arXiv subjects

Nimrod Moiseyev

Publications and source records attributed to Nimrod Moiseyev.

At least 19 recordsLinked to original sources

arXiv2026

The Quantum Polariton Hamiltonian that Reproduces the Same Mean Acceleration High-Harmonic Generation Spectra as the Classical Hamiltonian in Strong Laser Fields

This manuscript investigates the possibility of defining a quantum Hamiltonian that leads to the same mean acceleration high-harmonic generation (HHG) spectra as those predicted by Floquet theory in the regime where the number of infrared (IR) pump photons is much larger than the number of emitted UV photons. The key assumption underlying our derivation is that the intensity of the emitted high harmonics is many orders of magnitude smaller than that of the IR laser, and that the emission of the Nnt harmonic results from the absorption of N IR photons.

quant-ph↗

arXiv2025

Spontaneous Emission from Electronic Metastable Resonance States

We demonstrate that calculating the spontaneous emission decay rate from metastable resonance states (states with finite lifetimes embedded in the continuum) requires considering transitions to all continuum states, not just to lower states. This holds even when the lifetimes of the metastable states are very long and might be effectively considered as bound states in the continuum. However, employing complex-scaling transformations, this computationally prohibitive task becomes feasible by utilizing methods originally designed for excited bound states for calculation of complex poles of the scattering matrix. As an illustrative example, these methods are applied to calculate the spontaneous emission decay rates of metastable resonance states in a double-barrier potential. The rapid numerical convergence of this approach highlights a new avenue for studying spontaneous emission from metastable states in real-life systems, particularly in many-electron systems, where calculation of the spontaneous emission decay rate from metastable resonances (e.g., autoionization states) is computationally difficult, if not impossible, using the standard (Hermitian) formalism of quantum mechanics.

quant-ph↗

arXiv2025

Conditions for Suppression of Gas Phase Chemical Reactions inside a Dark Infrared Cavity: O$_{2}$ + 2NO$\to$ 2NO$_{2}$ as an example

The ability to slow down chemical reactions using a seemingly simple setup reactions confined within a cavity formed by two parallel mirrors is fascinating. However, theory and experiment have not yet fully converged. In this work, we provide the conditions and guidelines for selecting reactions that can be suppressed in a dark cavity. The primary requirement is that the reaction's potential energy surface must contain two saddle points (and not one saddle point as required for enhancement). This condition enables a reduction in the reaction rate. Specifically, we demonstrate that the reaction rate of O$_{2}$ + 2NO$\to$ 2NO$_{2}$ can be suppressed by a dark cavity composed of two parallel mirrors. We show that the suppression of the reaction rate depends on the distance between the mirrors, which determines the cavity parameters, and on the number of molecules in the transition state configuration that simultaneously interact with the cavity.

quant-ph↗

arXiv2025

Cavity-Controlled High Harmonic Generation

Employing non-Hermitian Floquet theory, the strong-field process of high harmonic (HH) generation by a classical continuous-wave field irradiating ground-state atom is discovered to be controllable by placing the irradiated atom inside a single-mode quantum cavity initiated even with a single photon. Judicious cavity coupling of cavity-free photo-induced atomic Floquet states forms polaritonic Floquet states that generate side harmonics around the (standard no-cavity) odd harmonics. The different possible cavity-controlled HH spectra, including also the ones resulting from several cavities in a row, enable attosecond-pulse sequences different from the one produced without a cavity. Moreover, the present study sets the framework and opens the way for further cavity control over the HH generation process as well as over other strong-field processes

quant-ph↗

arXiv2025

Accelerated Hydrogen Exchange Reaction in a Dark Cavity: A Benchmark for Bridging the Gap Between Theory and Experiment

The gas-phase hydrogen exchange reaction (HER) is the most fundamental chemical process for benchmarking quantum reaction dynamics. In this Letter, we focus on controlling HER by means of strong light-matter coupling inside a resonant cavity, an approach often called polariton chemistry. In particular, we focus on the isotopic variation of HER involving collisions between molecular hydrogen H$_2$ and deuterium atom D, i.e., H$_2$+D$\to$HD+H. We find that the asymmetry introduced by the different isotopes, despite being small, enables strong cavity-induced modifications of reaction rates. Outside of the cavity the reaction is as usual D+H${_2}$$\to$DH+H. However, inside the cavity another type of reactions take place where D+H$_2$$\to$DH+H+E$_{photon}$, where E$_{photon}$=$\hbarω_{cav}$. Our results show that HER is an ideal platform to make a significant step toward closing the gap between theory and experiment in polariton chemistry.

