arXiv subjects

Charlie Nation

Publications and source records attributed to Charlie Nation.

17 recordsLinked to original sources

arXiv2026

Graph automorphisms to obtain Clifford symmetries in open and closed qudit models

In the recent article [arXiv:2605.18966], we demonstrated that finding Clifford symmetries can be mapped to a Graph Automorphism (GA) problem. Here, we provide an algorithm to obtain such symmetries on general qudit systems, that works on the principle of encoding Clifford invariants of a Hamiltonian onto properties of a graph. Labelling Hamiltonian terms as vertices, a permutation of such vertices that respects the Clifford invariants (a GA) is both a valid Clifford, and a symmetry up to phase correction checks. We test this on multiple physical models and discuss the scaling with respect to the number of qudits and Pauli strings, as well as various strategies for optimisation in different regimes. We further show that the graph automorphism representation of Clifford symmetries can be expanded to open quantum systems.

quant-ph↗

arXiv2026

Attention-based optimizer for symmetry finding

Finding symmetries is crucial for understanding physical models. In this work, we present an optimization framework that searches Pauli symmetries of Hamiltonians, merging the fields of machine learning with automated symmetry finding. Built on a Set-Transformer architecture, our framework uses self-attention to encode the pairwise and higher-order correlations among the Pauli-Strings. The relations are then decoded as a candidate, which is further optimized with a custom commutation-based objective, and mapped to a symmetry of the input Hamiltonian. We apply our method to random Pauli Hamiltonians, periodic one and two dimensional transverse-field Ising model and the Toric code. We show that for physical Hamiltonians (Ising and Toric), our framework succeeds with near-deterministic probability while providing substantial advantage compared to state-of-the-art strategies. For random Pauli Hamiltonians, we estimate the required computational resources, specifically the number of parallel starts and the number of GPUs, to find a symmetry with high success probability under fixed design specifications.

quant-ph↗

arXiv2026

Clifford symmetries in quantum many-body systems

Obtaining the symmetries of a model is a critical step towards developing an understanding and ultimately analytically or numerically solving the model. However, finding symmetries is generally extremely complicated, often being the result of insightful thinking. In this work, we complement human ingenuity with an algorithm. We leverage the classically efficient Clifford group to find symmetries for arbitrary many-body Hamiltonians via a graph representation. We demonstrate our method on random and physical Hamiltonians, with instances of up to one thousand qubits and demonstrate how our approach can provide deeper understanding of the model.

quant-ph↗

arXiv2026

Probing the ergodicity breaking transition via violations of random matrix theoretic predictions for local observables

Quantum many-body systems can exhibit distinct regimes where dynamics is either ergodic, dynamically exploring an extensive region of available state-space, or non-ergodic, where the dynamics may be restricted. An example is the many-body localization (MBL) transition, where disorder induces non-ergodic behaviour. Most measures of ergodicity notably rely on global quantities, such as level spacing statistics. We explore the ability for a subsystem to probe the ergodicity of dynamics via measurement of local observables, and use expected results from random matrix theory (RMT) as a benchmark for the ergodic regime. We exploit two predictions from RMT as ergodicity is broken: the time evolution of the quantum Fisher information, and a fluctuation-dissipation relation. These are investigated in three different ergodicity breaking mechanisms, namely, as a consequence of transition to integrability, MBL, and Quantum Many-Body Scars (QMBS). We show that the predicted behaviour from RMT can be used as a potential witness for transition to non-ergodic behaviour from the measurement of local observables alone.

quant-ph↗

arXiv2025

Analytical determination of multi-time correlation functions in quantum chaotic systems

The time-dependence of multi-point observable correlation functions are essential quantities in analysis and simulation of quantum dynamics. Open quantum systems approaches utilize two-point correlations to describe the influence of an environment on a system of interest, and in studies of chaotic quantum system, the out-of-time-ordered correlator (OTOC) is used to probe chaoticity of dynamics. In this work we analytically derive the time dependence of multi-point observable correlation functions in quantum systems from a random matrix theoretic approach, with the highest order function of interest being the OTOC. We find in each case that dynamical contributions are related to a simple function, related to the Fourier transform of coarse-grained wave-functions. We compare the predicted dynamics to exact numerical experiments in a spin chain for various physical observables. We comment on implications towards the emergence of Markovianity and quantum regression in closed quantum systems, as well as relate our results to known bounds on chaotic dynamics.

