Abstract

Quantum chemistry calculations such as the prediction of molecular properties and modeling of chemical reactions are a few of the critical areas where near-term quantum computers can showcase quantum advantage. We present a method to calculate energy derivatives for both ground state and excited state energies with respect to the parameters of a chemical system based on the framework of the variational quantum eigensolver (VQE). A low-depth implementation of quantum circuits within the hybrid variational paradigm is designed, and their computational costs are analyzed. We showcase the effectiveness of our method by incorporating it in some key quantum chemistry applications of energy derivatives, such as to perform minimum energy configuration search and estimate molecular response properties estimation of H2 molecule, and also to find the transition state of H2 + H ↔ H + H2 reaction. The obtained results are shown to be in complete agreement with their respective full configuration interaction (FCI) values. Keywords: Quantum Computing, Quantum Chemistry, Variational Quantum Eigensolver, Hybrid QuantumClassical Algorithms

Abstract

Near-term quantum hardware can support two-qubit operations only on the qubits that can interact with each other. Therefore, to execute an arbitrary quantum circuit on the hardware, compilers have to frst perform the task of qubit routing, i.e., to transform the quantum circuit either by inserting additional SWAP gates or by reversing existing CNOT gates to satisfy the connectivity constraints of the target topology. The depth of the transformed quantum circuits is minimized by utilizing the Monte Carlo tree search (MCTS) to perform qubit routing by making it both construct each action and search over the space of all actions. It is aided in performing these tasks by a Graph neural network that evaluates the value function and action probabilities for each state. Along with this, we propose a new method of adding mutex-lock like variables in our state representation which helps factor in the parallelization of the scheduled operations, thereby pruning the depth of the output circuit. Overall, our procedure (referred to as QRoute) performs qubit routing in a hardware agnostic manner, and it outperforms other available qubit routing implementations on various circuit benchmarks.

Abstract

Machine learning (ML) has recently facilitated many advances in solving problems related to many-body physical systems. Given the intrinsic quantum nature of these problems, it is natural to speculate that quantum-enhanced machine learning will enable us to unveil even greater details than we currently have. With this motivation, this paper examines a quantum machine learning approach based on shallow variational ansatz inspired by tensor networks for supervised learning tasks. In particular, we first look at the standard image classification tasks using the FashionMNIST dataset and study the effect of repeating tensor network layers on ansatz’s expressibility and performance. Finally, we use this strategy to tackle the problem of quantum phase recognition for the transverse-field Ising and Heisenberg spin models in one and two dimensions, where we were able to reach ≥ 98% test-set accuracies with both multi-scale entanglement renormalization ansatz (MERA) and tree tensor network (TTN) inspired parametrized quantum circuits. Keywords: Quantum Computing, Quantum Machine Learning, Quantum Many-body Systems, QuantumClassical Algorithms

Abstract

Synaptic integration is a prominent aspect of neuronal information processing. The detailed mechanisms that modulate synaptic inputs determine the computational properties of any given neuron. We study a simple model for the summation of excitatory inputs from synapses and illustrate its use by characterizing some functional properties of postsynaptic neurons. In this regard, we study the response of postsynaptic neurons as defined by the model to two well known noise driven processes: stochastic and coherence resonance. The model requires a small number of parameters and is especially useful to isolate the role of integration mechanisms that rely on summation of inputs with little dendritic processing.

Abstract

Density functional theory (DFT) based calculations are carried out to study effect of molecular charge on both the binding energy as well as chemical Raman enhancement of deprotonated 4-Mercaptopyridine (ligand) bound to the semiconducting nanocluster, Zn3Se3 and metal substituted nanoclusters, Zn2MSe3 (M: Ag, Cu). Change of molecular charge from 0 to −1 increases binding energy of the ligand for Zn3Se3 cluster and decreases Raman activity of vibrational modes. On the other hand, the ligand bound to the metal substituted nanoclusters, Zn2MSe3 shows minimal decrease in binding energy and a noteworthy enhancement in Raman activity of the vibrational modes, on varying the molecular charge from 0 to −1. Static Raman activities of vibrational modes are analyzed using change of molecular polarizability components with normal coordinates of the modes. An APT (Atomic Polar Tensor) charge based descriptor is used for characterization of the chemical Raman enhancement. The descriptor measures substrate induced charge concentration per unit displacement of an atom involved in a mode of vibration. We also show that significantly enhanced intensity of vibrational modes is associated with atoms of relatively greater magnitudes of the APT charge based descriptor

