arXiv subjects

Petr Klenovský

Publications and source records attributed to Petr Klenovský.

At least 19 recordsLinked to original sources

arXiv2026

From Projected Subspaces to Full-Space Implementations: Representation-Equivalence Audits for Open-Shell ADAPT-VQE

A variational state optimized in a symmetry-projected space need not be the state prepared by exponentiating the corresponding unprojected parent generators. For an orthogonal target-space projector $P$, $Q=I-P$, and an anti-Hermitian generator $A$, exact equivalence for all amplitudes requires target-space invariance, $QAP=0$. We formulate this condition as a representation-equivalence audit for open-shell ADAPT-VQE. For a canonical CAS(15e,9o) type-1 Cu(II) Hamiltonian, 274 of 323 bare Hartree--Fock-origin single, double, and triple excitations violate this condition. Projected-doublet ADAPT reaches 1.5719 m$E_h$ with 121 operators, whereas the same amplitudes in the corresponding full-space sequence give 464.730 m$E_h$ error, 0.07343 fidelity with the projected state, and 54.45\% quartet weight. Reoptimizing the same 121 parent generators in the same order under the hard constraint $p_D\ge0.999$ reduces the best converged error to 8.482 m$E_h$, showing that the catastrophic same-angle failure is primarily a representation/parameter-transfer failure but is not completely repaired by full-space reoptimization of the fixed sequence. An independent 12-qubit NO benchmark exhibits the same mechanism more mildly. A spin-preserving generalized full-space construction removes the representation ambiguity and reaches 1.493 and 1.181 m$E_h$ at 18 and 24 qubits. For the 18-qubit positive control, selectively refining only four Suzuki-2 factors and re-transpiling with a fixed compiler seed passes the predeclared circuit-equivalence gate at 19,660 controlled-X (CX) gates and depth 23,016. Circuit synthesis and final-energy measurement therefore constitute separate validation layers. The contribution is a validation framework, not a new spin-adapted ADAPT algorithm.

quant-ph↗

arXiv2026

Strain-Engineered Deterministic Quantum Dots for Telecom O-Band Emission Using Buried Stressors

The deterministic realization of quantum light sources operating at telecom wavelengths is essential for long-distance fiber-based quantum communication and distributed quantum computing. In this work, we demonstrate that telecom O-band emission can be achieved from site-controlled InGaAs/GaAs quantum dots (QDs). Our concept utilizes a buried AlAs/Al$_2$O$_3$ stressor layer with the unique feature that induces a well-defined and controllable tensile strain field at the growth surface, enabling both a redshift of QD emission to the $\sim$1.3~μm range and site-selective nucleation at the mesa centers. This concept eliminates not only the need for strain-reducing layers (SRLs), which are known to degrade optical coherence, but also provides spatial control and spectral tunability. The grown telecom QDs show pure single-photon emission with $g^{(2)}(τ) = (5.0 \pm 1.0) \times 10^{-2}$ at 4 K and $(2.8 \pm 0.3) \times 10^{-1}$ at 77~K, demonstrating the quantum nature and thermal stability of the emitters. The emission characteristics of complex excitonic states are analyzed using 8-band $k \cdot p$ and configuration-interaction modeling, which quantitatively reproduces the experimental observations. Finally, we present a theory-supported strategy to further redshift the emission toward the center of the O-band and beyond by employing a multi-buried-stressor approach. This combined framework of experiment and theory establishes the buried stressor concept as a scalable route toward highly coherent, position-controlled O-band quantum emitters compatible with industrial photonic integration.

cond-mat.mes-hall↗

arXiv2026

Buried Stressor Engineering for Position-Controlled InGaAs Quantum Dots with Local Density Variation for Integrated Quantum Photonics

