arXiv subjects

Andrei Piryatinski

Publications and source records attributed to Andrei Piryatinski.

At least 19 recordsLinked to original sources

arXiv2025

Scattering theory of frequency-entangled biphoton states facilitated by cavity polaritons

The use of quantum light to probe exciton properties in semiconductor and molecular nanostructures typically occurs in the low-intensity regime. A substantial enhancement of exciton-photon coupling can be achieved with photonic cavities, where excitons hybridize with cavity modes to form polariton states. To provide a theoretical framework for interpreting emerging experimental efforts in this direction, we develop a scattering theory describing the interaction of frequency-entangled photon pairs with cavity polariton and bipolariton states under various coupling regimes. Employing the Tavis-Cummings model in combination with our scattering approach, we present a quantitative analysis of how the interaction of the entangled photon pair with the polariton/bipolariton modifies its joint spectral amplitude (JSA). Specifically, we examine the effects of the cavity-mode steady-state population, exciton-cavity coupling strength, and different forms of the input photon JSA. Our results show that the entanglement entropy of the scattered photons is highly sensitive to the interplay between the input JSA and the spectral line shapes of the polariton resonances, emphasizing the cavity filtering effects. We suggest that biphoton scattering quantum light spectroscopy best serves as a sensitive probe of polariton and bipolariton states in the photon-vacuum cavity state. Our approach is not only robust to various regimes of cavity-exciton coupling, but also amenable to extensions beyond the Tavis-Cummings model, enabling the representation of a broad class of molecular systems and solid state quantum materials.

quant-ph↗

arXiv2025

Topology and Spectral Entanglement in Cavity-Mediated Photon Scattering

We develop a diagrammatic theory of cavity-mediated photon-photon interactions in a topological insulator using the SSH model. The fourth-order vertex $\Gamma^{(4)}(\omega_1,\omega_2)$ governs spectral entanglement and Kerr nonlinearity, leading to a nonlinear topological phase diagram. We also compute the electronic self-energy from vacuum photon exchange and identify symmetry-imposed limits on band renormalization. These results link band geometry to the emergence of light-matter correlations.

quant-ph↗

arXiv2025

Tensorial Spin-Phonon Relaxation Reveals Mode-Selective Relaxation Pathways in a Single-Molecule Magnet

Understanding and controlling spin relaxation in molecular qubits is essential for developing chemically tunable quantum information platforms. We present a fully first-principles framework for computing the spin relaxation tensor in a single-molecule magnet, \ce{VOPc(OH)8}, by combining density functional theory with a mode-resolved open-system formalism. By expanding the spin Hamiltonian in vibrational normal modes and evaluating both linear and quadratic spin-phonon coupling tensors via finite differences of the $g$-tensor, we construct a relaxation tensor that enters a Lindblad-type quantum master equation. Our formalism captures both direct (one-phonon) and resonant-Raman (two-phonon) relaxation processes. Numerical analysis reveals a highly mode-selective structure: only three vibrational modes dominate longitudinal ($T_1$) decoherence, while a single mode accounts for the majority of transverse ($T_2$) relaxation. The computed relaxation times show excellent agreement with experimental measurements, without any empirical fitting. These results demonstrate that first-principles spin-phonon tensors can provide predictive insight into decoherence pathways and guide the rational design of molecular qubits.

quant-ph↗

arXiv2024

Robust Quantum Gate Preparation in Open Environments

We develop an optimal control algorithm for robust quantum gate preparation in open environments with the state of the quantum system represented using the Lindblad master equation. The algorithm is based on adaptive linearization and iterative quadratic programming to progressively shape the control signal into an optimal form. Robustness is achieved with exponential rates of convergence by introducing uncertain parameters into the master equation and expanding the parameterized state over the basis of Legendre polynomials. We prove that the proposed control algorithm reduces to GRadient Ascent Pulse Engineering (GRAPE) when the robustness portion of the algorithm is bypassed and signal restrictions are relaxed. The control algorithm is applied to prepare Controlled NOT and SWAP gates with high precision. Using only second order Legendre polynomials, the examples showcase unprecedented robustness to 100% parameter uncertainty in the interaction strength between the qubits, while simultaneously compensating for 20% uncertainty in signal intensity. The results could enable new capabilities for robust implementation of quantum gates and circuits subject to harsh environments and hardware limitations.

