Quantum Dynamics Research Articles

Dynamical maps for accelerating detectors

We study the open quantum dynamics using the language of dynamical maps of a two-level particle detector that starts accelerating through Minkowski vacuum weakly coupled to a massless scalar field. We consider a detector with non-zero size and study its time evolution for the case where it is initially in inertial motion and subsequently a constant acceleration is switched on for a finite time. We study the dynamical maps that describe the evolution of such a system and show that the dynamics is not completely positive (NCP). The inertial motion prior to the acceleration can entangle the detector and field leading to the NCP dynamics. We examine the nature of the open dynamics during the accelerated phase as a function of the duration of prior inertial motion and the magnitude of the acceleration.

Toward Efficient Modeling of Nonradiative Decay in Extended INVEST: Overcoming Computational Challenges in Quantum Dynamics Simulations.

In recent years, an increasing number of fully organic molecules capable of thermally activated delayed fluorescence (TADF) have been reported, often with very small or even inverted singlet-triplet (INVEST) energy gaps. These molecules typically exhibit complex photophysics due to the close energy levels of multiple singlet and triplet states, which create various transition pathways toward emission. A predictive model for the rates of these transitions is thus essential for assessing the suitability of new materials for light-emitting devices. Quantum Dynamics (QD) calculations are ideal for this purpose, as they include quantum effects, without the limitations of first-order perturbative approaches, also allowing taking into account more than two electronic states at once. However, the huge computational demands of QD methodologies, especially for large molecules, currently limit their use as a standard tool. To address this problem, we here employ a strategy that allows us to include almost the whole set of the vibrational coordinates by selecting the key elements of the Hilbert space that significantly impact dynamics, thereby hugely reducing the computational burden. Application of this protocol to two relatively large INVEST molecules reveals that internal conversion in these systems is very fast, making indirect emissive pathways a possible channel for the population of the S1 state. More importantly, this study demonstrates that the dynamics can be accurately described even with a significantly reduced vibrational space, thus allowing quantum dynamics calculations that yield accurate transition rates in a few minutes of computational time.

Complex stochastic optimal control foundation of quantum mechanics

Abstract Recent studies have extended the use of the stochastic Hamilton-Jacobi-Bellman (HJB) equation to include complex variables for deriving quantum mechanical equations. However, these studies often assume that it is valid to apply the HJB equation directly to complex numbers, an approach that overlooks the fundamental problem of comparing complex numbers when finding optimal controls. This paper explores the application of the HJB equation in the context of complex variables. It provides an in-depth investigation of the stochastic movement of quantum particles within the framework of stochastic optimal control theory. We obtain the complex diffusion coefficient in the stochastic equation of motion using the Cauchy-Riemann theorem, considering that the particle’s stochastic movement is described by two perfectly correlated real and imaginary stochastic processes. During the development of the covariant form of the HJB equation, we demonstrate that if the temporal stochastic increments of the two processes are perfectly correlated, then the spatial stochastic increments must be perfectly anti-correlated, and vice versa. The diffusion coefficient we derive has a form that enables the linearization of the HJB equation. The method for linearizing the HJB equation, along with the subsequent derivation of the Dirac equation, was developed in our previous work [V. Yordanov, Scientific Reports 14, 6507 (2024)]. These insights deepen our understanding of quantum dynamics and enhance the application of stochastic optimal control theory to quantum mechanics.

Uhlmann quench and geometric dynamic quantum phase transition of mixed states

Uhlmann quench and geometric dynamic quantum phase transition of mixed states

State-resolved quantum dynamics and reaction mechanisms of the Ne + HD+(v = 1) reaction

In the work, accurate converged theoretical simulations of the Ne + HD + ( v = 1 ) reaction are presented with the time-dependent wave packet (TDWP) method using the accurate potential energy surface by Lv (J. Chem. Phys. 2010, 132, 014303). In particular, the effect of intramolecular substitution and the translational collision energy on the reactive dynamics is investigated in detail to discern the reaction mechanisms. Total and state-to-state integral cross sections (ICSs) and differential cross sections (DCSs) of the two channels are derived and compared over a wide range of collision energies up to 1.2 eV. The analysis of the ICSs in terms of the total angular momentum contributions shows that the contribution of the J partial wave varies in the two reactive channels at different collision energies. The product state-resolved DCSs reveal that two opposite mechanisms co-exist in the two reactive channels with different ratios at low and high collision energies. Overall, NeH + + D channel appears to form the products via collision complex formation, which survives long enough leading to a forward–backward symmetric total DCS at low collision energies. In comparison, the NeD + + H proceeds through a direct stripping reaction, with predominantly forward scattering in the DCSs.