We report on the monolithic, two-step epitaxial growth of site-controlled InGaAs quantum dots via the buried stressor method with local quantum dot density variation. As a result of high fabrication accuracy, we achieve low lateral displacements of the individual buried stressor apertures of $ 17^{+19}_{-17}$~nm from mesa centers. We provide extensive micro-photoluminescence and cathodoluminescence characterization of the site-controlled quantum dots and give theoretical calculations, explaining the effect of the stressor aperture on the quantum dot emission properties, positioning, and density. We show reproducibility of the nucleation process for apertures of the same size and achieve precisely-positioned, low- and high-density quantum dot nucleation within one active layer growth step. The results presented in this work demonstrate the significant potential of the buried stressor concept in fabricating single photonic chips, simultaneously combining single-photon sources and microlasers featuring different local densities of site-controlled quantum dots, paving the way for highly functional source modules with applications in photonic quantum technology.

cond-mat.mtrl-sci↗

arXiv2026

Coulomb correlated multi-particle states of weakly confining GaAs quantum dots

We compute the electronic and emission properties of Coulomb-correlated multi-particle states (X$^0$, X$^\pm$, XX) in weakly confining GaAs/AlGaAs quantum dots using an 8-band $\mathbf{k}\!\cdot\!\mathbf{p}$ model coupled to continuum elasticity and configuration interaction (CI). We evaluate polarization-resolved oscillator strengths and radiative rates both in the dipole approximation (DA) and in a quasi-electrostatic beyond-dipole (BDA) longitudinal formulation implemented via a Poisson reformulation exactly equivalent to the dyadic Green-tensor kernel. For the dots studied, BDA yields lifetimes in quantitative agreement with experiment, e.g., $τ^X=0.279\,\mathrm{ns}$ vs $0.267\,\mathrm{ns}$ (exp.) and $τ^{XX}=0.101\,\mathrm{ns}$ vs $0.115\,\mathrm{ns}$ (exp.). The framework also reproduces electric-field tuning of the multi-particle electronic structure and emission-including the indistinguishability inferred from $P=τ^X/(τ^X+τ^{XX})$-and we assess sensitivity to CI-basis size and to electron-electron and hole-hole exchange.

cond-mat.mes-hall↗

arXiv2025

Combined tight-binding and configuration interaction study of unfolded electronic structure of G-color center in Si

We have theoretically studied the G-center in bulk silicon material using the empirical tight-binding model for calculations of unfolded band structures with configuration interaction correction for the exciton at $Γ$ point of the Brillouin zone. The G-center in B configuration (emissive) being a candidate structure as the telecom single- and entangled-photon source has two substitutional carbons and one interstitial atom embedded into the bulk in six equally possible configurations. Taking the advantage of the low computation effort of the tight-binding and unfolding approach, it is possible to calculate and analyze the behavior of a variety of the electronic configurations. Our tight-binding model is able to describe not only the behavior of the G-center in the silicon bulk but using the unfolding approach it can also pinpoint the contributions of different elements of the supercell on the final pseudo-band structure. Moreover, the configuration interaction correction with single-particle basis states computed by our unfolded tight-binding model predicts a very small fine-structure splitting of the ground state exciton both for bright and dark doublet in the studied system. That underscores the possibility of the silicon G-center to become a very good emitter of single and entangled photons for quantum communication and computation applications.

cond-mat.mes-hall↗

arXiv2023

Anomalous luminescence temperature dependence of (In,Ga)(As,Sb)/GaAs/GaP quantum dots overgrown by a thin GaSb capping layer for nanomemory applications

We study (In,Ga)(As,Sb)/GaAs quantum dots embedded in a GaP (100) matrix, which are overgrown by a thin GaSb capping layer with variable thickness. Quantum dot samples are studied by temperature-dependent photoluminescence, and we observe that the quantum dot emission shows anomalous temperature dependence,~i.e., increase of energy with temperature increase from 10~K to $\sim$70~K, followed by energy decrease for larger temperatures. With the help of fitting of luminescence spectra by Gaussian bands with energies extracted from eight band ${\bf k}\cdot{\bf p}$ theory with multiparticle corrections calculated using the configuration interaction method, we explain the anomalous temperature dependence as mixing of momentum direct and indirect exciton states. We also find that the ${\bf k}$-indirect electron-hole transition in type-I regime at temperatures $<70$~K is optically more intense than ${\bf k}$-direct. Furthermore, we identify a band alignment change from type-I to type-II for QDs overgrown by more than one monolayer of GaSb. Finally, we predict the retention time of (In,Ga)(As,Sb)/GaAs/AlP/GaP quantum dots capped with GaSb layers with varying thickness, for usage as storage units in the QD-Flash nanomemory concept and observe that by using only a 2~ML-thick GaSb capping layer, the projected storage time surpasses the non-volatility limit of 10 years.

