arXiv subjects

Fedor Šimkovic

Publications and source records attributed to Fedor Šimkovic.

16 recordsLinked to original sources

arXiv2026

Noise-Robust Estimation of Quantum Observables in Noisy Hardware

Error mitigation is essential for extracting reliable results from quantum computations performed on noisy intermediate-scale quantum hardware. Here we introduce Noise-Robust Estimation (NRE), a noise-agnostic framework that suppresses estimation bias through a two-stage post-processing protocol. The method combines measurement data from a target circuit and a corresponding noise-canceling companion circuit to construct a baseline estimator with reduced sensitivity to noise. We show that the residual bias of this estimator is governed by the variation of an auxiliary quantity across amplified noise realizations, motivating the use of a measurable diagnostic quantity: the normalized dispersion of this auxiliary estimator. When the dispersion approaches zero, contributions arising from imperfect noise amplification vanish and the remaining bias terms are expected to diminish for smooth stationary noise profiles. Leveraging this relationship, NRE performs a final extrapolation to the zero-dispersion limit using bootstrapped measurement data. We experimentally validate the method on a 20-qubit IQM superconducting quantum processor using circuits containing up to 480 entangling CZ gates. Across a variety of circuits and noise levels, NRE consistently achieves substantially reduced bias compared to existing mitigation techniques while maintaining moderate sampling overhead. These results establish NRE as a practical and broadly applicable error-mitigation strategy for quantum computations on noisy hardware.

quant-ph↗

arXiv2025

A Semi-Empirical Formula for Two-Neutrino Double-Beta Decay

We propose a semi-empirical formula (SEF) for calculating the nuclear matrix elements (NMEs) for two-neutrino double-beta decay. The SEF's dependence on the proton and neutron numbers, the pairing, isospin, and deformation properties of the initial and final nuclei is inspired by the insights offered by nuclear many-body methods and trends in experimental data. Compared with the previous phenomenological and nuclear models, the SEF yields the best agreement with the experimental NMEs. Its stability and predictive power are cross-validated, and predictions are provided for nuclear systems of experimental interest.

nucl-th↗

arXiv2024

Radiative and exchange corrections for two-neutrino double-beta decay

We investigate the impact of radiative and atomic exchange corrections in the two-neutrino double-beta ($2νββ$)-decay of $^{100}$Mo. In the calculation of the exchange correction, the electron wave functions are obtained from a modified Dirac-Hartree-Fock-Slater self-consistent framework that ensures orthogonality between continuum and bound states. The atomic exchange correction causes a steep increase in the low-energy region of the single-electron spectrum, consistent with previous studies on $β$-decay, while the radiative correction primarily accounts for a 5\% increase in the decay rate of $^{100}$Mo. When combined, the radiative and exchange effects cause a leftward shift of approximately 10 keV in the maximum of the summed electron spectrum. This shift may impact current constraints on parameters governing potential new physics scenarios in $2νββ$-decay. The exchange and radiative corrections are introduced on top of our previous description of $2νββ$-decay, where we used a Taylor expansion for the lepton energy parameters within the nuclear matrix elements denominators. This approach results in multiple components for each observable, controlled by the measurable $ξ_{31}$ and $ξ_{51}$ parameters. We explore the effects of different $ξ_{31}$ and $ξ_{51}$ values, including their experimental measurements, on the total corrected spectra. These refined theoretical predictions can serve as precise inputs for double-beta decay experiments investigating standard and new physics scenarios within $2νββ$-decay.

nucl-th↗

arXiv2024

Nuclear shell model study of neutrinoless double beta decay under Left-Right symmetric model

We use the large scale nuclear shell model to calculate the nuclear matrix elements for the neutrino mediated neutrinoless double beta decay within the Left-Right symmetric model for four nuclei: $^{76}$Ge, $^{82}$Se, $^{130}$Te and $^{136}$Xe. We perform a systematic analysis on the general magnitude of different terms for related mechanisms. For the $η$ mechanism, we find that the weak magnetism $R$ term dominates the decay rate while the $p$-wave effect is suppressed. While for the $λ$ mechanism, the $ω$ and the $q$ terms are with equal importance. For the latter $q$ term, important contributions from weak-magnetism MM part are observed. Finally, we give the constraints on the new physics parameters $m_{ββ}$, $λ$ and $η$ from current experiments.