Digital-Analog Counterdiabatic Quantum Optimization with Trapped Ions

Abstract We introduce a hardware-specific, problem-dependent digital-analog quantum algorithm of a counterdiabatic
quantum dynamics tailored for optimization problems. Specifically, we focus on trapped-ion architectures,
taking advantage from global Mølmer-Sørensen gates as the analog interactions complemented by digital gates,
both of which are available in the state-of-the-art technologies. We show an optimal configuration of analog
blocks and digital steps leading to a substantial reduction in circuit depth compared to the purely digital
approach. This implies that, using the proposed encoding, we can address larger optimization problem instances,
requiring more qubits, while preserving the coherence time of current devices. Furthermore, we study the
minimum gate fidelity required by the analog blocks to outperform the purely digital simulation, finding that it
is below the best fidelity reported in the literature. To validate the performance of the digital-analog encoding,
we tackle the maximum independent set problem, showing that it requires fewer resources compared to the
digital case. This hybrid co-design approach paves the way towards quantum advantage for efficient solutions
of quantum optimization problems.

Dynamic quantum secret sharing with identity verification based on quantum walks

Abstract A dynamic quantum secret sharing protocol with authentication is proposed based on quantum walks. The protocol can share specific secrets and also update the secret information periodically based on the existing quantum walks structure. Participants can be added or removed while keeping the original secret unchanged. This is more flexible in practical applications. Before the secrets are shared, each participant is authenticated to ensure the legitimacy of all participants' identities. In addition, the protocol uses single photons as quantum resources, avoiding the complex preparation of entangled states, which makes the protocol more advantageous and potential in practical applications. Security analyses show that the protocol is resistant to collusion attack and other common attacks.

Collective quantum dynamics with distant quantum emitters in slow-wave nanoplasmonic waveguides

We consider a slow-wave nanoplasmonic waveguide system with spatially separated (distant) quantum emitters. Based on a nanoplasmonic waveguide quantum electrodynamic theory the emerging non-Markovian collective plasmon-polariton dynamics directly reflects the spatial positioning of the quantum emitters. A phase-space analysis allows us to distinguish between collectivity and cooperativity and the transition between these regimes. For distant emitters, temporal decoherence is reflected in anomalous phase-space evolution. In the spectral domain, collectivity emerges as a resonant single Lorentzian peak with two weak sidebands, while cooperativity manifests as a Fano-like resonance normal-mode splitting. Remarkably, even for distant quantum emitters, we achieve collective multiple quantum emitter dynamics with non-vanishing excitation and vanishing instantaneous emission, establishing an interaction-based quantum nanoplasmonic memory with key relevance in quantum nanoplasmonic networks.

Study on Quantum Dynamics of the Na + Na2 Exchanged Reaction and Lifetime Prediction of Na3 Complex Based on the Neural Network Potential Energy Surface.

A high-precision global potential energy surface (PES) is constructed for the Na3 system based on high-level ab initio calculations and the fundamental invariant neural network (FI-NN) method. The root-mean-square error (RMSE) of the PES is 2.88 cm-1. The state-resolved quantum dynamics of the ground-state Na + Na2 (v = 0, j = 0) → Na2 (v', j') + Na reaction is studied using the time-dependent wave packet (TDWP) method on the new PES. Analysis of the relevant integral cross sections revealed a complicated energy-transfer mechanism during collisions. Similarly, the characteristics of the differential cross sections indicate that the complex-forming mechanism plays a dominant role in the reaction, providing conditions for a comprehensive exploration of the lifetimes of the complexes. Based on the Rice-Ramsperger-Kassel-Marcus (RRKM) theory, the calculated lifetime of the Na3 complex is approximately 3.9 ns.