cond-mat.mes-hall↗

arXiv2022

Excitonic fine structure of epitaxial Cd(Se,Te) on ZnTe type-II quantum dots

The structure of the ground state exciton of Cd(Se,Te) quantum dots embedded in ZnTe matrix is studied experimentally using photoluminescence spectroscopy and theoretically using ${\bf k}\cdot{\bf p}$ and configuration interaction methods. The experiments reveal a considerable reduction of fine-structure splitting energy of the exciton with increase of Se content in the dots. That effect is interpreted by theoretical calculations to originate due to the transition from spatially direct (type-I) to indirect (type-II) transition between electrons and holes in the dot induced by increase of Se. The trends predicted by the theory match those of the experimental results very well.The theory identifies that the main mechanism causing elevated fine-structure energy in particular in type-I dots is due to the multipole expansion of the exchange interaction. Moreover, the theory reveals that for Se contents in the dot $>0.3$, there exist also a {\bf peculiar type of confinement showing signatures of both type~I and type~II} and which exhibits extraordinary properties, such as almost purely light hole character of exciton and toroidal shape of hole states.

cond-mat.mes-hall↗

arXiv2022

Electric field induced tuning of electronic correlation in weakly confining quantum dots

We conduct a combined experimental and theoretical study of the quantum-confined Stark effect in GaAs/AlGaAs quantum dots obtained with the local droplet etching method. In the experiment, we probe the permanent electric dipole and polarizability of neutral and positively charged excitons weakly confined in GaAs quantum dots by measuring their light emission under the influence of a variable electric field applied along the growth direction. Calculations based on the configuration-interaction method show excellent quantitative agreement with the experiment and allow us to elucidate the role of Coulomb interactions among the confined particles and -- even more importantly -- of electronic correlation effects on the Stark shifts. Moreover, we show how the electric field alters properties such as built-in dipole, binding energy, and heavy-light hole mixing of multiparticle complexes in weakly confining systems, underlining the deficiencies of commonly used models for the quantum-confined Stark effect.

cond-mat.mes-hall↗

arXiv2021

Interplay between multipole expansion of exchange interaction and Coulomb correlation of exciton in colloidal II-VI quantum dots

We study the effect of Coulomb correlation on the emission properties of the ground state exciton in zincblende CdSe/ZnS core-shell and in wurtzite ZnO quantum dots. We validate our theory model by comparing results of computed exciton energies of CdSe/ZnS quantum dots to photoluminescence and scanning near-field optical microscopy measurements. We use that to estimate the diameter of the quantum dots using a simple model based on infinitely deep quantum well and compare the results with the statistics of the atomic force microscopy scans of CdSe/ZnS dots, obtaining excellent agreement. Thereafter, we compute the energy fine structure of exciton, finding striking difference between properties of zincblende CdSe/ZnS and wurtzite ZnO dots. While in the former the fine structure is dominated by the dipole terms of the multipole expansion of the exchange interaction, in the latter system that is mostly influenced by Coulomb correlation. Furthermore, the correlation sizeably influences also the exciton binding energy and emission radiative rate in~ZnO dots.

cond-mat.mes-hall↗

arXiv2021

On the importance of antimony for temporal evolution of emission from self-assembled (InGa)(AsSb)/GaAs quantum dots on GaP(001)

Understanding the carrier dynamics of nanostructures is the key for development and optimization of novel semiconductor nano-devices. Here, we study the optical properties and carrier dynamics of (InGa)(AsSb)/GaAs/GaP quantum dots (QDs) by means of non-resonant energy and temperature modulated time-resolved photoluminescence. Studying this material system is important in view of the ongoing implementation of such QDs for nano memory devices. Our set of structures contains a single QD layer, QDs overgrown by a GaSb capping layer, and solely a GaAs quantum well, respectively. Theoretical analytical models allow us to discern the common spectral features around the emission energy of 1.8 eV related to GaAs quantum well and GaP substrate. We observe type-I emission from QDs with recombination times between 2 ns and 10 ns, increasing towards lower energies. The distribution suggests the coexistence of momentum direct and indirect QD transitions. Moreover, based on the considerable tunability of the dots depending on Sb incorporation, we suggest their utilization as quantum photonic sources embedded in complementary metal-oxide-semiconductor (CMOS) platforms, since GaP is almost lattice-matched to Si. Finally, our analysis confirms the nature of the pumping power blue-shift of emission originating from the charged-background induced changes of the wavefunction topology.