nucl-th↗

arXiv2024

Theoretical analysis and predictions for the double electron capture of $^{124}$Xe

We provide a complete theoretical description of the two-neutrino electron capture in $^{124}$Xe, improving both the nuclear and the atomic structure calculations. We improve the general formalism through the use of the Taylor expansion method, leading to higher order terms in the decay rate of the process. The nuclear part is treated with pn-QRPA and interacting shell model (ISM) methods. The nuclear matrix elements (NMEs) are calculated with the pn-QRPA method with spin restoration by fixing the input parameters so that the experimental decay rate is reproduced, resulting in values significantly lower than in previous calculations. The validity of the pn-QRPA NMEs is tested by showing their values to be comparable with the ones for double-beta decay with emission of two electrons of $^{128,130}$Te, which have similar pairing features. Within the ISM, we reproduce the total experimental half-life within a factor of two and predict the capture fraction to the KK channel of about 74\%. We also predict the capture fractions to other decay channels and show that for the cumulative decay to the $\rm{KL_{1}}$-$\rm{KO_{1}}$ channels, a capture fraction of about 24\% could be observed experimentally. On the atomic side, calculations are improved by accounting for the Pauli blocking of the decay of innermost nucleon states and by considering all $s$-wave electrons available for capture, expanding beyond the K and L$_1$ orbitals considered in previous studies. We also provide improved atomic relaxation energies of the final atomic states of $^{124}$Te, which may be used as input for background modeling in liquid Xenon experiments.

nucl-th↗

arXiv2021

Fast Principal Minor Algorithms for Quantum Many Body Systems

The computation of determinants plays a central role in diagrammatic Monte Carlo algorithms for strongly correlated systems. The evaluation of large numbers of determinants can often be the limiting computational factor determining the number of attainable diagrammatic expansion orders. In this work we build upon the algorithm presented in [\emph{Linear Algebra and its applications} 419.1 (2006), 107-124] which computes all principal minors of a matrix in $O(2^n)$ operations. We present multiple generalizations of the algorithm to the efficient evaluation of certain subsets of all principal minors with immediate applications to Connected Determinant Diagrammatic Monte Carlo within the normal and symmetry-broken phases as well as Continuous-time Quantum Monte Carlo. Additionally, we improve the asymptotic scaling of diagrammatic Monte Carlo formulated in real-time to $O(2^n)$ and report speedups of up to a factor $25$ at computationally realistic expansion orders. We further show that all permanent-principal-minors, corresponding to sums of bosonic Feynman diagrams, can be computed in $O(3^n)$, making diagrammatic Monte Carlo for bosonic and mixed systems a viable path worth exploring.

cond-mat.str-el↗

arXiv2021

Two-Neutrino Double Beta Decay with Sterile Neutrinos

Usually considered a background for experimental searches for the hypothetical neutrinoless double beta decay process, two-neutrino double beta decay nevertheless provides a complementary probe of physics beyond the Standard Model. In this paper we investigate how the presence of a sterile neutrino, coupled to the Standard Model either via a left-handed or right-handed current, affects the energy distribution and angular correlation of the outgoing electrons in two-neutrino double beta decay. We pay particular attention on the behaviour of the energy distribution at the kinematic endpoint and we estimate the current limits on the active-sterile mixing and effective right-handed coupling using current experimental data as a function of the sterile neutrino mass. We also investigate the sensitivities of future experiments. Our results complement the corresponding constraints on sterile neutrinos from single beta decay measurements in the 0.1 - 10 MeV mass range.

hep-ph↗

arXiv2021

Many-Configuration Markov-Chain Monte Carlo

We propose a minimal generalization of the celebrated Markov-Chain Monte Carlo algorithm which allows for an arbitrary number of configurations to be visited at every Monte Carlo step. This is advantageous when a parallel computing machine is available, or when many biased configurations can be evaluated at little additional computational cost. As an example of the former case, we report a significant reduction of the thermalization time for the paradigmatic Sherrington-Kirkpatrick spin-glass model. For the latter case, we show that, by leveraging on the exponential number of biased configurations automatically computed by Diagrammatic Monte Carlo, we can speed up computations in the Fermi-Hubbard model by two orders of magnitude.

