arXiv subjects

Peter Schmitteckert

Publications and source records attributed to Peter Schmitteckert.

At least 19 recordsLinked to original sources

arXiv2026

Interaction-driven dynamics in graphene flakes as a benchmark for quantum simulation

We study interaction-driven ultrafast dynamics in finite graphene flakes following an optical pump quench in an interacting tight-binding model. By comparing exact real-time evolution with simulations restricted to particle-hole excitation subspaces, we assess when relaxation can be captured by low-order many-body processes and when this is not sufficient. The single-particle orbital entropy provides a compact diagnostic for dynamic correlation growth. For the systems studied here, periodic graphene flakes are well described by low-order excitations, whereas confined geometries require substantial higher-order contributions even for relatively small interaction strengths. The quench protocol combines simple initial-state preparation with strongly correlated dynamics, identifying a promising benchmark problem for future quantum-computing simulations.

cond-mat.str-el↗

arXiv2026

Quantum Simulation of Magnetic Materials: from Ab-Initio to NISQ

Quantum computers are increasingly accessible, yet demonstrations of physically meaningful simulations for real materials remain scarce. In our work we simulate low-energy magnetic excitations, specifically spin-wave spectra, of chromium tri-halide monolayers. Starting from ab-initio electronic structure calculations for these two-dimensional magnets, we derive an effective spin model and simulate low-energy spin excitations using a real-time propagation of the spin system on the commercial quantum computing cloud platform IQM Resonance. The results for systems with up to 48 qubits are validated against classical benchmarks. While some spectral features remain challenging for today's NISQ devices, our simulation achieves good agreement at quasi-constant wall-time scaling, compared to the exponential scaling of classical methods. Our results demonstrate that, even in the absence of quantum advantage, useful quantum simulations of real materials are becoming possible for domain experts via commercial cloud access to quantum computers.

quant-ph↗

arXiv2026

System-bath model for quantum chemistry

We propose an approximate mapping of a molecular Hamiltonian to a Hamiltonian of qubits, which allows for high accuracy quantum chemistry calculations of vertical excitation energies of some molecules. The mapping is based on separating of a very small active space of only two orbitals and on modeling the electronic excitations in the remaining orbitals by a set of qubits or, equivalently, by a set of oscillators. This approach is inspired by the Random Phase Approximation (RPA), in which the excitations of electron gas are described by bosonic degrees of freedom. As a result, the Hamiltonian of the molecule is reduced to that of a system-bath model. The "system" part of the Hamiltonian describes the two molecular orbitals -- the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) -- which are populated by two electrons. Two qubits are sufficient to encode the Hamiltonian of such a system. The "bath" consists of oscillators or, equivalently, of two level systems with each of them corresponding to an electron excitation from a doubly occupied orbital below the Fermi level to an empty orbital above the Fermi level. We hope that this mapping can inspire new approaches and algorithms aimed at calculating excitation energies of molecules on near term quantum computers.

quant-ph↗

arXiv2025

Can a Quantum Computer Simulate Nuclear Magnetic Resonance Spectra Better than a Classical One?

The simulation of the spectra measured in nuclear magnetic resonance (NMR) spectroscopy experiments is a computationally non-trivial problem which, due to its natural interpretation as a quantum spin problem, maps in a straightforward way to a quantum computer. As such, it represents a problem for which such a device may provide some practical advantage over traditional computing methods. In order to understand the extent to which such problems may indeed provide examples of useful quantum advantage, it is important to understand the limitations of existing classical simulation methods. In this work, we benchmark our own classical solver designed to study such problems. This solver uses a clustering approximation to achieve a resource scaling which is linear in the total number of nuclear spins in a given molecule, for a fixed cluster size. The success of such an approximation would present a stark repudiation to the common claim that such problems require an exponential scaling of resources, the very claim which makes simulating an NMR spectra a candidate for quantum advantage. Our benchmarking results indicate that our approximation performs well throughout, and even somewhat beyond, the more typical experimental regimes. We discuss what implications this may have for future efforts to demonstrate quantum advantage in the context of NMR.

quant-ph↗

arXiv2025

A method to derive material-specific spin-bath model descriptions of materials displaying prevalent spin physics

Magnetism and spin physics are true quantum mechanical effects and their description usually requires multi reference methods and is often hidden in the standard description of molecules in quantum chemistry. In this work we present a twofold approach to the description of spin physics in molecules and solids. First, we present a method that identifies the single-particle basis in which a given subset of the orbitals is equivalent to spin degrees of freedom for models and materials which feature significant spin physics at low energies. We introduce a metric for the spin-like character of a basis orbital, of which the optimization yields the basis containing the optimum spin-like basis orbitals. Second, we demonstrate an extended Schrieffer-Wolff transformation method to derive the effective Hamiltonian acting on the subspace of the Hilbert space in which the charge degree of freedom of electron densities in the spin-like orbitals is integrated out. The method then yields an effective Hamiltonian describing spins coupled to a fermionic environment. This extended Schrieffer-Wolff transformation is applicable to a wide range of Hamiltonians and has been utilized in this work for model Hamiltonians as well as the Hamiltonian describing the active orbital space of molecular chromium bromide. This is achieved by reformulating the highly non-linear Schrieffer-Wolff equations into a linear set of equations corresponding to an operator basis.