math.OC↗

arXiv2024

A quantum analog of Huygen's clock: noise-induced synchronization

We propose a quantum analogue of the Huygens clock, in which the phases of two spins achieve synchronization through their interaction with a shared environment. The environment functions analogously to the escapement mechanism in a mechanical clock, regulating the gear train and permitting the advancement of timing in discrete intervals. In our proposed model, the relative phase of the two spins become synchronized through interaction with a mutual, correlated, environment. We show that for a system of qubits, several arguments can be made that significantly reduce the cardinality of the set of allowed measurements and, hence, the complexity of the problem. We present a numerically efficient method to calculate the degree of quantumness that exists in the correlations of our final density matrix. This method also provides a tight upper bound for when the system is described by rank-3 and rank-4 density matrices.

quant-ph↗

arXiv2024

Spin/Phonon Dynamics in Single Molecular Magnets: II. spin/phonon entanglemen

We introduce a new quantum embedding method to explore spin-phonon interactions in molecular magnets. This technique consolidates various spin/phonon couplings into a limited number of collective degrees of freedom, allowing for a fully quantum mechanical treatment. By precisely factorizing the entire system into "system" and "bath" sub-ensembles, our approach simplifies a previously intractable problem, making it solvable on modest-scale computers. We demonstrate the effectiveness of this method by studying the spin relaxation and dephasing times of the single-molecule qubit \ce{VOPc(OH)8}, which features a lone unpaired electron on the central vanadium atom. By using this mode projection method, we are able to perform numerical exact quantum dynamical calculation on this system which allows us to follow the flow of quantum information from the single spin qubit into the projected phonon degrees of freedom. Our results demonstrate both the utility of the method and suggest how one can engineer the environment as to further optimize the quantum properties of a qubit system.

quant-ph↗

arXiv2024

Spin/Phonon Dynamics in Single Molecular Magnets: I. quantum embedding

Single molecular magnets (SMMs) and Metal-Organic Frameworks (MOFs) attract significant interest due to their potential in quantum information processing, scalable quantum computing, and extended lifetimes and coherence times. The limiting factor in these systems is often the spin dephasing caused by interactions and couplings with the vibrational motions of the molecular framework. This work introduces a systematic projection/embedding scheme to analyze spin-phonon dynamics in molecular magnets. This scheme consolidates all spin/phonon couplings into a few collective degrees of freedom. quantum mechanically. Using parameters obtained from ab initio methods for spin/phonon coupling via Zeeman interaction, we apply this approach to compute the electronic spin relaxation times for a single-molecule qubit \ce{VOPc(OH)8}, which features a single unpaired electron localized on the central vanadium atom. However, our general embedding scheme can be applied to any single-molecule magnet or qubit MOF with any coupling/interaction Hamiltonian. This development offers a crucial tool for simulating spin relaxation in complex environments with significantly reduced computational complexity.

quant-ph↗

arXiv2024

Potential Applications of Quantum Computing at Los Alamos National Laboratory

The emergence of quantum computing technology over the last decade indicates the potential for a transformational impact in the study of quantum mechanical systems. It is natural to presume that such computing technologies would be valuable to large scientific institutions, such as United States national laboratories. However, detailed descriptions of what these institutions would like to use these computers for are limited. To help provide some initial insights into this topic, this report develops detailed use cases of how quantum computing technology could be utilized to enhance a variety of quantum physics research activities at Los Alamos National Laboratory, including quantum magnetic materials, high-temperature superconductivity and nuclear astrophysics simulations. The report discusses how current high-performance computers are used for scientific discovery today and develops detailed descriptions of the types of quantum physics simulations that Los Alamos National Laboratory scientists would like to conduct, if a sufficient computing technology became available. While the report strives to highlight the breadth of potential application areas for quantum computation, this investigation has also indicated that many more use cases exist at Los Alamos National Laboratory, which could be documented in similar detail with sufficient time and effort.

