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1. From Coherence to Function: Exploring the Connection in Chemical Systems

   https://doi.org/10.1021/acs.accounts.4c00312  

   Rather, Shahnawaz R; Scholes, Gregory D; Chen, Lin X (September 2024, Accounts of Chemical Research)

   Conspectus: The role of quantum mechanical coherences or coherent superposition states in excited state processes has received considerable attention in the last two decades largely due to advancements in ultrafast laser spectroscopy. These coherence effects hold promise for enhancing the efficiency and robustness of functionally relevant processes, even when confronted with strong energy disorder and environmental fluctuations. Understanding coherence deeply drives us to unravel mechanisms and dynamics controlled by order and synchronization at a quantum mechanical level, envisioning optical control of coherence to enhance functions or create new ones in molecular and material systems. In this frontier, the interplay between electronic and vibrational dynamics, specifically the influence of vibrations in directing electronic dynamics, has emerged as the leading principle. Here, two energetically disparate quantum degrees of freedom work in-sync to dictate the trajectory of an excited state reaction. Moreover, with the vibrational degree being directly related to the structural composition of molecular or material systems, new molecular designs could be inspired by tailoring certain structural elements. In the realm of chemical kinetics, our understanding of the dynamics of chemical transformations is underpinned by fundamental theories such as transition state theory, activated rate theory, and Marcus theory. These theories elucidate reaction rates by considering the energy barriers that must be overcome for reactants to transform into products. Those barriers are surmounted by the stochastic nature of energy gap fluctuations within reacting systems, emphasizing that the reaction coordinate—the pathway from reactants to products—is not rigidly defined by a specific vibrational motion but encompasses a diverse array of molecular motions. While less is known about the involvement of specific intramolecular vibrational modes, their significance in certain cases cannot be overlooked. In this Account, we summarize key experimental findings that offer deeper insights into the complex electronic-vibrational trajectories encompassing excited states afforded from state-of-the-art ultrafast laser spectroscopy in three exemplary processes: photo-induced electron transfer, singlet-triplet intersystem crossing, and intramolecular vibrational energy flow in molecular systems. We delve into rapid decoherence—loss of phase and amplitude correlations—of vibrational coherences along promoter vibrations during a sub-picosecond intersystem crossing dynamics in a series of binuclear platinum complexes. This rapid decoherence illustrates the vibration-driven reactive pathways from Franck-Condon state to the curve crossing region. We also explore the generation of new vibrational coherences induced by impulsive reaction dynamics—rather than by the laser pulse—in these systems, which sheds light on specific energy dissipation pathways and thereby on the progression of the reaction trajectory in the vicinity of the curve crossing on the product side. Another property of vibrational coherences, amplitude, reveals how energy can flow from one vibration to another in the electronic excited state of a terpyridine-molybdenum complex hosting a nonreactive dinitrogen substrate. A slight change in vibrational energy triggers a quasi-resonant interaction, leading to constructive wavepacket interference and ultimately intramolecular vibrational redistribution from a Franck-Condon active terpyridine vibration to dinitrogen stretching vibration, energizing the dinitrogen bond. 

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2. Probing electronic decoherence with high-resolution attosecond photoelectron interferometry

   https://doi.org/10.1140/epjd/s10053-022-00438-y  

   Busto, David; Laurell, Hugo; Finkelstein-Shapiro, Daniel; Alexandridi, Christiana; Isinger, Marcus; Nandi, Saikat; Squibb, Richard J.; Turconi, Margherita; Zhong, Shiyang; Arnold, Cord L.; et al (July 2022, The European Physical Journal D)

   AbstractQuantum coherence plays a fundamental role in the study and control of ultrafast dynamics in matter. In the case of photoionization, entanglement of the photoelectron with the ion is a well-known source of decoherence when only one of the particles is measured. Here, we investigate decoherence due to entanglement of the radial and angular degrees of freedom of the photoelectron. We study two-photon ionization via the 2s2p autoionizing state in He using high spectral resolution photoelectron interferometry. Combining experiment and theory, we show that the strong dipole coupling of the 2s2p and 2p$$^2$$ ${}^2$states results in the entanglement of the angular and radial degrees of freedom. This translates, in angle-integrated measurements, into a dynamic loss of coherence during autoionization. Graphic Abstract 

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3. Quantum interference in atom-exchange reactions

   https://doi.org/10.1126/science.adl6570  

   Liu, Yi-Xiang; Zhu, Lingbang; Luke, Jeshurun; Houwman, J_J Arfor; Babin, Mark C; Hu, Ming-Guang; Ni, Kang-Kuen (June 2024, Science)