quant-ph↗

arXiv2025

Enhanced measurements on quantum computers via the simultaneous probing of non-commuting Pauli operators

Measuring the state of quantum computers is a highly non-trivial task, with implications for virtually all quantum algorithms. We propose a novel scheme where identical copies of a quantum state are measured jointly so that all Pauli operators within the considered observable can be simultaneously assessed. We use Bayesian statistics to accurately estimate the average and error, and develop an adaptive shot-allocation algorithm that preferentially samples the most uncertain Pauli terms. In regimes with many non-commuting Pauli operators, our ``double'' scheme can outperform the state-of-the-art measurement protocol in minimizing total shots for a given precision.

quant-ph↗

arXiv2024

Non-perturbative exciton transfer rate analysis of the Fenna-Matthews-Olson photosynthetic complex under reduced and oxidised conditions

Two-dimensional optical spectroscopy experiments have shown that exciton transfer pathways in the Fenna-Matthews-Olson (FMO) photosynthetic complex differ drastically under reduced and oxidised conditions, suggesting a functional role for collective vibronic mechanisms that may be active in the reduced form but attenuated in the oxidised state. Higgins et al. [PNAS 118 (11) e2018240118 (2021)] used Redfield theory to link the experimental observations to altered exciton transfer rates due to oxidative onsite energy shifts that detune excitonic energy gaps from a specific vibrational frequency of the bacteriochlorophyll (BChl) a. Using a memory kernel formulation of the hierarchical equations of motion, we present non-perturbative estimations of transfer rates that yield a modified physical picture. Our findings indicate that onsite energy shifts alone cannot reproduce the observed rate changes in oxidative environments, either qualitatively or quantitatively. By systematically examining combined changes both in site energies and the local environment for the oxidised complex, while maintaining consistency with absorption spectra, our results suggest that vibronic tuning of transfer rates may indeed be active in the reduced complex. However, we achieve qualitative, but not quantitative, agreement with the experimentally measured rates. Our analysis indicates potential limitations of the FMO electronic Hamiltonian, which was originally derived by fitting spectra to second-order cumulant and Redfield theories. This suggests that reassessment of these electronic parameters with a non-perturbative scheme, or derived from first principles, is essential for a consistent and accurate understanding of exciton dynamics in FMO under varying redox conditions.

physics.chem-ph↗

arXiv2024

Equivalence of dynamics of disordered quantum ensembles and semi-infinite lattices

We develop a formalism for mapping the exact dynamics of an ensemble of disordered quantum systems onto the dynamics of a single particle propagating along a semi-infinite lattice, with parameters determined by the probability distribution of disorder realizations of the original heterogeneous quantum ensemble. This mapping provides a geometric interpretation on the loss of coherence when averaging over the ensemble and allows computation of the exact dynamics of the entire disordered ensemble in a single simulation. Alternatively, by exploiting the reverse map, one can obtain lattice dynamics by averaging over realisations of disorder. The potential of this equivalence is showcased with examples of the map in both directions: obtaining dephasing of a qubit via mapping to a lattice model, and solving a simple lattice model via taking an average over realizations of disorder of a unit cell.

quant-ph↗

arXiv2024

Photon correlation time-asymmetry and dynamical coherence in multichromophoric systems

We theoretically investigate polarization-filtered two-photon correlations for the light emitted by a multichromophoric system undergoing excitation transport under realistic exciton-phonon interactions, and subject to continuous incoherent illumination. We show that for a biomolecular aggregate, such as the Fenna-Matthews Olson (FMO) photosynthetic complex, time-asymmetries in the cross-correlations of photons corresponding to different polarizations can be exploited to probe both quantum coherent transport mechanisms and steady-state coherence properties, which are not witnessed by zero-delay correlations. A classical bound on correlation asymmetry is obtained, which FMO is shown to violate using exact numerical calculations. Our analysis indicates that the dominant contributions to time-asymmetry in such photon cross-correlations are population to coherence transfer for Frenkel-Exciton models. Our results therefore put forward photon correlation asymmetry as a promising approach to investigate coherent contributions to excited-stated dynamics in molecular aggregates and other many-site quantum emitters.

