, enabling new ultrafast electronics for superfast computers in the future.
• the establishment of new spectroscopic techniques to study quantum materials such as strongly correlated materials, topological insulators and magnetic materials
• new facets to our modern views on material and reaction design that will provide with efficient tools to tackle modern-day problems such as energy harvesting and capturing

The tools developed within this project are immediately made available to the entire scientific community through the Octopus project, allowing its members to address fundamental questions in energy transfer phenomena in realistic materials.
In the course of the 54 months of the project, we’ve been able to further develop our theoretical framework and have gained insight in many of the process and interactions as stated above. Following you find a summary on our most relevant findings:
• It is known that the chemical properties of atoms and molecules are determined by the electromagnetic interaction between their negatively charged electrons and the positively
charged nuclei. Usually the quantum nature of light does not play an important role in this state. However, upon placing a molecule between two strongly reflecting mirrors, in a so-called optical cavity, single photons can interact unusually strongly with the molecule, and one can no longer distinguish between molecule and photons. The properties of this new state of matter can be very different to the bare molecule. This phenomenon has already been observed in experiments, but theoretical predictions of the chemical properties of such states were possible only to a limited extend. Common quantum-chemical methods do not take into account the quantum nature of light. We have successfully extended some of these methods to include the coupling to the photons, showing among others how strong coupling to photons in an optical cavity changes chemical properties of molecules, like its bond length or its absorption. In a next step we want to apply their developed theoretical methods to more complex molecules.
• In the standard model of particle physics, the fundamental particles that make up all matter around us – electrons and quarks – are so-called fermions. Quantum theory predicts that elementary fermions could exist as three different kinds: Dirac, Weyl, and Majorana fermions. So far only Dirac fermions had been observed as elementary particles in nature. Thanks to the discovery of graphene, the existence of Weyl fermions was first verified in recent years. While any known material only hosts one kind of these fermions in its equilibrium state, we’ve demonstrated how one can transform the fermion nature within specific materials by using tailored light pulses. Dirac and Weyl fermions differ by their chirality. Just like our left and right hands, Weyl fermions occur in pairs, where one particle is a mirrored version of the other. The two partners are almost identical, yet they cannot be superimposed. Dirac fermions, by contrast, do not have this property. By means of the Floquet theory and using high-level computational simulations of material properties we’ve shown how optical transformation via laser-driven systems can change the topology of a material, i.e. its chirality, changing from Dirac fermions to Weyl, in a real three- dimensional material.
We have demonstrated that the long-sought magnetic Weyl semi-metallic state can be induced by ultrafast laser pulses in a three-dimensional class of magnetic materials dubbed pyrochlore iridates. These results could enable high-speed magneto-optical topological switching devices for next-generation electronics.
• The generation of high-order harmonics in gases is nowadays routinely used in many different areas of sciences, ranging from physics, to chemistry and biology. Despite the growing interest in this phenomenon in solids, the mechanism behind the conversion of light is still under debate for solid materials. Together with our collaborators at CFEL, we have demonstrated the possibility of using a new knob to control and optimize the generation of high-order harmonics in bulk materials. Using extensive first-principles simulations, we’ve researched how two-dimensional crystals react to a strong electric field polarized perpendicularly to the crystal, demonstrating that two-dimensional materials can generate high-order harmonics with the same mechanism as atoms and molecules. This makes two-dimensional crystals an attractive alternative to atoms and molecules for the generation of high-energy photons and ultrashort light pulses, which offer more control possibilities than gases. Our latest work elucidates how high-order harmonics from solids with controlled polarization states can be created, taking advantage of both crystal symmetry and attosecond electronic dynamics. The new technique might find intriguing applications in petahertz electronics and for spectroscopic studies of novel quantum materials
• Microscopically, materials consist of electrons and nuclei. The shape and magnitude of the nuclei vibrations, which is specifically called a phonon, are a major factor determining the material’s properties besides the charge and spin of the electrons. We’ve demonstrated that phonons can be used to induce and control magnetic responses in non-magnetic two-dimensional layers of transition metal dichalcogenides.
• Using state-of-the-art quantum dynamical methods to calculate the quantum of spin or charge Hall conductivity, we’ve successfully classified the intrinsic topological natures of insulators. We found that the entire classification of these materials can be achieved by their conductivity itself instead of the mathematically devised topological number. By calculating the time-dependent quantum mechanical equation, we were able to numerically quantify the electrons’ velocity, which summation is to be quantized and the detailed intrinsic quantum mechanical structure can be labelled by those quantum numbers. In short, we proposed to use physical quantity, less contaminated by mathematical notion, to characterize the materials’ properties, which provide a more direct link to experimental measurements.
• Through computer simulations we’ve shown that the transfer of energy and charge between molecules can be drastically enhanced and controlled with virtual photons. For example, placing two ordinary mirrors close to each other and changing their distance can alter the course of chemical reactions occurring in between them, not because the molecules interact with the mirrors’ surfaces but solely because the mirrors force the vacuum to behave in a specific way “controlled vacuum”. Experiments and theoretical calculations prove that those seemingly incredible effects exist and that they can present a vital control-mechanism. Control over the vacuum then provides control over chemical reactions, lets particles communicate with each other highly efficiently over long distances (‘spooky interactions’), and enforces their positions.
QSpec-NewMat aims to develop the theoretical and computational framework that will enable the first principles quantum simulations of the joint electronic and ionic dynamics of atomic, molecular or extended systems coupled to quantized electromagnetic field and thereby set the stage for a new era in the first-principle computational modelling of light-matter interactions. This will enable reaching the ultimate goal of probing, following, and controlling/manipulating molecular complex systems. In QSpec-NewMat we are bringing together the recent advances in electronic structure, quantum optics and the nonlinear spectroscopy of many-body systems to study the complex dynamics in quantum systems beyond classical limits and explore the fundamentals of the light-matter interactions of molecular systems that are embedded in nanostructured surroundings, such as interfaces, cavities, or metamaterials, both in weak and strong-coupling regimes.
The expected results can be highlighted into the following novel and unique concepts:
• QED-chemistry: “changing and controlling molecular chemistry”, For example, entanglement between reactants, intersystem crossings.
• QED-materials: Discovery light-induced novel states of matter: Floquet and Quantum-cavity engineering of materials
• How to control/modify decoherence/dissipation? Under what conditions can we design long-lived coherent states for the coupled photon-matter system?
• Using photon orbital angular momentum, photon spin and multi-photon correlations to imprint novel correlations onto molecular systems
• Generate a condensate of entangled photons
• Nanoplasmonics: look for chiral-plasmon- exciton and new class of quantum Hall states, etc.
As it is clear from the description of the scientific achievements above, we have already achieved fundamental contributions in all those points already in the first half of the project.
Interaction of a laser field (red) with a crystalline solid (white) @Harms/MPSD
Schematic of energy transfer between two molecules outside and inside a controlled vacuum @Harms/MPD
Strong coupling of light-vibrational waves to electrons in a 2D atomically thin material @Harms/MPSD