Exact quantum dynamics for two-level systems with time-dependent driving

Abstract It is well known that the time-dependent Schrrödinger equation can only be solved exactly in very rare cases, even for two-level quantum systems. Thus, finding the exact quantum dynamics under a time-dependent Hamiltonian is not only fundamentally important in quantum physics but also facilitates active quantum manipulations for quantum information processing. In this work, we present a method for generating nearly infinite analytically assisted solutions to the Schrrödinger equation for a qubit under time-dependent driving. These analytically assisted solutions feature free parameters with only boundary restrictions, making them applicable in a variety of precise quantum manipulations. Due to the general form of the time-dependent Hamiltonian in our approach, it can be readily implemented in various experimental setups involving qubits. Consequently, our scheme offers new solutions to the Schrrödinger equation, providing an alternative analytical framework for precise control over qubits.

The development and applications of multidimensional biomolecular spectroscopy illustrated by photosynthetic light harvesting.

The parallel and synergistic developments of atomic resolution structural information, new spectroscopic methods, their underpinning formalism, and the application of sophisticated theoretical methods have led to a step function change in our understanding of photosynthetic light harvesting, the process by which photosynthetic organisms collect solar energy and supply it to their reaction centers to initiate the chemistry of photosynthesis. The new spectroscopic methods, in particular multidimensional spectroscopies, have enabled a transition from recording rates of processes to focusing on mechanism. We discuss two ultrafast spectroscopies - two-dimensional electronic spectroscopy and two-dimensional electronic-vibrational spectroscopy - and illustrate their development through the lens of photosynthetic light harvesting. Both spectroscopies provide enhanced spectral resolution and, in different ways, reveal pathways of energy flow and coherent oscillations which relate to the quantum mechanical mixing of, for example, electronic excitations (excitons) and nuclear motions. The new types of information present in these spectra provoked the application of sophisticated quantum dynamical theories to describe the temporal evolution of the spectra and provide new questions for experimental investigation. While multidimensional spectroscopies have applications in many other areas of science, we feel that the investigation of photosynthetic light harvesting has had the largest influence on the development of spectroscopic and theoretical methods for the study of quantum dynamics in biology, hence the focus of this review. We conclude with key questions for the next decade of this review.

Quantum and classical dynamics correspondence and the brachistochrone problem

Quantum and classical dynamics correspondence and the brachistochrone problem

Quantum-coherence-assisted dynamical phase transition in the one-dimensional transverse-field Ising model

Quantum coherence will undoubtedly play a fundamental role in understanding the dynamics of quantum many-body systems; therefore, to be able to reveal its genuine contribution is of great importance. In this paper, we focus our discussions on the one-dimensional transverse field quantum Ising model initialized in the coherent Gibbs state, and investigate the effects of quantum coherence on dynamical quantum phase transition (DQPT). After quenching the strength of the transverse field, the effects of quantum coherence are studied using Fisher zeros and the rate function of the Loschmidt echo. We find that quantum coherence not only recovers DQPT destroyed by thermal fluctuations, but also generates some entirely new DQPTs, which are independent of the equilibrium quantum critical point. We also find that the Fisher zero cutting the imaginary axis is not sufficient to generate DQPT because it also requires the Fisher zeros to be tightly bound close enough to the neighborhood of the imaginary axis. It can be manifested that DQPTs are rooted in quantum fluctuations. This work reveals new information on the fundamental connection between quantum critical phenomena and quantum coherence.

A threshold changeable dynamic quantum secret sharing scheme with cheating identification

A threshold changeable dynamic quantum secret sharing scheme with cheating identification

A-Site Doping Promotes CO2 Activation and Prolongs Charge Carrier Lifetimes in SrTiO3: Insight from Quantum Dynamics.

Using time-dependent density functional theory and nonadiabatic molecular dynamics, we systematically investigated the effect of A-site doping on the CO2 activation and charge carrier lifetimes in SrTiO3(STO). Our simulations revealed that A-site doping significantly enhances the chemical adsorption of CO2 on SrTiO3 surfaces, which is beneficial for promoting CO2 activation. Moreover, we found that A-site doping can efficiently stabilize the lowest unoccupied molecular orbital (LUMO) of CO2 near the conduction band minimum of STO, promoting the photogenerated electron transfer from the conduction band of STO to the CO2 LUMO. Importantly, A-site doping causes a significant nonadiabatic coupling reduction and prolongs the charge recombination time by a factor of 1.86 compared to the pristine STO. Our study clarifies the influencing mechanism of A-site doping on CO2 activation and charge carrier lifetimes and suggests important principles for the design of high-performance photocatalytic semiconductors.