physics.chem-ph↗

arXiv2025

Enantiosensitive exceptional points in open chiral systems

Exceptional points (EPs) are remarkable spectral degeneracies in a non-Hermitian system's parameter space, where both eigenvalues and eigenstates coalesce. Here, we show that in non-Hermitian molecular chiral systems the position of EPs in the parameter space is enantiomer-specific. First, we show that encircling the EP of one enantiomer drives robust topological population transfer in the chiral molecule while its mirror twin remains unaffected, offering a new route for selective chiral control. Second, we reveal how resonant excitation of EPs in chiral molecules can amplify weak chiral effects, offering an alternative approach to the enhancement of chiral interactions. Third, we demonstrate that a twisted chiral fiber immersed in a liquid solution of chiral molecules exhibits topologically different behavior depending on the solution's enantiomeric excess, offering a new approach to the detection of molecular chirality. Our results combine high enantiosensitivity with topological robustness in chiral discrimination and control, paving the way for new approaches in the exploration of non-Hermitian and chiral phenomena.

quant-ph↗

arXiv2024

Photon Statistics from Non-Hermitian Floquet Theory: High Harmonic Generation and Above-Threshold Ionization Spectra Detected via IR Detectors

Although it seems that obtaining quantum properties of light from classical calculations is a self-contradictory claim, it is shown here that a unified mechanism governs the three distinct measurements of high harmonic generation spectra (HGS), above-threshold ionization (ATI), and IR photon number distribution, none of which require the quantization of the electromagnetic field. Here, the conditions that enable the calculations of HGS and ATI spectra for atoms interacting with high-intensity laser fields from photon statistics are first derived. Through the non-Hermitian theoretical simulation, the regimes where there is correspondence between the HHG and ATI spectra and annihilated pump photons (with post-selection) are identified. Consequently, the HGS and ATI spectra, as detected by XUV detectors, can be obtained by monitoring the fluctuations of the infrared absorbed photons.

quant-ph↗

arXiv2024

Conditions for enhancement of chemical reactions in gas phase inside a dark cavity

Enhancing chemical reactions, such as $A+B \to [\textit{activated complex}]^\# \to C+D$, in gas phase through its coupling to quantum-electrodynamics (QED) modes in a dark cavity is investigated. The main result is that the enhancement of the reaction rate by a dark cavity is for asymmetric reactions (products different from reactants.) Notice that in addition to the cavity been dark, the reactants are in their ground electronic and vibrational states, i.e., it is indeed dark. Theoretical derivation, utilizing the non-Hermitian formalism of quantum mechanics (NHQM), provides conditions and guidelines for selecting the proper type of reactions that can be enhanced by a dark cavity. Nevertheless, the time-dependent simulations of such experiments can be carried out using the standard (Hermitian) scattering theory (but including the conditions derived via NHQM). We believe that this work opens a gate to new types of studies and hopefully helps to close the gap between theory and experiments in this fascinating, relatively new field of research. As an example, we demonstrate that the asymmetric reaction rates of $O+D_2\to [ODD]^{\#} \to OD+D$ and $H+ArCl \to [ArHCl]^{\#} \to H+Ar+Cl$ can be enhanced by a dark cavity. Contrary, the dark cavity effect on the symmetric reaction of hydrogen exchange in methane will be negligible.

quant-ph↗

arXiv2023

Enantiosensitive exceptional points

We show that the position of the exceptional points (EPs) in the parameter space of a chiral molecule coupled to the photoionization continuum by a three-color field is enantiosensitive. Using a minimal model of a three-level system driven by a three-color field to form a cyclic loop transition, we investigate the enantiosensitivity of the EPs with respect to the system parameters and exploit the asymmetric switch mechanism associated with the encirclement of an EP in parameter space in an enantio-selective way. Our work paves the way for future applications of enantiosensitive EPs in chiral systems.