Abstract

We report photo-isomerization of malonaldehyde in its electronic ground state (S0), mediated by coupled 1nπ∗ (S1) - 1ππ∗ 10 (S2) electronic excited states, accomplished with the aid of optimally 11 designed ultraviolet (UV)-laser pulses. In particular, the control of H-transfer from a configuration 12 predominantly located in the left well (say, reactant) to that in the right well (say, product) of the 13 electronic ground S0 potential energy surface is achieved by a pump-dump mechanism including 14 the nonadiabatic interactions between the excited S1 and S2 states. An interplay between the 15 nonadiabatic coupling due to conical intersection of the S1 and S2 states and the laser-molecule 16 interaction is found to be imprinted in the time-dependent electronic population. The latter is also 17 examined by employing optimal fields of varying intensity and frequency of the UV laser pulses. 18 For the purpose of the present study, we constructed a three-states and two-modes coupled diabatic 19 Hamiltonian with the help of adiabatic electronic energies and transition dipole moments calculated 20 by ab initio quantum chemistry methods. The electronic diabatic model is developed using the 21 calculated adiabatic energies of the two excited electronic states (S1 and S2) in order to carry out the 22 dynamics study. The optimal fields for achieving the controlled isomerization are designed within 23 the framework of optimal control theory employing the optimization technique of a multitarget 24 functional using the genetic algorithm. The laser-driven dynamics of the system is treated by 25 numerically solving the time-dependent Schr¨odinger equation within the dipole approximation. 26 Time-averaged yield of the target prod

Abstract

Optimal initiation of quantum dynamics of N-H photodissociation of pyrrole on the S0- 1πσ∗( 1A2) coupled electronic states by UV-laser pulses in an effort to guide the subsequent dynamics to dissociation limits is studied theoretically. Specifically, the task of designing optimal laser pulses that act on initial vibrational states of the system for an effective UV-photodissociation is considered by employing optimal control theory. The associated control mechanism(s) for the initial state dependent photodissociation dynamics of pyrrole in the presence of control pulses is examined and discussed in detail. The initial conditions determine implicitly the variation in the dissociation probabilities for the two channels, upon interaction with the field. The optimal pulse corresponds to the objective fixed as maximization of overall reactive flux subject to constraints of reasonable fluence and quantum dynamics. The simple optimal pulses obtained by the use of genetic algorithm based optimization are worth an experimental implementation given the experimental relevance of πσ∗-photochemistry in recent times.

Abstract

A first principles quantum dynamics study of N–H photodissociation of pyrrole on the coupled electronic states is carried out with the aid of an optimally designed UV-laser pulse. A new photodissociation path, as compared to the conventional barrier crossing on the state, opens up upon electronic transitions under the influence of pump-dump laser pulses, which efficiently populate both the dissociation channels. The interplay of electronic transitions due both to vibronic coupling and the laser pulse is observed in the control mechanism and discussed in detail. The proposed control mechanism seems to be robust, and not discussed in the literature so far, and is expected to trigger future experiments on the photochemistry of molecules of chemical and biological importance. The design of the optimal pulses and their application to enhance the overall dissociation probability is carried out within the framework of optimal control theory. The quantum dynamics of the system in the presence of pulse is treated by solving the time-dependent Schr ̈ odinger equation in the semi-classical dipole approximation

Abstract

We will present our work on controlling nuclear and electronic motion in molecules using laser pulses designed using Optimal control theory (OCT). The control problem is formulated as the maximization of cost functional made of an objective function along with the constraints on the field fluence. The objective function has the prescribed dynamical goals. A detailed analysis of the optimal pulse reveals complex path-ways to arrive at seemingly simple goals. We have used OCT to obtain infrared laser pulses for the selective vibrational excitation of several different systems, from a simple diatomic to triatomic systems located in complex molecular environments. Controlled initiation of intramolecular H-transfer in malonaldehyde using ultrashort uv-laser pulses through an optically bright electronic excited state as a mediator will be discussed. Finally, our attempts to control the quantum dynamics of N-H photodissociation of pyrrole via a counterintuitive coupling of electronic states using uv-laser pulses will be discussed.

Abstract

DNA molecules tagged to metal nanoparticles, especially gold nanoparticles (AuNPs), have been shown to have potential applications in the design and fabrication of novel electronic nano-devices, but the binding mechanism between gold nanoparticles and DNA bases and its implications are not completely understood. In this work, a comprehensive study to examine the effect of structural perturbations caused to DNA base pairs in terms of size expansion and adsorption on a gold cluster (Au3) has been carried out using density functional theory. The geometric and electronic features of these complexes provide evidence for the distortion of certain base pairs depending on the binding site of the cluster. This is further substantiated via normal mode, natural bond orbital (NBO) and atoms in molecules (AIM) analyses. The natural population analysis (NPA) and NBO analysis indicate that complexation greatly affects …