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arXiv:2610.37589

Dynamical quantum simulations as laboratories for quantum information

Su Yeon Chang, Hailey S. Murray, Brian Barch, Greeshma Oruganti, Luis Pedro García-Pintos, Martin Larocca, Mark M. Wilde, M. Cerezo

quant-phcond-mat.mtrl-sciSubmitted Oct 27, 2026• Announced Oct 28, 2026• 0 citations
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Abstract

Dynamical quantum simulations are commonly designed to access a particular response function or correlation function. Yet the underlying protocol prepares a family of many-body states that contains information well beyond the observable for which the simulation was originally designed. In this work, we explore the idea of using these states as laboratories for quantum information. As a case study, we consider the local-pulse protocols used to compute Green functions in finite XX and XXX spin chains, and we interpret opposite source rotations as encoding a classical bit into the many-body system. This allows us to follow how the resulting record propagates, how it is encoded across local subsystems, and how readily it can be accessed with different measurements. We also find a clear separation between representation and readout. While fermionic covariance data exactly reconstruct the signed response in the XX chain, simple spin measurements recover a larger fraction of the locally available information in the XXX chain. This reversed ordering persists when we move from finite-pulse discrimination to local source-angle estimation. Going beyond the source record, the same reduced states also allow us to reinterpret results in terms of Bell state recovery and quantum communication rates, as well as providing direct access to the evolving bipartite and genuine multipartite entanglement of the system. Our results show that dynamical quantum simulations developed for response calculations can naturally be used to ask a much broader set of quantum-information questions about the many-body states they already prepare.

AI Summary

The paper asks what quantum-information properties can be extracted from states already prepared by dynamical response simulations. It studies opposite local $z$ quarter-turns applied to ground states of open, 50-spin XX and XXX chains, interpreting the pulse sign as a classical bit. Matrix-product-state simulations provide reduced density matrices on blocks of up to seven spins, analyzed using trace distance, Holevo information, restricted measurements, Fisher information, and entanglement measures. Larger blocks retain source information when single-site responses are small. Multisite records become noncommuting and exhibit joint information across separated flanks, including separable two-qubit examples. The main comparison separates representation from readout: covariance matrices reconstruct XX branches exactly, whereas the median normalized signed-response error reaches about $0.70$ for seven-site XXX blocks. Nevertheless, optimized product-Pauli measurements recover median fractions $0.660$ versus $0.605$ of optimal discrimination bias, and $0.370$ versus $0.307$ of Holevo information, in XXX versus XX. This ordering survives signal-strength matching; angle-estimation comparisons favor XXX after matching local quantum Fisher information, although raw trajectories can favor XX. A coherent-control extension gives exact Bell-recovery fidelity $F_{\mathrm{Bell}}=(1+\sqrt{F(\rho_+^{E},\rho_-^{E})})/2$ and, using degradability and spin-flip symmetry, quantum capacity $Q=1-\chi_E$, with the receiver retaining the control and the environment complement. Five-site states remain genuinely multipartite entangled while internal entanglement generally decreases and block–exterior entropy increases. The numerical evidence concerns these finite chains and small blocks, not scaling laws. Recoveries allow unrestricted joint operations; small-block branch data alone do not determine complementary-region recovery. Mixed preparations or noise require including additional environments.

Likely largely AI-generatedAI use disclosedIncrementalUnclear novelty
The authors acknowledge the use of Codex and ChatGPT 5.6 Sol for code development and for writing and reviewing the manuscript.

Error found: The abstract says the reversed readout ordering persists for source-angle estimation without specifying that the demonstrated advantage for XXX is a matched-local-QFI aggregate comparison at theta=0 for widths three and five; the displayed unmatched angle-dependent trajectory generally favors XX.

Written by gpt-6.1-sol from the paper's full text. Automated: it can be wrong.