physics.chem-ph↗

arXiv2024

Two-colour photon correlations probe coherent vibronic contributions to electronic excitation transport under incoherent illumination

Identifying signatures of quantum coherent behaviour in photoactive systems that are maintained in stationary states away from thermal equilibrium is an open problem of wide interest in a variety of physical scenarios, including single photosynthetic complexes subjected to continuous incoherent illumination. Here we consider a prototype light-harvesting heterodimer exhibiting coherent and collective exciton-vibration interactions and show that the second-order frequency-filtered correlations of fluorescence photons provide insightful information on the influence of such coherent interactions for different transitions, thereby yielding fundamentally different photon-counting statistics. Furthermore, we show that coherent vibronic mechanisms strongly affect the asymmetries characteristic of time-resolved photon cross-correlations and manifest themselves in a time-dependent violation of the Cauchy-Schwarz inequality bounding cross-correlations for classically fluctuating fields. We finally discuss how such second-order correlation asymmetry establishes important connections between coherent vibronic interactions, directional exciton population transport, and violation of quantum detailed balance. Our work then indicates that measurement of two-colour photon correlation asymmetry can be an important avenue to investigate quantum behaviour of single photoactive biomolecular and chemical systems under incoherent illumination conditions.

physics.chem-ph↗

arXiv2024

Random Matrix Theory Approach to Quantum Fisher Information in Quantum Many-Body Systems

We theoretically investigate parameter quantum estimation in quantum chaotic systems. Our analysis is based on an effective description of non-integrable quantum systems in terms of a random matrix Hamiltonian. Based on this approach we derive an analytical expression for the time evolution of the quantum Fisher information. We test our random matrix theory prediction with the exact diagonalization of a non-integrable spin system, focusing on the estimation of a local magnetic field by measurements of the many-body state. Our numerical calculations agree with the effective random matrix theory approach and show that the information on the local Hamiltonian parameter is distributed throughout the quantum system during the quantum thermalization process. Our analysis shows a first stage in which the initial information spread is quadratic in time which quickly passes into linear increase with slope determine by the decay rate of the measured spin observable. When the information is fully spread among all degrees of freedom a second quadratic time scale determines the long time behaviour of the quantum Fisher information.

quant-ph↗

arXiv2023

Multi-level Purcell effect and the impact of vibrational modes in molecular quantum optics

The increased decay rate of a two-level system weakly coupled to an optical cavity, known as the Purcell effect, is a cornerstone of cavity QED. However, the effect of cavity coupling is not well understood if the two-level system is replaced by a multi-level interacting system. Motivated by experiments looking to characterise molecular systems via exploiting a cavity interaction, we study a manifestation of the Purcell effect in a bio-inspired photosynthetic dimer. We focus in particular on how molecular vibrational modes, thought to play an important role in photosynthetic exciton transport, impact the system-cavity behaviour in the Purcell regime. We provide a theoretical picture in terms of an effective non-Hermitian Hamiltonian, which extends the simple picture of a Jaynes-Cummings model to the description of a `multi-level' Purcell effect, where different levels have differing Purcell factors, with effective cooperativities mediated by coherent vibrational interactions.

quant-ph↗

arXiv2020

Taking snapshots of a quantum thermalization process: emergent classicality in quantum jump trajectories

We investigate theoretically the emergence of classical statistical physics in a finite quantum system that is either totally isolated or otherwise subjected to a quantum measurement process. We show via a random matrix theory approach to nonintegrable quantum systems that the set of outcomes of the measurement of a macroscopic observable evolve in time like stochastic variables, whose variance satisfies the celebrated Einstein relation for Brownian diffusion. Our results show how to extend the framework of eigenstate thermalization to the prediction of properties of quantum measurements on an otherwise closed quantum system. We show numerically the validity of the random matrix approach in quantum chain models.