quant-ph↗

arXiv2024

Harnessing Complexity: Nonlinear Optical Phenomena in L-Shapes, Nanocrescents, and Split-Ring Resonators

We conduct systematic studies of the optical characteristics of plasmonic nanoparticles that exhibit C2v symmetry. We analyze three distinct geometric configurations: an L-type shape, a crescent, and a split-ring resonator. Optical properties are examined using the FDTD method. It is demonstrated that all three shapes exhibit two prominent plasmon bands associated with the two axes of symmetry. This is in addition to a wide range of resonances observed at high frequencies corresponding to quadrupole modes and peaks due to sharp corners. Next, to facilitate nonlinear analysis, we employ a semiclassical hydrodynamic model where the electron pressure term is explicitly accounted for. Employing this model enables us to rigorously examine the second-order angular resolved nonlinear optical response of these nanoparticles in each of the three configurations. For CW pumping, we explore properties of the SHG. Polarization and angle-resolved SHG spectra are obtained, revealing strong dependence on the nanoparticle geometry and incident wave polarization. For pulsed excitations, we discuss the phenomenon of broadband THz generation induced by the DFG. It is shown that the THz emission spectra exhibit unique features attributed to the plasmonic resonances and symmetry of the nanoparticles. The polarization of the generated THz waves is also examined, revealing interesting patterns tied to the nanoparticle geometry. To gain deeper insight, we propose a simple analytical theory that agrees very well with the numerical experiments. An expression for the far-field THz intensity is derived in terms of the incident pulse parameters and the nonlinear response tensor of the nanoparticle. The results presented in this work offer new insights into the linear and nonlinear optical properties of nanoparticles with C2v symmetry.

physics.optics↗

arXiv2024

Interplay of gain and loss in arrays of nonlinear plasmonic nanoparticles: toward parametric downconversion and amplification

With the help of a theoretical model and finite-difference-time-domain simulations based on the hydrodynamic-Maxwell model, we examine the effect of difference frequency generation in an array of L-shaped metal nano-particles characterized by intrinsic plasmonic nonlinearity. The outcomes of the calculations reveal the spectral interplay of the gain and loss in the vicinity of the fundamental frequency of the localized surface-plasmon resonances. Subsequently, we identify different array depths and pumping regimes facilitating parametric amplification and parametric down-conversion. Our results suggest that the parametric amplification regime becomes feasible on a scale of hundreds of nanometers and parametric downconversion on the scale of tens of nanometers, opening up new exciting opportunities for developing building blocks of photonic metasurfaces.

physics.optics↗

arXiv2023

Correlated noise enhances coherence and fidelity in coupled qubits

It is generally assumed that environmental noise arising from thermal fluctuations is detrimental to preserving coherence and entanglement in a quantum system. In the simplest sense, dephasing and decoherence are tied to energy fluctuations driven by coupling between the system and the normal modes of the bath. Here, we explore the role of noise correlation in an open-loop model quantum communication system whereby the ``sender'' and the ``receiver'' are subject to local environments with various degrees of correlation or anticorrelation. We introduce correlation within the spectral density by solving a multidimensional stochastic differential equations and introduce these into the Redfield equations of motion for the system density matrix. We find that correlation can enhance both the fidelity and purity of a maximally entangled (Bell) state. Moreover, by comparing the evolution of different initial Bell states, we show that one can effectively probe the correlation between two local environments. These observations may be useful in the design of high-fidelity quantum gates and communication protocols.