cond-mat.mes-hall↗

arXiv2020

Theory of magneto-optical properties of neutral and charged excitons in GaAs/AlGaAs quantum dots

Detailed theoretical study of the magneto-optical properties of weakly confining GaAs/AlGaAs quantum dots is provided. We focus on the diamagnetic coefficient and the $g$-factor of the neutral and the charged excitonic states, respectively, and their evolution with various dot sizes for the magnetic fields applied along $[001]$ direction. For the calculations we utilize the combination of ${\bf k}\cdot{\bf p}$ and the configuration interaction methods. We decompose the theory into four levels of precision,~i.e., (i) single-particle electron and hole states, (ii) non-interacting electron-hole pair, (iii) electron-hole pair constructed from the ground state of both quasiparticles and interacting via the Coulomb interaction (i.e. with minimal amount of correlation), and (iv) that including the effect of correlation. The aforementioned approach allows us to pinpoint the dominant influence of various single- and multi-particle effects on the studied magneto-optical properties, allowing the characterization of experiments using models which are as simple as possible, yet retaining the detailed physical picture.

cond-mat.mes-hall↗

arXiv2019

Resolving the temporal evolution of line broadening in quantum emitters

Light emission from solid-state quantum emitters is inherently prone to environmental decoherence, which results in an inhomogeneous line broadening and in the deterioration of photon indistinguishability. Here we employ photon correlation Fourier spectroscopy (PCFS) to study the temporal evolution of such a broadening for the biexciton and exciton emission in resonantly driven GaAs quantum dots. Differently from previous experiments, the time scales we probe range from a few nanoseconds to milliseconds and, simultaneously, the spectral resolution we achieve can be as small as 2 $μ$eV. We find pronounced differences in the temporal evolution of the two lines, which we attribute to differences in their homogeneous linewidth and sensitivity to charge noise. We then analyze the effect of irradiation with additional white light, which reduces blinking at the cost of enhanced charge noise. Due to its robustness against experimental imperfections and its high temporal resolution and bandwidth, PCFS outperforms established spectroscopy techniques, such as Michelson interferometry. We discuss its practical implementation, its limitations, and the possibility to use it to estimate the indistinguishability of consecutively emitted single photons for applications in quantum communication and photonic-based quantum information processing.

cond-mat.mes-hall↗

arXiv2019

Electronic states of (InGa)(AsSb)/GaAs/GaP quantum dots

Detailed theoretical studies of the electronic structure of (InGa)(AsSb)/GaAs/GaP quantum dots are presented. This system is unique since it exhibits concurrently direct and indirect transitions both in real and momentum space and is attractive for applications in quantum information technology, showing advantages as compared to the widely studied (In,Ga)As/GaAs dots. We proceed from the inspection of the confinement potentials for ${\bf k}\neq 0$ and ${\bf k}= 0$ conduction and ${\bf k}= 0$ valence bands, through the formulation of ${\bf k}\cdot{\bf p}$ calculations for ${\bf k}$-indirect transitions, up to the excitonic structure of $Γ$-transitions. Throughout this process we compare the results obtained for dots on both GaP and GaAs substrates enabling us to make a direct comparison to the (In,Ga)As/GaAs quantum dot system. We also discuss the realization of quantum gates.

cond-mat.mes-hall↗

arXiv2019

Single-Particle-Picture Breakdown in laterally weakly confining GaAs Quantum Dots