   Chemical reactions, in which bonds break and form, are highly dynamic quantum processes. A fundamental question is whether coherence can be preserved in chemical reactions and then harnessed to generate entangled products. Here we investigated this question by studying the 2KRb $\to$ ${\text{K}}_2$+ Rb₂reaction at 500 nanokelvins, focusing on the nuclear spin degrees of freedom. We prepared the initial nuclear spins in KRb (potassium-rubidium) in an entangled state by lowering the magnetic field to where the spin-spin interaction dominates and characterized the preserved coherence in nuclear spin wave function after the reaction. We observed an interference pattern that is consistent with full coherence at the end of the reaction, suggesting that entanglement prepared within the reactants could be redistributed through the atom-exchange process. 

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4. Single-Shot Readout and Weak Measurement of a Tin-Vacancy Qubit in Diamond

   https://doi.org/10.1103/PhysRevX.14.041008  

   Rosenthal, Eric I; Biswas, Souvik; Scuri, Giovanni; Lee, Hope; Stein, Abigail J; Kleidermacher, Hannah C; Grzesik, Jakob; Rugar, Alison E; Aghaeimeibodi, Shahriar; Riedel, Daniel; et al (October 2024, Physical Review X)

   The negatively charged tin-vacancy center in diamond ( ${\mathrm{SnV}}^{-}$) is an emerging platform for building the next generation of long-distance quantum networks. This is due to the ${\mathrm{SnV}}^{-}$’s favorable optical and spin properties including bright emission, insensitivity to electronic noise, and long spin coherence times at temperatures above 1 K. Here, we demonstrate measurement of a single ${\mathrm{SnV}}^{-}$electronic spin with a single-shot readout fidelity of 87.4%, which can be further improved to 98.5% by conditioning on multiple readouts. In the process, we develop understanding of the relationship between strain, magnetic field, spin readout, and microwave spin control. We show that high-fidelity readout is compatible with rapid microwave spin control, demonstrating a favorable parameter regime for use of the ${\mathrm{SnV}}^{-}$center as a high-quality spin-photon interface. Finally, we use weak quantum measurement to study measurement-induced dephasing; this illuminates the fundamental interplay between measurement and decoherence in quantum mechanics, and provides a universal method to characterize the efficiency of color-center spin readout. Taken together, these results overcome an important hurdle in the development of the ${\mathrm{SnV}}^{-}$-based quantum technologies and, in the process, develop techniques and understanding broadly applicable to the study of solid-state quantum emitters. Published by the American Physical Society2024 

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5. Low rank representations for quantum simulation of electronic structure

   https://doi.org/10.1038/s41534-021-00416-z  

   Motta, Mario; Ye, Erika; McClean, Jarrod R.; Li, Zhendong; Minnich, Austin J.; Babbush, Ryan; Chan, Garnet Kin-Lic (May 2021, npj Quantum Information)

   Abstract The quantum simulation of quantum chemistry is a promising application of quantum computers. However, forNmolecular orbitals, the$${\mathcal{O}}({N}^{4})$$ $O\left(N^4\right)$gate complexity of performing Hamiltonian and unitary Coupled Cluster Trotter steps makes simulation based on such primitives challenging. We substantially reduce the gate complexity of such primitives through a two-step low-rank factorization of the Hamiltonian and cluster operator, accompanied by truncation of small terms. Using truncations that incur errors below chemical accuracy allow one to perform Trotter steps of the arbitrary basis electronic structure Hamiltonian with$${\mathcal{O}}({N}^{3})$$ $O\left(N^3\right)$gate complexity in small simulations, which reduces to$${\mathcal{O}}({N}^{2})$$ $O\left(N^2\right)$gate complexity in the asymptotic regime; and unitary Coupled Cluster Trotter steps with$${\mathcal{O}}({N}^{3})$$ $O\left(N^3\right)$gate complexity as a function of increasing basis size for a given molecule. In the case of the Hamiltonian Trotter step, these circuits have$${\mathcal{O}}({N}^{2})$$ $O\left(N^2\right)$depth on a linearly connected array, an improvement over the$${\mathcal{O}}({N}^{3})$$ $O\left(N^3\right)$scaling assuming no truncation. As a practical example, we show that a chemically accurate Hamiltonian Trotter step for a 50 qubit molecular simulation can be carried out in the molecular orbital basis with as few as 4000 layers of parallel nearest-neighbor two-qubit gates, consisting of fewer than 10⁵non-Clifford rotations. We also apply our algorithm to iron–sulfur clusters relevant for elucidating the mode of action of metalloenzymes. 

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