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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.

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.

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.

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.

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.

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., Comment: 5 pages, 3 figures, journal article

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.

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.

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.

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)., Comment: 8 pages, 3 figures

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., Comment: arXiv admin note: substantial text overlap with arXiv:2008.08211

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., Comment: 20 pages, 4 figures

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{\^o} 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., Comment: Replaces previous incorrect posting. In press, J. Chem. Phys

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., Comment: 20 pages,4 figures

We report on theoretical study of second-harmonic generation (SHG) in plasmonic nanostructures interacting with two-level quantum emitters (QE) under incoherent energy pump. We generalize driven-dissipative Tavis-Cummings model by introducing anharmonic surface plasmon-polariton (SPP) mode coupled to QEs and examine physical properties of corresponding SPP-QE polariton states. Our calculations of the SHG efficiency for strong QE-SPP coupling demonstrate orders of magnitude enhancement facilitated by the polariton gain. We further discuss time-domain numerical simulations of SHG in a square lattice comprised of Ag nanopillars coupled to QEs utilizing fully vectorial nonperturbative nonlinear hydrodynamic model for conduction electrons coupled to Maxwell-Bloch equations for QEs. The simulations support the idea of gain enhanced SHG and show orders of magnitude increase in the SHG efficiency as the QEs are tuned in resonance with the lattice plasmon mode and brought above the population inversion threshold by incoherent pump. By varying pump frequency and tuning QEs to a localized plasmon mode, we demonstrate further enhancement of the SHG efficiency facilitated by strong local electric fields. The incident light polarization dependence of the SHG is examined and related to the symmetries of participating plasmon modes., Comment: 10 pages, 6 figures

Quantum entangled photons provide a sensitive probe of many-body interactions and offer an unique experimental portal for quantifying many-body correlations in a material system. In this paper, we present a theoretical demonstration of how photon-photon entanglement can be generated via interactions between coupled qubits. Here we develop a model for the scattering of an entangled pair of photons from a molecular dimer. We develop a diagrammatic theory for the scattering matrix and show that one can correlate the von Neumann entropy of the outgoing bi-photon wave function to exciton exchange and repulsion interactions. We conclude by discussing possible experimental scenarios for realizing these ideas.

Hybrid photonic-plasmonic nanostructures allow one to engineer coupling of quantum emitters and cavity modes accounting for the direct coherent and environment mediated dissipative pathways. Using generalized plasmonic Dicke model, we explore the non-equilibrium phase diagram with respect to these interactions. The analysis shows that their interplay results in the extension of the superradiant and regular lasing states to the dissipative coupling regime and an emergent lasing phase without population inversion having boundary with the superradiant and normal states. Calculated photon emission spectra are demonstrated to carry distinct signatures of these phases., Comment: 13 pages, 4 figures

An effective mass based model accounting for the conduction band quantization in a high aspect ratio semiconductor nanotip is developed to describe injected electron transport and subsequent electron emission from the nanotip. A transfer matrix formalism is used to treat electron scattering induced by the variation in the tip diameter and the electron emission. Numerical analysis of the scattering and emission probabilities is performed for the diamond parametrized nanotip model. Our scattering and emission models are further combined with a Monte Carlo (MC) approach to simulate electron transport through the nanotip. The MC simulations, also accounting for the electron-phonon scattering and externally applied electric field, are performed for a minimal nanotip model and an equivalent width diamond slab. An effect of the level quantization, electron scattering due to the nanotip diameter variation, and electron-phonon scattering on the nanotip emission properties is identified and compared with the case of bulk slab., Comment: 14 pages, 14 figures

Recent theoretical and experiments have explored the use of entangled photons as a spectroscopic probe of material systems. We develop here a theoretical description for entropy production in the scattering of an entangled biphoton state within an optical cavity. We develop this using perturbation theory by expanding the biphoton scattering matrix in terms of single-photon terms in which we introduce the photon-photon interaction via a complex coupling constant, $\xi$. We show that the von Neumann entropy provides a succinct measure of this interaction. We then develop a microscopic model and show that in the limit of fast fluctuations, the entanglement entropy vanishes whereas in the limit the coupling is homogeneous broadened, the entanglement entropy depends upon the magnitude of the fluctuations and reaches a maximum.