Comments
23+23 pages, 13+11 figures, 1 table

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The paper asks what quantum-information properties can be extracted from states already prepared by dynamical response simulations. It studies opposite local zzz quarter-turns applied to ground states of open, 50-spin XX and XXX chains, interpreting the pulse sign as a classical bit. Matrix-product-state simulations provide reduced density matrices on blocks of up to seven spins, analyzed using trace distance, Holevo information, restricted measurements, Fisher information, and entanglement measures.
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A coherent-control extension gives exact Bell-recovery fidelity FBell=(1+F(ρ+E,ρ−E))/2F_{\mathrm{Bell}}=(1+\sqrt{F(\rho_+^{E},\rho_-^{E})})/2FBell​=(1+F(ρ+E​,ρ−E​)​)/2 and, using degradability and spin-flip symmetry, quantum capacity Q=1−χEQ=1-\chi_EQ=1−χE​, with the receiver retaining the control and the environment complement. Five-site states remain genuinely multipartite entangled while internal entanglement generally decreases and block–exterior entropy increases.
The numerical evidence concerns these finite chains and small blocks, not scaling laws. Recoveries allow unrestricted joint operations; small-block branch data alone do not determine complementary-region recovery. Mixed preparations or noise require including additional environments.
Quantum simulations, where controllable quantum systems are used to simulate other quantum systems, originally proposed by Richard Feynman, are one of the most remarkable applications of quantum information science. Compared to computation, quantum simulations require much less number of qubits for the m to be practical. In the work described in this thesis, we have performed a few quantum simulations of small quantum systems using Nuclear Magnetic Resonance(NMR) techniques. These simulations have been used to experimentally demonstrate the underlying interesting quantum protocols. All the experiments presented have been carried out using liquid-state or liquid crystal NMR. Numerical pulse optimization techniques have been utilized in some of the experiments, to achieve better control over the spin systems. The first chapter contains “Introduction” to quantum information processing, NMR, and numerical pulse optimization techniques. In chapter 2, we describe quantum simulation of a 3-spin Heisenberg-XY spin chain having only nearest neighbour interactions. Recently, spin chains having pre-engineered short-range interactions have been proposed to efficiently transfer quantum information between different parts of a quantum information processor. Other important proposals involving these spin chains include generating entangled states and universal quantum computation. However, such engineered interactions do not occur naturally in any system. In such a scenario, the experimental viability of these proposals can be tested by simulating the spin chains in other controllable quantum systems. In this work, we first theoretically study the time evolution of bipartite and tripartite entanglement measures for a 3-spin open ended XY spin chain. Then, by simulating the XY interactions in a 3-spin nuclear spin system, we experimentally generate, (i)a bipartite maximally(pseudo-)entangled state(Bell state) between end qubits, and(ii) multipartite(pseudo-)entangled states(Wand GHZ states),starting from separable pseudo-pure states. Bell state has been generated by using only the natural unitary evolution of the XY spin chain. W-state and GHZ-state have been generated by applying a single-qubit rotation to the second qubit, and a global rotation of all the three qubits respectively after the unitary evolution of the spin chain. In chapter 3, we simulate a 3-spin quantum transverse Ising spin system in a triangular configuration, and show that multipartite quantum correlations can be used to distinguish between the frustrated and non-frustrated regimes in the ground state of this spin system. The ground state of the spin system has been prepared by using adiabatic state preparation method. Gradient ascent pulse engineering technique has been utilized to efficiently realize the adiabatic evolution of the spin system. To analyse the experimental ground state of the system, we employ two different multipartite quantum correlation measures, generated from monogamy studies of bipartite quantum correlations. Chapter 4 contains a digital…
We present a symmetry-enabled direct quantum protocol for computing many-body Green's functions, a central tool for studying strongly correlated quantum systems. Our protocol relies only on native time evolution and straightforward measurements available on current hardware platforms. By exploiting parity symmetry—satisfied by a broad class of Hamiltonians in condensed matter physics and quantum chemistry, including the Fermi–Hubbard and Heisenberg models—we introduce a tailored quench spectroscopy scheme that recovers both the real and imaginary parts of two-point time correlators, from which Green's functions can be reconstructed via efficient classical signal processing. We further develop a tailored symmetric quantum Gibbs sampler that prepares parity-resolved (symmetric and antisymmetric) thermal states, enabling finite-temperature extensions within the same framework. Finally, we show that the same symmetry-based measurement primitive extends naturally to out-of-time-ordered correlators (OTOCs). Our results provide a practical route to estimating symmetry-resolved dynamical correlators on near-term and early fault-tolerant quantum hardware.