physics.comp-ph↗

arXiv2020

Searching for New Physics in Two-Neutrino Double Beta Decay

Motivated by non-zero neutrino masses and the possibility of New Physics discovery, a number of experiments search for neutrinoless double beta decay. While hunting for this hypothetical nuclear process, a significant amount of two-neutrino double beta decay data has become available. Although these events are regarded and studied mostly as the background of neutrinoless double beta decay, they can be also used to probe physics beyond the Standard Model. In this paper we show how the presence of right-handed leptonic currents would affect the energy distribution and angular correlation of the outgoing electrons in two-neutrino double beta decay. Consequently, we estimate constraints imposed by currently available data on the existence of right-handed neutrino interactions without having to assume their nature. In this way our results complement the bounds coming from the non-observation of neutrinoless double beta decay as they limit also the exotic interactions of Dirac neutrinos. We perform a detailed calculation of two-neutrino double beta decay under the presence of exotic (axial-)vector currents and we demonstrate that current experimental searches can be competitive to existing limits.

hep-ph↗

arXiv2018

$0νββ$ nuclear matrix elements, neutrino potentials and $\mathrm{SU}(4)$ symmetry

Intimate relation between the Gamow-Teller part of the matrix element $M^{0ν}_\mathrm{GT}$ and the $2νββ$ closure matrix element $M^{2ν}_\mathrm{cl}$ is explained and explored. If the corresponding radial dependence $C^{2ν}_\mathrm{cl}(r)$ would be known, $M^{0ν}$ corresponding to any mechanism responsible for the $0νββ$ decay can be obtained as a simple integral. However, the $M^{2ν}_\mathrm{cl}$ values sensitively depend on the properties of higher lying $1^+$ states in the intermediate odd-odd nuclei. We show that the $β^-$ and $β^+$ amplitudes of such states typically have opposite relative signs, and their contributions reduce severally the $M^{2ν}_\mathrm{cl}$ values. Vanishing values of $M^{2ν}_\mathrm{cl}$ are signs of a partial restoration of the spin-isospin $\mathrm{SU}(4)$ symmetry. We suggest that demanding that $M^{2ν}_\mathrm{cl}$ = 0 is a sensible way, within the method of the Quasi-particle Random Phase Approximation (QRPA), of determining the amount of renormalization of isoscalar particle-particle interaction strength $g^{T=0}_{pp}$. Using such prescription, the matrix elements $M^{0ν}$ are evaluated; their values are not very different ($\le$ 20\%) from the usual QRPA values when $g^{T=0}_{pp}$ is related to the known $2νββ$ half-lives.

nucl-th↗

arXiv2018

Improved description of the $2νββ$-decay and a possibility to determine the effective axial-vector coupling constant

An improved formalism of the two-neutrino double-beta decay ($2νββ$-decay) rate is presented, which takes into account the dependence of energy denominators on lepton energies via the Taylor expansion. Till now, only the leading term in this expansion has been considered. The revised $2νββ$-decay rate and differential characteristics depend on additional phase-space factors weighted by the ratios of $2νββ$-decay nuclear matrix elements with different powers of the energy denominator. For nuclei of experimental interest all phase-space factors are calculated by using exact Dirac wave functions with finite nuclear size and electron screening. For isotopes with measured $2νββ$-decay half-life the involved nuclear matrix elements are determined within the quasiparticle random phase approximation with partial isospin restoration. The importance of correction terms to the $2νββ$-decay rate due to Taylor expansion is established and the modification of shape of single and summed electron energy distributions is discussed. It is found that the improved calculation of the $2νββ$-decay predicts slightly suppressed $2νββ$-decay background to the neutrinoless double-beta decay signal. Further, a novel approach to determine the value of effective weak-coupling constant in nuclear medium $g^{\rm eff}_{\rm A}$ is proposed.