We present an input/output analysis of photon-correlation experiments whereby a quantum mechanically entangled bi-photon state interacts with a material sample placed in one arm of a Hong-Ou-Mandel (HOM) apparatus. We show that the output signal contains detailed information about subsequent entanglement with the microscopic quantum states in the sample. In particular, we apply the method to an ensemble of emitters interacting with a common photon mode within the open-system Dicke Model. Our results indicate considerable dynamical information concerning spontaneous symmetry breaking can be revealed with such an experimental system.

We develop a theoretical background to treat exciton states of semiconductor single-walled carbon nanotubes (SWCNTs) in presence of a periodic potential induced by the surface acoustic wave (SAW) propagating along semiconducting SWCNT. The formalism naturally accounts for the electronic bands splitting into the Floquet sub-bands brought about by the Bragg scattering on the SAW potential. Optically induced transitions within the Floquet states and formation of correlated electron-hole pairs, i.e., exciton states, are examined numerically. We discuss dynamical formation of new van Hove singularities within electron-hole continuum and associated reduction of the exciton oscillator strengths and its effect on the photoluminescence quenching in presence of the SAW. We argue that SAW induced dynamical gaps in the single particle dispersion leads to redistribution of the oscillator strength from excitons to the Floquet edge states. The simulations also confirm exciton energy Stark red shift as well as reduction in the binding energy. Comparison of our results with previous theoretical and experimental studies is provided., Comment: 9 pages, 3 figures

We study the collective, superradiant behavior in the system of emitter-dressed Ag nanorods. Starting from the Drude model for the plasmon oscillations, we arrive at a semi-empirical Hamiltonian describing the coupling between quantized surface plasmon modes and the quantum emitters that can be controlled by manipulating their geometry, spacing, and orientation. Further, identifying the lowest polariton mode as SP-states dressed by excitons in the vicinity of $k=0$, we examine conditions allowing for the polariton quantum phase transition. While the system is formally a 1D array, we show that the polariton states of interest can undergo a quantum phase transition to form a Bose condensate at finite temperatures for physically accessible parameter ranges., Comment: 9 pages, 5 figures

Pump-dependent photoluminescence imaging and 2nd order photon correlation studies have been performed on individual single-walled carbon nanotubes (SWCNTs) at room temperature that enable the extraction of both the exciton diffusion constant and the Auger recombination coefficient. A linear correlation between these is attributed to the effect of environmental disorder in setting the exciton mean free-path and capture-limited Auger recombination at this lengthscale. A suppression of photon antibunching is attributed to creation of multiple spatially non-overlapping excitons in SWCNTs whose diffusion length is shorter than the laser spot size. We conclude that complete antibunching at room temperature requires an enhancement of exciton-exciton annihilation rate that may become realizable in SWCNTs allowing for strong exciton localization., Comment: Revised version (30 pages, 11 figures) accepted for publication in Phys. Rev. Lett. including Supplementary materials

To examine thermoelectric (TE) properties of a semiconductor nanowire (NW) network, we propose a theoretical approach mapping the TE network on a two-port network. In contrast to a conventional single-port (i.e., resistor) network model, our model allows for large scale calculations showing convergence of TE figure of merit, $ZT$, with an increasing number of junctions. Using this model, numerical simulations are performed for the Bi$_2$Te$_3$ branched nanowire (BNW) and Cayley tree NW (CTNW) network. We find that the phonon scattering at the network junctions plays a dominant role in enhancing the network $ZT$. Specifically, disordered BNW and CTNW demonstrate an order of magnitude higher ZT enhancement compared to their ordered counterparts. Formation of preferential TE pathways in CTNW makes the network effectively behave as its BNW counterpart. We provide formalism for simulating large scale nanowire networks hinged upon experimentally measurable TE parameters of a single T-junction., Comment: Revised version: 9 pages, 5 figures

We formulate a real-space renormalization group (RG) approach for efficient numerical analysis of the low-temperature hopping dynamics in energy-disordered lattices. The approach explicitly relies on the time-scale separation of the trapping/escape dynamics. This time-scale separation allows to treat the hopping dynamics as a hierarchal process, an RG step being a transformation between the levels of the hierarchy. We apply the proposed RG approach to analyze hopping dynamics in one- and two-dimensional lattices with varying degree of energy disorder, and find the approach to be more accurate at low temperatures and computationally much faster, than the direct diagonalization. Applicability criteria of the proposed approach with respect to the time-scale separation and the maximum number of hierarchy levels are formulated. RG flows of energy distribution and preexponents of the Miller-Abrahams model are analyzed.