Establishing quantum simulations as reliable tools for materials research requires quantitative validation against experiments and, where available, classical many-body calculations. For dynamical spin response, such validation remains challenging because experiment, classical simulation, and quantum simulation do not produce the same native observables. This challenge is increasingly relevant as quantum simulation protocols for dynamical response progress from theory to hardware-level benchmarking against neutron scattering data. The longer-term goal is to extend validation to classically intractable regimes accessible through future fault-tolerant implementations. Here, we develop a cross-pipeline validation framework for quantum simulation of dynamical spin response, using inelastic neutron scattering and classical many-body simulation as complementary experimental and computational anchors. The framework brings these pipelines to common momentum- and energy-resolved comparison levels through explicit forward and inverse observable maps, propagates uncertainty via covariance matrices or resampling, tests robustness to structured distortion, and evaluates agreement using a hierarchy of complementary metric families. It distinguishes stochastic uncertainty from robustness-induced distortion, carries both through the comparison chain, and uses the resulting metric-level uncertainty and distortion information to support layered validation at the pipeline, solver, and model levels. We also introduce actuator-aware feedback logic to improve agreement without obscuring the physical origin of any remaining mismatch. We close by outlining future extensions of this methodology, including upstream uncertainty and distortion modeling, adaptive feedback, asymmetric validation beyond full classical benchmarking, fault-tolerant workflows, and community infrastructure for reproducible validation.
We present a symmetry-enabled direct quantum protocol for computing many-body Green's functions, a central tool for studying strongly correlated quantum systems. Our protocol relies only on native time evolution and straightforward measurements available on current hardware platforms. By exploiting parity symmetry -- satisfied by a broad class of Hamiltonians in condensed matter physics and quantum chemistry, including the Fermi--Hubbard and Heisenberg models -- we introduce a tailored quench spectroscopy scheme that recovers both the real and imaginary parts of two-point time correlators, from which Green's functions can be reconstructed via efficient classical signal processing. We further develop a tailored symmetric quantum Gibbs sampler that prepares parity-resolved (symmetric and antisymmetric) thermal states, enabling finite-temperature extensions within the same framework. Finally, we show that the same symmetry-based measurement primitive extends naturally to out-of-time-ordered correlators (OTOCs). Our results provide a practical route to estimating symmetry-resolved dynamical correlators on near-term and early fault-tolerant quantum hardware.
The field of quantum information processing seeks to transform modern information technology, with applications such as quantum computing and distributed quantum networks illustrating its potential. The key ingredient underlying these applications is entanglement, a purely quantum property that allows disjoint systems to become correlated beyond classical limitations. Entanglement enables core protocols including quantum teleportation and entanglement swapping, which are well studied in few-body settings. However, extending these concepts to many-body systems is non-trivial, and forms the primary focus of this thesis. Concretely, the first half of this thesis focuses on the characterization and quantification of entanglement in generic quantum settings. These ideas culminate in the formulation of a many-body entanglement swapping protocol, describing the redistribution of entanglement between extended systems that have no prior causal connection. To test this protocol, a classical simulation of up to 202020 qubits is carried out, and the resulting entanglement is analyzed using various quantities, most notably von Neumann entropy, mutual information, and observable spin correlation functions. The simulations realize entanglement swapping between causally disconnected many-body systems, induced by measurements on auxiliary subsystems. This is captured by an increase in post-measurement mutual information and spin correlation functions when spin-conserving systems are considered. When the spin conservation requirement is relaxed, observable spin correlations become suppressed while mutual information remains largely unchanged, highlighting the regimes where observable correlation measures remain informative probes of the underlying entanglement dynamics.
Local observables can lose sensitivity to an initial state during strongly interacting many-body evolution even though the global dynamics remain unitary. We show that this sensitivity can be enhanced through a time-dependent Hamiltonian readout. Two orthogonal product states are first evolved under a slanted-field Ising Hamiltonian, where their distinction becomes strongly suppressed as observed through several local observables, including subsystem magnetizations and correlation functions, and are then quenched to the transverse-field Ising model at a tunable time. Exact simulations of chains up to N=12N=12N=12 show that the optimized time-averaged separation after the switch exceeds the residual slanted-field baseline for every observable and system size tested. In the strongest channels, the standardized readout separation remains robust over the accessible size range, with no clear systematic suppression at larger NNN. The enhancement recurs in widely separated late-time windows and persists qualitatively for open boundaries. These results establish Hamiltonian switching as an observable-selective mechanism for enhancing initial-state sensitivity without time reversal or implying recovery of the full reduced state.