quant-ph↗

arXiv2023

QuDPy: A Python-Based Tool For Computing Ultrafast Non-linear Optical Responses

Nonlinear Optical Spectroscopy is a well-developed field with theoretical and experimental advances that have aided multiple fields including chemistry, biology and physics. However, accurate quantum dynamical simulations based on model Hamiltonians are need to interpret the corresponding multi-dimensional spectral signals properly. In this article, we present the initial release of our code, QuDPy (quantum dynamics in python) which addresses the need for a robust numerical platform for performing quantum dynamics simulations based on model systems, including open quantum systems. An important feature of our approach is that one can specify various high-order optical response pathways in the form of double-sided Feynman diagrams via a straightforward input syntax that specifies the time-ordering of ket-sided or bra-sided optical interactions acting upon the time-evolving density matrix of the system. We use the quantum dynamics capabilities of QuTip for simulating the spectral response of complex systems to compute essentially any n-th-order optical response of the model system. We provide a series of example calculations to illustrate the utility of our approach.

quant-ph↗

arXiv2023

Correlated Excitonic Signatures in a Nanoscale van der Waals Antiferromagnet

Composite quasi-particles with emergent functionalities in spintronic and quantum information science can be realized in correlated materials due to entangled charge, spin, orbital, and lattice degrees of freedom. Here we show that by reducing the lateral dimension of correlated antiferromagnet NiPS3 flakes to tens of nanometers, we can switch-off the bulk spin-orbit entangled exciton in the near-infrared (1.47 eV) and activate visible-range (1.8 to 2.2 eV) transitions with charge-transfer character. These ultra-sharp lines (<120 ueV at 4.2 K) share the spin-correlated nature of the bulk exciton by displaying a Neel temperature dependent linear polarization. Furthermore, exciton photoluminescence lineshape analysis reveals a polaronic character via coupling with at-least 3 phonon modes and a comb-like Stark effect through discretization of charges in each layer. These findings augment the knowledge on the many-body nature of excitonic quasi-particles in correlated antiferromagnets and also establish the nanoscale platform as promising for maturing integrated magneto-optic devices.

cond-mat.mtrl-sci↗

arXiv2023

Degenerate parametric down-conversion facilitated by exciton-plasmon polariton states in nonlinear plasmonic cavity

We study the effect of degenerate parametric down-conversion (DPDC) in an ensemble of two-level quantum emitters (QEs) coupled via near-field interactions to a single surface plasmon (SP) mode of a nonlinear plasmonic cavity. For this purpose, we develop a quantum driven-dissipative model capturing non-equilibrium dynamics of the system in which incoherently pumped QEs have transition frequency tuned near the second-harmonic response of the SPs. Considering the strong coupling regime, i.e., the SP-QE interaction rate exceeds system dissipation rates, we find a critical SP-QE coupling attributed to the phase transition between normal and lasing steady states. Examining fluctuations above the system's steady states, we predict new elementary excitations, namely, the exciton-plasmon polaritons formed by the two-SP quanta and single-exciton states of QEs. The contribution of two-SP quanta results in the linear scaling of the SP-QE interaction rate with the number of QEs, ${\cal N}_o$, as opposed to the $\sqrt{{\cal N}_o}$-scaling known for the Dicke and Tavis-Cummings models. We further examine how SP-QE interaction scaling affects the polariton dispersions and power spectra in the vicinity of the critical coupling. For this purpose, we compare the calculation results assuming a finite ensemble of QEs and the model thermodynamic limit. The calculated power spectra predict an interplay of coherent photon emission by QEs near the second-harmonic frequency and correlated photon-pair emission at the fundamental frequency by the SPs (i.e., the photonic DPDC effect).

physics.optics↗

arXiv2022

Stochastic exciton-scattering theory of optical lineshapes: Renormalized many-body contributions