nucl-th↗

arXiv2018

The $λ$ mechanism of the $0νββ$-decay

The $λ$ mechanism ($W_L$-$W_R$ exchange) of the neutrinoless double beta decay ($0νββ$-decay), which has origin in left-right symmetric model with right-handed gauge boson at TeV scale, is investigated. The revisited formalism of the $0νββ$-decay, which includes higher order terms of nucleon current, is exploited. The corresponding nuclear matrix elements are calculated within quasiparticle random phase approximation with partial restoration of the isospin symmetry for nuclei of experimental interest. A possibility to distinguish between the conventional light neutrino mass ($W_L$-$W_L$ exchange) and $λ$ mechanisms by observation of the $0νββ$-decay in several nuclei is discussed. A qualitative comparison of effective lepton number violating couplings associated with these two mechanisms is performed. By making viable assumption about the seesaw type mixing of light and heavy neutrinos with the value of Dirac mass $m_D$ within the range $1~ \textrm{MeV} < m_D < 1~ \textrm{GeV}$, it is concluded that there is a dominance of the conventional light neutrino mass mechanism in the decay rate.

hep-ph↗

arXiv2018

The $0νββ$-decay nuclear matrix element for light and heavy neutrino mass mechanisms from deformed QRPA cacluations for $^{76}$Ge, $^{82}$Se, $^{130}$Te, $^{136}$Xe and $^{150}$Nd with isospin restoration

In this work, with restored isospin symmetry, we evaluated the neutrinoless double beta decay nuclear matrix elements for $^{76}$Ge, $^{82}$Se, $^{130}$Te, $^{136}$Xe and $^{150}$Nd for both the light and heavy neutrino mass mechanisms using the deformed QRPA approach with realistic forces. We give detailed decompositions of the nuclear matrix elements over different intermediate states and nucleon pairs, and discuss how these decompositions are affected by the model space truncations. Compared to the spherical calculations, our results show reductions from $30\%$ to about $60\%$ of the nuclear matrix elements for the calculated isotopes mainly due to the presence of BCS overlap factor between the initial and final ground states. The comparison between different nucleon-nucleon forces with corresponding Short-Range-Correlations (src) shows, that the choice of the NN force gives roughly $20\%$ deviations for light exchange neutrino mechanism and much larger deviations for the heavy neutrino exchange mechanism.

nucl-th↗

arXiv2013

$0νββ$ and $2νββ$ nuclear matrix elements, QRPA, and isospin symmetry restoration

Within QRPA we achieve partial restoration of the isospin symmetry and hence fulfillment of the requirement that the $2νββ$ Fermi matrix element $M^{2ν}_F$ vanishes, as it should, unlike in the previous version of the method. This is accomplished by separating the renormalization parameter $g_{pp}$ of the particle-particle proton-neutron interaction into the isovector and isoscalar parts. The isovector parameter $g_{pp}^{T=1}$ need to be chosen to be essentially equal to the pairing constant $g_{pair}$, so no new parameter is needed. For the $0νββ$ decay the Fermi matrix element $M^{0ν}_F$ is substantially reduced, while the full matrix element $M^{0ν}$ is reduced by $\approx$ 10%. We argue that this more consistent approach should be used from now on in the proton-neutron QRPA and in analogous methods.

nucl-th↗

arXiv2011

Relation between the $2νββ$ and $0νββ$ nuclear matrix elements

A formal relation between the GT part of the nuclear matrix elements $M^0ν}_{GT}$ of $0νββ$ decay and the closure matrix elements $M^{2ν}_{cl}$ of $2νββ$ decay is established. This relation is based on the integral representation of these quantities in terms of their dependence on the distance $r$ between the two nucleons undergoing transformation. We also discuss the difficulties in determining the correct values of the closure $2νββ$ decay matrix elements.

nucl-th↗

arXiv2010

Relation between the $0νββ$ and $2νββ$ nuclear matrix elements revisited

We show that the dominant Gamow-Teller part, $M^{0ν}_{GT}$, of the nuclear matrix element governing the neutrinoless $ββ$ decay is related to the matrix element $M^{2ν}_{cl}$ governing the allowed two-neutrino $ββ$ decay. That relation is revealed when these matrix elements are expressed as functions of the relative distance $r$ between the pair of neutrons that are transformed into a pair of protons in the $ββ$ decay. Analyzing this relation allows us to understand the contrasting behavior of these matrix elements when $A$ and $Z$ is changed; while $M^{0ν}_{GT}$ changes slowly and smoothly, $M^{2ν}$ has pronounced shell effects. We also discuss the possibility of phenomenological determination of the $M^{2ν}_{cl}$ and from them of the $M^{0ν}_{GT}$ values from the experimental study of the $β^{\pm}$ strength functions.

nucl-th↗