cond-mat.str-el↗

arXiv2024

Generalized Josephson effect with arbitrary periodicity in quantum magnets

Easy-plane quantum magnets are strikingly similar to superconductors, allowing for spin supercurrent and an effective superconducting phase stemming from their $U(1)$ rotation symmetry around the $z$-axis. We uncover a generalized fractional Josephson effect with a periodicity that increases linearly with system size in one-dimensional spin-$1/2$ chains at selected anisotropies and phase-fixing boundary fields. The effect combines arbitrary integer periodicities in a single system, exceeding the $4\pi $ and $8\pi $ periodicity of superconducting Josephson effects of Majorana zero modes and other exotic quasiparticles. We reveal a universal energy-phase relation and connect the effect to the recently discovered phantom helices.

cond-mat.str-el↗

arXiv2024

Understanding Radicals via Orbital Parities

We introduce analysis of orbital parities as a concept and a tool for understanding radicals. Based on fundamental reduced one- and two-electron density matrices, our approach allows us to evaluate a total measure of radical character and provides spin-like orbitals to visualize real excess spin or odd electron distribution of singlet polyradicals. Finding spin-like orbitals aumotically results in their localization in the case of disjoint (zwitterionic) radicals and so enables radical classification based on spin-site separability. We demonstrate capabilities of the parity analysis by applying it to a number of polyradicals and to prototypical covalent bond breaking.

physics.chem-ph↗

arXiv2023

Applicability and limitations of cluster perturbation theory for Hubbard models

We present important use cases and limitations when considering results obtained from Cluster Perturbation Theory (CPT). CPT combines the solutions of small individual clusters of an infinite lattice system with the Bloch theory of conventional band theory in order to provide an approximation for the Green's function in the thermodynamic limit. To this end we are investigating single-band and multi-band Hubbard models in one- and two-dimensional systems. A special interest is taken in the supposed pseudo gap regime of the two-dimensional square lattice at half filling and intermediate interaction strength ($U \leq 3t$) as well as the metal-insulator transition. We point out that the finite-size level spacing of the cluster limits the resolution of spectral features within CPT. This restricts the investigation of asymptotic properties of the metal-insulator transition, as it would require much larger cluster sizes that are beyond computational capabilities.

cond-mat.str-el↗

arXiv2022

Local density of states of the interacting resonant level model at zero temperature

We present results of the impurity local density of states of the interacting resonant level model at zero temperature. We concentrate on low-energy properties and predominantly use the numerical renormalisation group technique. As interaction is increased, we find that the resonance peak at zero energy disappears, while two new peaks at finite energy emerge. This is in the absence of any field breaking the resonance. We further show that the height of the spectral function does not scale in the same way as the width, and in fact defines a second distinct exponent. We back up our results with analytic strong-coupling calculations as well as an analytic diagrammatic renormalisation group calculation that rather surprisingly gets the second exponent exactly, even for strong interactions.

cond-mat.str-el↗

arXiv2020

Observation of separated dynamics of charge and spin in the Fermi-Hubbard model

Strongly correlated quantum systems give rise to many exotic physical phenomena, including high-temperature superconductivity. Simulating these systems on quantum computers may avoid the prohibitively high computational cost incurred in classical approaches. However, systematic errors and decoherence effects presented in current quantum devices make it difficult to achieve this. Here, we simulate the dynamics of the one-dimensional Fermi-Hubbard model using 16 qubits on a digital superconducting quantum processor. We observe separations in the spreading velocities of charge and spin densities in the highly excited regime, a regime that is beyond the conventional quasiparticle picture. To minimize systematic errors, we introduce an accurate gate calibration procedure that is fast enough to capture temporal drifts of the gate parameters. We also employ a sequence of error-mitigation techniques to reduce decoherence effects and residual systematic errors. These procedures allow us to simulate the time evolution of the model faithfully despite having over 600 two-qubit gates in our circuits. Our experiment charts a path to practical quantum simulation of strongly correlated phenomena using available quantum devices.

quant-ph↗

arXiv2020

Learning DFT

We present an extension of reverse engineered Kohn-Sham potentials from a density matrix renormalization group calculation towards the construction of a density functional theory functional via deep learning. Instead of applying machine learning to the energy functional itself, we apply these techniques to the Kohn-Sham potentials. To this end we develop a scheme to train a neural network to represent the mapping from local densities to Kohn-Sham potentials. Finally, we use the neural network to up-scale the simulation to larger system sizes.

cond-mat.dis-nn↗

arXiv2020

High-temperature spin dynamics in the Heisenberg chain: Magnon propagation and emerging KPZ-scaling in the zero-magnetization limit