We report on systematic numerical study of carrier multiplication (CM) processes in spherically symmetric nanocrystal (NC) and bulk forms of PbSe and PbS representing the test bed for understanding basic aspects of CM dynamics. The adopted numerical method integrates our previously developed Interband Exciton Scattering Model (IESM) and the effective mass based electronic structure model for the lead chalocogenide semiconductors. The analysis of the IESM predicted CM pathways shows complete lack of the pathway interference during the biexciton photogeneration. This allows us to interpret the biexciton photogeneration as a single impact ionization (II) event and to explain major contribution of the multiple II events during the phonon-induced population relaxation into the total quantum efficiency (QE). We investigate the role of quantum confinement on QE, and find that the reduction in the biexciton density of states (DOS) overruns weak enhancement of the Coulomb interactions leading to lower QE values in NCs as compared to the bulk on the absolute photon energy scale. However, represented on the photon energy scale normalized by corresponding band gap energies the trend in QE is opposite demonstrating the advantage of NCs for photovoltaic applications. Comparison to experiment allows us to interpret the observed features and to validate the applicability range of our model. Modeling of QE as a function of pulse duration shows weak dependence for the gaussian pulses. Finally, comparison of the key quantities determining QE in PbSe and PbS demonstrates the enhancement of II rate in the latter materials. However, the fast phonon-induced carrier relaxation processes in PbS can lead to the experimentally observed reduction in QE in NCs as compared to PbSe NCs., Comment: 18 pages, 12 figures

Employing the interband exciton scattering model, we have performed a numerical study of the direct photogeneration and population relaxation processes contributing to carrier multiplication (CM) in nanocrystalline and bulk PbSe. We argue that in both cases the impact ionization is the main mechanisms of CM. This explains weak contribution of the direct photogeneration to the total quantum efficiency (QE). Investigated size scaling of QE in NCs and comparison to the bulk limit provide microscopic insight in the experimentally observed trends., Comment: 4 pages, 3 figures (revised version) Accepted for publication in Physical Review Letters

Using previously developed exciton scattering model accounting for the interband, i.e., exciton-biexciton, Coulomb interactions in semiconductor nanocrystals (NCs), we derive a closed set of equations for 2D double-quantum coherence signal. The signal depends on the Liouville space pathways which include both the interband scattering processes and the inter- and intraband optical transitions. These processes correspond to the formation of different cross-peaks in the 2D spectra. We further report on our numerical calculations of the 2D signal using reduced level scheme parameterized for PbSe NCs. Two different NC excitation regimes considered and unique spectroscopic features associated with the interband Coulomb interactions are identified., Comment: 11 pages, 5 figures

The effect of carrier multiplication (CM) in semiconductor nanocrystals is systematically treated by employing an exciton scattering approach. Using projection operators, we reduce the Coulomb coupled multi-exciton dynamics to scattering dynamics in the space spanning both single- and bi-exciton states. We derive a closed set of equations determining the scattering matrix elements. This allows us to interpret CM dynamics as a series of odd-order interband scattering events. Using the time-dependent density matrix formalism, we provide a rigorous description of the CM dynamics induced by a finite-time pump pulse. Within this approach, both processes of single- and bi-exciton photogeneration and the consequent population relaxation are treated on the same footing. This approach provides a framework for numerical calculations and for comparisons of the quantum efficiencies associated with each process. For applications, the limit of weak interband Coulomb coupling is considered. Finally, we demonstrate that three previously used theoretical models can be recovered as limiting cases of our exciton scattering model., Comment: 20 pages, 8 figures, Revised version, Accepted for publication in the Journal of Chemical Physics