Spectral line-shapes provide a window into the local environment coupled to a quantum transition in the condensed phase. In this paper, we build upon a stochastic model to account for non-stationary background processes produced by broad-band pulsed laser stimulation. In particular, we consider the contribution of pair-fluctuations arising from the full bosonic many-body Hamiltonian within a mean-field approximation, treating the coupling to the system as a stochastic noise term. Using the It{ô} transformation, we consider two limiting cases for our model which lead to a connection between the observed spectral fluctuations and the spectral density of the environment. In the first case, we consider a Brownian environment and show that this produces spectral dynamics that relax to form dressed excitonic states and recover an Anderson-Kubo-like form for the spectral correlations. In the second case, we assume that the spectrum is Anderson-Kubo like, and invert to determine the corresponding background. Using the Jensen inequality, we obtain an upper limit for the spectral density for the background. The results presented here provide the technical tools for applying the stochastic model to a broad range of problems.

cond-mat.mes-hall↗

arXiv2022

The Optical Signatures of Stochastic Processes in Many-Body Exciton Scattering

We review our recent quantum stochastic model for spectroscopic lineshapes in the presence of a co-evolving and non-stationary background population of excitations. Starting from a field theory description for interacting bosonic excitons, we derive a reduced model whereby optical excitons are coupled to an incoherent background via scattering as mediated by their screened Coulomb coupling. The Heisenberg equations of motion for the optical excitons are then driven by an auxiliary stochastic population variable, which we take to be the solution of an Ornstein-Uhlenbeck process. Here we discuss an overview of the theoretical techniques we have developed as applied to predicting coherent non-linear spectroscopic signals. We show how direct (Coulomb) and exchange coupling to the bath give rise to distinct spectral signatures and discuss mathematical limits on inverting spectral signatures to extract the background density of states.

cond-mat.mtrl-sci↗

arXiv2022

Manipulating Interlayer Excitons for Ultra-pure Near-infrared Quantum Light Generation

Interlayer excitons (IXs) formed at the interface of atomically-thin semiconductors possess various novel properties. In a parallel development, nanoscale strain engineering has emerged as an effective means for creating 2D quantum light sources. Exploring the intersection of these two exciting areas, where strain and defects are exploited for the manipulation of IX toward the emergence of new functionalities, is currently at a nascent stage. Here, using MoS2/WSe2 heterostructure as a model system, we demonstrate how strain, defects, and layering can be utilized to create defect-bound IXs capable of bright, robust, and tunable quantum light emission in the technologically important near-infrared spectral range. We were able to achieve ultra-high single-photon purity with g(2)(0) = 0.01. Our strategy of creating site-controlled QEs from the defect-bound IXs represents a paradigm shift in 2D quantum photonics research, from engineering intralayer exciton in monolayer structures towards IXs at the interface of 2D heterostructures.

cond-mat.mes-hall↗

arXiv2022

Proximity Induced Chiral Quantum Light Generation in Strain-Engineered WSe2/NiPS3 Heterostructures

Quantum light emitters (QEs) capable of generating single photons of well-defined circular polarization could enable non-reciprocal single photon devices and deterministic spin-photon interfaces critical for realizing complex quantum networks. To date, emission of such chiral quantum light has been achieved via the application of intense external magnetic field electrical/optical injection of spin polarized carriers/excitons, or coupling with complex photonic/meta-structures. Here we report free-space generation of highly chiral single photons from QEs created in monolayer WSe2 - NiPS3 heterostructures at zero external magnetic field. These QEs emit in the 760-800 nm range with a degree of circular polarization and single photon purity as high as 0.71 and 80% respectively, independent of pump laser polarization. QEs are deterministically created by pressing a scanning probe microscope tip into a two-dimensional heterostructure comprising a WSe2 monolayer and a ~50 nm thick layer of the antiferromagnetic (AFM) insulator NiPS3. Temperature dependent magneto-photoluminescence studies indicate that the chiral quantum light emission arises from magnetic proximity interactions between localized excitons in the WSe2 monolayer and the out-of-plane magnetization of AFM defects in NiPS3, both of which are co-localized by the strain field arising from the nanoscale indentations.

cond-mat.mes-hall↗