We present a detailed investigation of different excitonic states weakly confined in single GaAs/AlGaAs quantum dots obtained by the Al droplet-etching method. For our analysis we make use of temperature-, polarization- and magnetic field-dependent $μ$-photoluminescence measurements, which allow us to identify different excited states of the quantum dot system. Besides that, we present a comprehensive analysis of g-factors and diamagnetic coefficients of charged and neutral excitonic states in Voigt and Faraday configuration. Supported by theoretical calculations by the Configuration interaction method, we show that the widely used single-particle Zeeman Hamiltonian cannot be used to extract reliable values of the g-factors of the constituent particles from excitonic transition measurements.

cond-mat.mes-hall↗

arXiv2019

Optical response of (InGa)(AsSb)/GaAs quantum dots embedded in a GaP matrix

The optical response of (InGa)(AsSb)/GaAs quantum dots (QDs) grown on GaP (001) substrates is studied by means of excitation and temperature-dependent photoluminescence (PL), and it is related to their complex electronic structure. Such QDs exhibit concurrently direct and indirect transitions, which allows the swapping of $Γ$ and $L$ quantum confined states in energy, depending on details of their stoichiometry. Based on realistic data on QD structure and composition, derived from high-resolution transmission electron microscopy (HRTEM) measurements, simulations by means of $\mathbf{k\cdot p}$ theory are performed. The theoretical prediction of both momentum direct and indirect type-I optical transitions are confirmed by the experiments presented here. Additional investigations by a combination of Raman and photoreflectance spectroscopy show modifications of the hydrostatic strain in the QD layer, depending on the sequential addition of QDs and capping layer. A variation of the excitation density across four orders of magnitude reveals a 50 meV energy blueshift of the QD emission. Our findings suggest that the assignment of the type of transition, based solely by the observation of a blueshift with increased pumping, is insufficient. We propose therefore a more consistent approach based on the analysis of the character of the blueshift evolution with optical pumping, which employs a numerical model based on a semi-self-consistent configuration interaction method.

cond-mat.mes-hall↗

arXiv2018

Effect of second order piezoelectricity on excitonic structure of stress-tuned InGaAs/GaAs quantum dots

We study the effects of the nonlinear piezoelectricity and the In distribution on the exciton energy, the electron-hole electric dipole moment, and the fine-structure splitting in stress-tunable InGaAs/GaAs quantum dots integrated onto a piezoelectric actuator. In particular, we investigate in detail the contributions of various elements of the expansion of the electrical polarization in terms of externally induced elastic strain on the latter two important quantum dot properties. Based on the comparison of the effects of first- and second-order piezoelectricity we provide a simple relation to estimate the influence of applied anisotropic stress on the quantum dot dipole moment for quantum dots significantly lattice mismatched to the host crystal.

cond-mat.mes-hall↗

arXiv2017

Inversion of the exciton built-in dipole moment in In(Ga)As quantum dots via nonlinear piezoelectric effect

We show that anisotropic biaxial stress can be used to tune the built-in dipole moment of excitons confined in In(Ga)As quantum dots up to complete erasure of its magnitude and inversion of its sign. We demonstrate that this phenomenon is due to piezoelectricity. We present a model to calculate the applied stress, taking advantage of the so-called piezotronic effect, which produces significant changes in the current-voltage characteristics of the strained diode-membranes containing the quantum dots. Finally, self-consistent k.p calculations reveal that the experimental findings can be only accounted for by the nonlinear piezoelectric effect, whose importance in quantum dot physics has been theoretically recognized although it has proven difficult to single out experimentally.

cond-mat.mes-hall↗

arXiv2016

Excitonic structure and pumping power dependent emission blue-shift of type-II quantum dots

In this work we study theoretically and experimentally the multi-particle structure of the so-called type-II quantum dots with spatially separated electrons and holes. Our calculations based on customarily developed full configuration interaction approach reveal that exciton complexes containing holes interacting with two or more electrons exhibit fairly large antibinding energies. This effect is found to be the hallmark of the type-II confinement. In addition, an approximate self-consistent solution of the multi-exciton problem allows us to explain two pronounced phenomena: the blue-shift of the emission with pumping and the large inhomogenous spectral broadening, both of those eluding explanation so far. The results are confirmed by detailed intensity and polarization resolved photoluminescence measurements on a number of type-II samples.

cond-mat.mes-hall↗