The large-scale dynamics of quantum integrable systems is often dominated by ballistic modes due to the existence of stable quasi-particles. We here consider as an archetypical example for such a system the spin-$\frac{1}{2}$ XXX Heisenberg chain that features magnons and their bound states. An interesting question, which we here investigate numerically, arises with respect to the fate of ballistic modes at finite temperatures in the limit of zero magnetization $m{=}0$. At a finite magnetization density $m$, the spin autocorrelation function $Π(x,t)$ (at high temperatures) typically exhibits a trimodal behavior with left- and right-moving quasi-particle modes and a broad center peak with slower dynamics. The broadening of the fastest propagating modes exhibits a sub-diffusive $t^{1/3}$ scaling at large magnetization densities, $m {\rightarrow} \frac{1}{2}$, familiar from non-interacting models; it crosses over into a diffusive scaling $t^{1/2}$ upon decreasing the magnetization to smaller values. The behavior of the center peak appears to exhibit a crossover from transient super-diffusion to ballistic relaxation at long times. In the limit $m{\to}0$, the weight carried by the propagating peaks tends to zero; the residual dynamics is carried only by the central peak; it is sub-ballistic and characterized by a dynamical exponent $z$ close to the value $\frac{3}{2}$ familiar from Kardar-Parisi-Zhang (KPZ) scaling. We confirm, employing elaborate finite-time extrapolations, that the spatial scaling of the correlator $Π$ is in excellent agreement with KPZ-type behavior and analyze the corresponding corrections.

cond-mat.stat-mech↗

arXiv2019

Exact equilibrium results in the Interacting Resonant Level Model

We present exact results for the susceptibility of the interacting resonant level model in equilibrium. Detailed simulations using both the Numerical Renormalization Group and Density Matrix Renormalization Group were performed in order to compare with closed analytical expressions. By first bosonizing the model and then utilizing the integrability of the resulting boundary sine-Gordon model, one finds an analytic expression for the relevant energy scale $T_K$ with excellent agreement to the numerical results. On the other hand, direct application of the Bethe ansatz of the interacting resonant level model does not correctly reproduce $T_K$ - however if the bare parameters in the model are renormalised, then quantities obtained via the direct Bethe ansatz such as the occupation of the resonant level as a function of the local chemical potential do match the numerical results. The case of one lead is studied in the most detail, with many results also extending to multiple leads, although there still remain open questions in this case.

cond-mat.str-el↗

arXiv2018

Inverse Mean Field Theory

In this work we discuss the extraction of mean field single particle Hamiltonians from a many body wave function of a fermionic system. It allows us to discuss the result of a many particle wave function in terms of a non-interacting description.

cond-mat.str-el↗

arXiv2018

Density matrix renormalization group for a highly degenerate quantum system: Sliding environment block approach

We present an infinite lattice DMRG sweeping procedure which can be used as a replacement for the standard infinite lattice blocking schemes. Although the scheme is generally applicable to any system, its main advantages are the correct representation of commensurability issues and the treatment of degenerate systems. As an example we apply the method to a spin chain featuring a highly degenerate ground state space where the new sweeping scheme provides an increase in performance as well as accuracy by many orders of magnitude compared to a recently published work.

cond-mat.str-el↗

arXiv2018

Towards verified numerical renormalization group calculations

Numerical approaches are an important tool to study strongly correlated quantum systems. However, their fragility with respect to rounding errors is not well studied and numerically verified enclosures of the results are not available. In this work we apply interval arithmetic to the well established numerical renormalization group scheme. This extension enables us to provide a numerically verified NRG excitation spectrum.

cond-mat.str-el↗

arXiv2018

A discrete energy space induced fermion parity breaking fixed point of the Kondo model

In this work we combine the well established Kondo problem with the more speculative field of a discrete space time. We show that a discrete energy space induces a flow towards a new fixed point by breaking the conservation of charge and spin and lifting the fermion parity. This parity lifting fixed point appears on a scale set by the discretization of energy space. In contrast to the Planck scale the associated energy scale is at very low energy scales, possibly given by the inverse size of the universe.

cond-mat.str-el↗

arXiv2017

Braiding errors in interacting Majorana quantum wires

Avenues of Majorana bound states (MBSs) have become one of the primary directions towards a possible realization of topological quantum computation. For a Y-junction of Kitaev quantum wires, we numerically investigate the braiding of MBSs while considering the full quasi-particle background. The two central sources of braiding errors are found to be the fidelity loss due to the incomplete adiabaticity of the braiding operation as well as the hybridization of the MBS. The explicit extraction of the braiding phase in the low-energy Majorana sector from the full many-particle Hilbert space allows us to analyze the breakdown of the independent-particle picture of Majorana braiding. Furthermore, we find nearest-neighbor interactions to significantly affect the braiding performance to the better or worse, depending on the sign and magnitude of the coupling.

cond-mat.mes-hall↗