Identifying challenges towards practical quantum advantage through resource estimation: the measurement roadblock in the variational quantum eigensolver

Recent advances in Noisy Intermediate-Scale Quantum (NISQ) devices have brought much attention to the potential of the Variational Quantum Eigensolver (VQE) and related techniques to provide practical quantum advantage in computational chemistry. However, it is not yet clear whether such algorithms, even in the absence of device error, could achieve quantum advantage for systems of practical interest and how large such an advantage might be. To address these questions, we have performed an exhaustive set of benchmarks to estimate number of qubits and number of measurements required to compute the combustion energies of small organic molecules to within chemical accuracy using VQE as well as state-of-the-art classical algorithms. We consider several key modifications to VQE, including the use of Frozen Natural Orbitals, various Hamiltonian decomposition techniques, and the application of fermionic marginal constraints. Our results indicate that although Frozen Natural Orbitals and low-rank factorizations of the Hamiltonian significantly reduce the qubit and measurement requirements, these techniques are not sufficient to achieve practical quantum computational advantage in the calculation of organic molecule combustion energies. This suggests that new approaches to estimation leveraging quantum coherence, such as Bayesian amplitude estimation [arxiv:2006.09350, arxiv:2006.09349], may be required in order to achieve practical quantum advantage with near-term devices. Our work also highlights the crucial role that resource and performance assessments of quantum algorithms play in identifying quantum advantage and guiding quantum algorithm design.Comment: 27 pages, 18 figure

Fieldlike and antidamping spin-orbit torques in as-grown and annealed Ta/CoFeB/MgO layers

We present a comprehensive study of the current-induced spin-orbit torques in perpendicularly magnetized Ta/CoFeB/MgO layers. The samples were annealed in steps up to 300 degrees C and characterized using x-ray absorption spectroscopy, transmission electron microscopy, resistivity, and Hall effect measurements. By performing adiabatic harmonic Hall voltage measurements, we show that the transverse (field-like) and longitudinal (antidamping-like) spin-orbit torques are composed of constant and magnetization-dependent contributions, both of which vary strongly with annealing. Such variations correlate with changes of the saturation magnetization and magnetic anisotropy and are assigned to chemical and structural modifications of the layers. The relative variation of the constant and anisotropic torque terms as a function of annealing temperature is opposite for the field-like and antidamping torques. Measurements of the switching probability using sub-{\mu}s current pulses show that the critical current increases with the magnetic anisotropy of the layers, whereas the switching efficiency, measured as the ratio of magnetic anisotropy energy and pulse energy, decreases. The optimal annealing temperature to achieve maximum magnetic anisotropy, saturation magnetization, and switching efficiency is determined to be between 240 degrees and 270 degrees C

De novo prediction of the ground state structure of transition metal complexes.

One of the main goals of computational methods is to identify reasonable geometries for target materials. Organometallic complexes have been investigated in this dissertation research, entailing a significant challenge based on transition metal diversity and the associated complexity of the ligands. A large variety of theoretical methods have been employed to determine ground state geometries of organometallic species. An impressive number of transition metals entailing diverse isomers (e.g., geometric, spin, structural and coordination), different coordination numbers, oxidation states and various numbers of electrons in d orbitals have been studied. Moreover, ligands that are single, double or triple bonded to the transition metal, exhibiting diverse electronic and steric effects, have been investigated. In this research, a novel de novo scheme for structural prediction of transition metal complexes was developed, tested and shown to be successful

Stability Studies of Transition-Metal Linkage Isomers Using Quantum Mechanical Methods, Groups 11 and 12 Transition Metals

Article discussing the stability studies of transition-metal linkage isomers using quantum mechanical methods and groups 11 and 12 transition metals

Low-Coordinate Chromium Siloxides: The "Box" [Cr(μ-OSitBu3)]4, Distorted Trigonal [(tBu3SiO)3Cr] [Na(benzene)] and [(tBu3SiO)3Cr] [Na(dibenzo-18-c-6)], and Trigonal (tBu3SiO)3Cr