Effective modeling of open quantum systems by low-rank discretization of structured environments.

The accurate description of the interaction of a quantum system with its environment is a challenging problem ubiquitous across all areas of physics and lies at the foundation of quantum mechanics theory. Here, we pioneer a new strategy to create discrete low-rank models of the system-environment interaction, by exploiting the frequency and time domain information encoded in the fluctuation-dissipation relation connecting the system-bath correlation function and the spectral density. We demonstrate the effectiveness of our methodology by combining it with tensor-network methodologies and simulating the quantum dynamics of complex excitonic systems in a highly structured bosonic environment. The new modeling framework sets the basis for a leap in the analysis of open quantum systems, providing controlled accuracy at significantly reduced computational costs, with benefits in all connected research areas.

Enhancing Extraction and Suppressing Cooling of Hot Electrons in Lead Halide Perovskites by Dipolar Surface Passivation.

Slowing hot carrier (HC) cooling and improving HC extraction are considered two pivotal factors for enhancing power conversion efficiency in emerging HC photovoltaic applications of perovskites and other materials. Employing ab initio quantum dynamics simulations, we demonstrate the simultaneous slow cooling and efficient extraction of hot electrons at the C60/CsPbI3 interface through dipolar surface passivation with phenethylammonium and 4-fluorophenethylammonium ligands. The passivation effectively suppresses I-Pb lattice vibrations, weakens the hot electron-phonon interaction in CsPbI3, and thus slows down the HC cooling. At the same time, the dipolar surface passivation elevates the LUMO + 1 state in C60 and reduces the energy gap for HC extraction. Concurrently, higher-frequency vibrations of the dipolar layer enhance the coupling between C60 and CsPbI3, promoting efficient HC extraction further. These phenomena are intensified with increased polarity of the dipolar layer. Furthermore, we find that dipolar passivation has the opposite influence on cold electron collection at the band edge, underscoring the fact that the observed improvement in photovoltaic performance stems preferentially from the effective utilization of HCs rather than cold electrons. The work provides a new strategy for achieving high-performance HC perovskite solar cells.

Quantum Dynamics and Quasi-Classical Trajectory Study for the Dissociative Chemisorption of NO on Cu(111)

Quantum Dynamics and Quasi-Classical Trajectory Study for the Dissociative Chemisorption of NO on Cu(111)

Snapshotting quantum dynamics at multiple time points

Measurement-induced state disturbance is a major challenge in obtaining quantum statistics at multiple time points. We propose a method to extract dynamic information from a quantum system at intermediate time points, namely snapshotting quantum dynamics. To this end, we apply classical post-processing after performing the ancilla-assisted measurements to cancel out the impact of the measurements at each time point. Based on this, we reconstruct a multi-time quasi-probability distribution (QPD) that correctly recovers the probability distributions at the respective time points. Our approach can also be applied to simultaneously extract exponentially many correlation functions with various time-orderings. We provide a proof-of-principle experimental demonstration of the proposed protocol using a dual-species trapped-ion system by employing 171Yb+ and 138Ba+ ions as the system and the ancilla, respectively. Multi-time measurements are performed by repeated initialization and detection of the ancilla state without directly measuring the system state. The two- and three-time QPDs and correlation functions are reconstructed reliably from the experiment, negativity and complex values in the QPDs clearly indicate a contribution of the quantum coherence throughout dynamics.

Including Photoexcitation Explicitly in Trajectory-Based Nonadiabatic Dynamics at No Cost.

Over the last decades, theoretical photochemistry has produced multiple techniques to simulate the nonadiabatic dynamics of molecules. Surprisingly, much less effort has been devoted to adequately describing the first step of a photochemical or photophysical process: photoexcitation. Here, we propose a formalism to include the effect of a laser pulse in trajectory-based nonadiabatic dynamics at the level of the initial conditions, with no additional cost. The promoted density approach (PDA) decouples the excitation from the nonadiabatic dynamics by defining a new set of initial conditions, which include an excitation time. PDA with surface hopping leads to nonadiabatic dynamics simulations in excellent agreement with quantum dynamics using an explicit laser pulse and highlights the strong impact of a laser pulse on the resulting photodynamics and the limits of the (sudden) vertical excitation. Combining PDA with trajectory-based nonadiabatic methods is possible for any arbitrary-sized molecules using a code provided in this work.