quant-ph↗

arXiv2022

Tuning quantum-classical correspondence of molecular systems in a cavity

We show that the correspondence between quantum and classical mechanics can be tuned by varying the coupling strength between the cavity modes and an atom or a molecule. In the acceleration gauge the cavity-matter system is represented by an effective Hamiltonian with a non-trivial coupling appearing in the potential, and a renormaized mass. Importantly, the acceleration-gauge coupling is non-monotonic with the strength cavity-matter interaction. As a result one obtain effective approximately decoupled field-matter dynamics for weak and strong interactions. In the weak coupling regime the effective mass is essentially the original mass. In contrast, the renormalized mass is increased as the interaction is increased. This results in acceleration gauge dynamics of atom/molecule with the original Hamiltonian and effective Planck's constant that is reduced when the interaction is increased. This approach might lead in particular to the possibility of studying the correspondence of "quantum-chaos" (quantum stochasticity) with classical chaos, as well as either enhancement or suppression of tunneling, by varying a controllable physical parameter. Physical realization of our findings is briefly discussed.

quant-ph↗

arXiv2022

Enhanced coupling of electron and nuclear spins by quantum tunneling resonances

Noble-gas spins feature hours long coherence times owing to their great isolation from the environment, and find practical usage in various applications. However, this isolation leads to extremely slow preparation times, relying on weak spin transfer from an electron-spin ensemble. Here we propose a controllable mechanism to enhance this transfer rate. We analyze the spin dynamics of helium-3 atoms with hot, optically-excited potassium atoms and reveal the formation of quasi-bound states in resonant binary collisions. We find a resonant enhancement of the spin-exchange cross section by up to six orders of magnitude and two orders of magnitude enhancement for the thermally averaged, polarization rate-coefficient. We further examine the effect for various other noble gases and find that the enhancement is universal. We outline feasible conditions under which the enhancement may be experimentally observed and practically utilized.

quant-ph↗

arXiv2021

Coalescence of two branch points in complex time marks the end of rapid adiabatic passage and the start of Rabi oscillations

We study theoretically the population transfer in two-level atoms driven by chirped lasers. It is known that in the Hermitian case, the rapid adiabatic passage (RAP) is stable for an above-critical chirp below which the final populations of states Rabi oscillate with varying laser power. We show that if the excited state is represented by a resonance, the separatrix marking this critical phenomenon in the space of the laser pulse parameters emanates from an exceptional point (EP) -- a non-Hermitian singularity formed in the atomic system by the fast laser field oscillations and encircled due to slow variations of the laser pulse envelope and instantaneous frequency. This critical phenomenon is neatly understood via extending the "slow" time variable into the complex plane, uncovering a set of branch points which encode non-adiabatic dynamics, where the switch between RAP and Rabi oscillations is triggered by a coalescence of two such branch points. We assert that the intriguing interrelation between the two different singularities -- the EP and the branch point coalescence in complex time plane -- can motivate feasible experiments involving laser driven atoms.

quant-ph↗

arXiv2021

Variational solutions for Resonances by a Finite-Difference Grid Method

We demonstrate that the finite difference grid method (FDM) can be simply modified to satisfy the variational principle and enable calculations of both real and complex poles of the scattering matrix. These complex poles are known as resonances and provide the energies and inverse lifetimes of the system under study (e.g., molecules) in metastable states. This approach allows incorporating finite grid methods in the study of resonance phenomena in chemistry. Possible applications include the calculation of electronic autoionization resonances which occur when ionization takes place as the bond lengths of the molecule are varied. Alternatively, the method can be applied to calculate nuclear predissociation resonances which are associated with activated complexes with finite lifetimes.

physics.comp-ph↗

arXiv2020

Encircling exceptional points as a non-Hermitian extension of rapid adiabatic passage

The efficient transfer of excitations between different levels of a quantum system is a task with many applications. Among the various protocols to carry out such a state transfer in driven systems, rapid adiabatic passage (RAP) is one of the most widely used. Here we show both theoretically and experimentally that adding a suitable amount of loss to the driven Hamiltonian turns a RAP protocol into a scheme for encircling an exceptional point including the chiral state transfer associated with it. Our work thus discloses an intimate connection between a whole body of literature on RAP and recent studies on the dynamics in the vicinity of an exceptional point, which we expect to serve as a bridge between the disjoint communities working on these two scenarios.