Article discussing low-coordinate chromium siloxides and the "box" [Cr(μ-Cl)(μ-OSiᵗBu₃)]₄, distorted trigonal [(ᵗBu₃SiO)₃Cr][Na(benzene)] and [(ᵗBu₃SiO)₃Cr][Na(dibenzo-18-c-6)], and trigonal (ᵗBu₃SiO)₃Cr

Consequences of Metal–Oxide Interconversion for C–H Bond Activation during CH₄ Reactions on Pd Catalysts

Mechanistic assessments based on kinetic and isotopic methods combined with density functional theory are used to probe the diverse pathways by which C–H bonds in CH₄ react on bare Pd clusters, Pd cluster surfaces saturated with chemisorbed oxygen (O*), and PdO clusters. C–H activation routes change from oxidative addition to H-abstraction and then to σ-bond metathesis with increasing O-content, as active sites evolve from metal atom pairs (*–*) to oxygen atom (O*–O*) pairs and ultimately to Pd cation-lattice oxygen pairs (Pd²⁺–O^2–) in PdO. The charges in the CH₃ and H moieties along the reaction coordinate depend on the accessibility and chemical state of the Pd and O centers involved. Homolytic C–H dissociation prevails on bare (*–*) and O*-covered surfaces (O*–O*), while C–H bonds cleave heterolytically on Pd²⁺–O^2– pairs at PdO surfaces. On bare surfaces, C–H bonds cleave via oxidative addition, involving Pd atom insertion into the C–H bond with electron backdonation from Pd to C–H antibonding states and the formation of tight three-center (H₃C···Pd···H)^⧧ transition states. On O*-saturated Pd surfaces, C–H bonds cleave homolytically on O*–O* pairs to form radical-like CH₃ species and nearly formed O–H bonds at a transition state (O*···CH₃^•···*OH)^⧧ that is looser and higher in enthalpy than on bare Pd surfaces. On PdO surfaces, site pairs consisting of exposed Pd²⁺ and vicinal O^2–, Pd_ox–O_ox , cleave C–H bonds heterolytically via σ-bond metathesis, with Pd²⁺ adding to the C–H bond, while O^2– abstracts the H-atom to form a four-center (H₃C^δ−···Pd_ox···H^δ+···O_ox)^⧧ transition state without detectable Pd_ox reduction. The latter is much more stable than transition states on *–* and O*–O* pairs and give rise to a large increase in CH₄ oxidation turnover rates at oxygen chemical potentials leading to Pd to PdO transitions. These distinct mechanistic pathways for C–H bond activation, inferred from theory and experiment, resemble those prevalent on organometallic complexes. Metal centers present on surfaces as well as in homogeneous complexes act as both nucleophile and electrophile in oxidative additions, ligands (e.g., O* on surfaces) abstract H-atoms via reductive deprotonation of C–H bonds, and metal–ligand pairs, with the pair as electrophile and the metal as nucleophile, mediate σ-bond metathesis pathways

3-Center-4-Electron Bonding in [(silox)2Mo=NtBu]2(μ-Hg) Controls Reactivity while Frontier Orbitals Permit a Dimolybdenum π-Bond Energy Estimate

Article describing research on 3-center-4-electron bonding in [(silox)2Mo=NtBu]2(mu-Hg)

Diffusion Mechanisms and Preferential Dynamics of Promoter Molecules in ZSM-5 Zeolite

The diffusion in ZSM-5 zeolite of methanol and of two series of promoters of the methanol to dimethyl ether reaction (linear methyl esters, benzaldehyde, 4-n-alkyl benzaldehydes) has been studied using classical molecular dynamics in the NVT ensemble. Whereas promoter diffusion coefficients decrease with increasing alkyl chain length in methyl esters, the aromatic aldehyde promoters all have similar diffusion coefficients. The lowest diffusion coefficient is that of benzaldehyde. All the promoters exhibit a preference for moving in the straight pore, a preference that is most pronounced for the 4-n-alkyl benzaldehydes and least for the longest aliphatic esters. A novel diffusion mechanism, a molecular ’3-point turn’, is observed. The diffusion coefficient of methanol is larger than that of all the promoters. The more catalytically active aromatic aldehyde promoters limit methanol diffusion less than the aliphatic esters