quant-ph↗

arXiv2019

Chaotic dynamics in a single excitation subspace: deviations from the ETH via long time correlations

In this work we study a scenario where dynamics is restricted to a single excitation manifold, for particular physical observables with support in the manifold, which we label a `correlated quench'. We ask how such dynamics may in general differ from predictions of the eigenstate thermalization hypothesis (ETH). We show that if thermalization occurs, it will not fulfil other key predictions of the ETH; instead following differing generic behaviours. We show this by analysing long-time fluctuations, two-point correlation functions, and the out-of-time-ordered correlator; analytically detailing deviation from ETH predictions. We derive instead an ETH-like relation, with non-random off-diagonals, for matrix elements of observables, with correlations that alter long-time behaviour and constrain dynamics. Further, we analytically compute the time-dependence of the decay to equilibrium, showing that it is proportional to the survival probability of the initial state. We finally note that the conditions studied are common in many physical scenarios, such as under the rotating-wave approximation. We show numerically that predictions are robust to perturbations that break this approximation.

quant-ph↗

arXiv2019

Ergodicity probes: using time-fluctuations to measure the Hilbert space dimension

Quantum devices, such as quantum simulators, quantum annealers, and quantum computers, may be exploited to solve problems beyond what is tractable with classical computers. This may be achieved as the Hilbert space available to perform such `calculations' is far larger than that which may be classically simulated. In practice, however, quantum devices have imperfections, which may limit the accessibility to the whole Hilbert space. We thus determine that the dimension of the space of quantum states that are available to a quantum device is a meaningful measure of its functionality, though unfortunately this quantity cannot be directly experimentally determined. Here we outline an experimentally realisable approach to obtaining the required Hilbert space dimension of such a device to compute its time evolution, by exploiting the thermalization dynamics of a probe qubit. This is achieved by obtaining a fluctuation-dissipation theorem for high-temperature chaotic quantum systems, which facilitates the extraction of information on the Hilbert space dimension via measurements of the decay rate, and time-fluctuations.

quant-ph↗

arXiv2018

Quantum Chaotic Fluctuation-Dissipation Theorem: Effective Brownian Motion in Closed Quantum Systems

We analytically describe the decay to equilibrium of generic observables of a non-integrable system after a perturbation in the form of a random matrix. We further obtain an analytic form for the time-averaged fluctuations of an observable in terms of the rate of decay to equilibrium. Our result shows the emergence of a Fluctuation-Dissipation theorem corresponding to a classical Brownian process, specifically, the Ornstein-Uhlenbeck process. Our predictions can be tested in quantum simulation experiments, thus helping to bridge the gap between theoretical and experimental research in quantum thermalization. We test our analytic results by exact numerical experiments in a spin-chain. We argue that our Fluctuation-Dissipation relation can be used to measure the density of states involved in the non-equilibrium dynamics of an isolated quantum system.

quant-ph↗

arXiv2018

Off-Diagonal Observable Elements from Random Matrix Theory: Distributions, Fluctuations, and Eigenstate Thermalization

We derive the Eigenstate Thermalization Hypothesis (ETH) from a random matrix Hamiltonian by extending the model introduced by J. M. Deutsch [Phys. Rev. A 43, 2046 (1991)]. We approximate the coupling between a subsystem and a many-body environment by means of a random Gaussian matrix. We show that a common assumption in the analysis of quantum chaotic systems, namely the treatment of eigenstates as independent random vectors, leads to inconsistent results. However, a consistent approach to the ETH can be developed by introducing an interaction between random wave-functions that arises as a result of the orthonormality condition. This approach leads to a consistent form for off-diagonal matrix elements of observables. From there we obtain the scaling of time-averaged fluctuations with system size for which we calculate an analytic form in terms of the Inverse Participation Ratio. The analytic results are compared to exact diagonalizations of a quantum spin chain for different physical observables in multiple parameter regimes.

cond-mat.stat-mech↗