physics.optics↗

arXiv2019

Linking scalar elastodynamics and non-Hermitian quantum mechanics

Recent years have seen a fascinating pollination of ideas from quantum theories to elastodynamics---a theory that phenomenologically describes the time-dependent macroscopic response of materials. Here, we open route to transfer additional tools from non-Hermitian quantum mechanics. We begin by identifying the differences and similarities between the one-dimensional elastodynamics equation and the time-independent Schrodinger equation, and finding the condition under which the two are equivalent. Subsequently, we demonstrate the application of the non-Hermitian perturbation theory to determine the response of elastic systems; calculation of leaky modes and energy decay rate in heterogenous solids with open boundaries using a quantum mechanics approach; and construction of degeneracies in the spectrum of these assemblies. The latter result is of technological importance, as it introduces an approach to harness extraordinary wave phenomena associated with non-Hermitian degeneracies for practical devices, by designing them in simple elastic systems. As an example of such application, we demonstrate how an assembly of elastic slabs that is designed with two degenerate shear states according to our scheme, can be used for mass sensing with enhanced sensitivity by exploiting the unique topology near the exceptional point of degeneracy.

cond-mat.soft↗

arXiv2019

Robust mode conversion in NV centers using exceptional points

We show that microwave-driven NV centers can function as topological mode switches by utilizing a special degeneracy called an exceptional point (EP). By tuning the intensities and frequencies of the driving fields, we find an EP---where two normal modes of the system coalesce---and, then, use it to simulate the dynamics and demonstrate topological and non-reciprocal mode switching. By comparing density matrices of the input and output states, we find that the quantum correlations decrease by three orders of magnitude at room temperature, and discuss ways for improving this result. This work extends the theory of topological mode switches (originally derived for pure states) to mixed states and is, therefore, applicable to general open quantum systems. Our theory enables exploring new phenomena (e.g., high-order EPs in low-dimensional systems) and presents a crucial step towards incorporating topological mode switches in quantum-information applications.

quant-ph↗

arXiv2019

Nonrigidity effects -- a missing puzzle piece in the description of low-energy anisotropic molecular collisions

Cold collisions serve as a very sensitive probe of the interaction potential. In the recent study of Klein et al. (Nature Phys. 13, 35-38 (2017)) the one-parameter scaling of the interaction potential was necessary to obtain agreement between theoretical and observed patterns of the orbiting resonances for excited metastable helium atoms colliding with hydrogen molecules. Here we show that the effect of nonrigidity of the H$_2$ molecule on the resonant structure, absent in the previous study, is critical to predict correct positions of the resonances in that case. We have complemented the theoretical description of the interaction potential and revised reaction rate coefficients by proper inclusion of the flexibility of the molecule. The calculated reaction rate coefficients are in remarkable agreement with the experimental data without empirical adjustment of the interaction potential. We have shown that even state-of-the-art calculations of the interaction energy cannot ensure agreement with the experiment if such an important physical effect as flexibility of the interacting molecule is neglected. Our findings about the significance of the nonrigidity effects can be especially crucial in cold chemistry, where the quantum nature of molecules is pronounced.

physics.chem-ph↗

arXiv2019

Ab-initio Theory of Photoionization via Resonances

We present an \emph{ab-initio} approach for computing the photoionization spectrum near autoionization resonances in multi-electron systems. While traditional (Hermitian) theories typically require computing the continuum states, which are difficult to obtain with high accuracy, our non-Hermitian approach requires only discrete bound and metastable states, which are accurately computed with advanced quantum chemistry tools. We derive a simple formula for the absorption lineshape near Fano resonances, which relates the asymmetry of the spectral peaks to the phase of the complex transition dipole moment. Additionally, we present a formula for the ionization spectrum of laser-driven targets and relate the `Autler-Townes' splitting of spectral lines to the existence of exceptional points in the Hamiltonian. We apply our formulas to compute the autoionization spectrum of helium, but our theory is also applicable for non-trivial multi-electron atoms and molecules.

physics.optics↗