Wen-ge Wang, G. Casati, Baowen Li
We study, analytically and numerically, the stability of quantum motion for a classically chaotic system. We show the existence of different regimes of fidelity decay which deviate from Fermi Golden rule and Lyapunov decay.
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We study the time evolution of two wave packets prepared at the same initial state, but evolving under slightly different Hamiltonians. For chaotic systems, we determine the circumstances that lead to an exponential decay with time of the wave packet overlap function. We show that for sufficiently weak perturbations, the exponential decay follows a Fermi golden rule, while by making the difference between the two Hamiltonians larger, the characteristic exponential decay time becomes the Lyapunov exponent of the classical system. We illustrate our theoretical findings by investigating numerical…
We study the crossover of the quantum Loschmidt echo (or fidelity) from the golden rule regime to the perturbation-independent exponential decay regime by using the kicked top model. It is shown that the deviation of the perturbation-independent decay of the averaged fidelity from the Lyapunov decay results from quantum fluctuations in individual fidelity, which are caused by the coherence in the initial coherent states. With an averaging procedure suppressing the quantum fluctuations effectively, the perturbation-independent decay is found to be close to the Lyapunov decay. We also show that …
We investigate experimentally a Fermi golden rule in two-edge and five-edge microwave networks with preserved time reversal invariance. A Fermi golden rule gives rates of decay of states obtained by perturbing embedded eigenvalues of graphs and networks. We show that the embedded eigenvalues are connected with the topological resonances of the analyzed systems and we find the trajectories of the topological resonances in the complex plane.
We present a Fermi golden rule giving rates of decay of states obtained by perturbing embedded eigenvalues of a quantum graph. To illustrate the procedure in a notationally simpler setting we also present a Fermi Golden Rule for boundary value problems on surfaces with constant curvature cusps. We also provide a resonance existence result which is uniform on compact sets of energies and metric graphs. The results are illustrated by numerical experiments.
The Gamow and the Fermi Golden Rules
本文利用动量表象中平方可积的Gamow态推导两体非相对论衰变的微分与总宽度，提出可统一描述能量和角度分布并在长寿命远离阈值时退化为Fermi黄金定则的Gamow黄金定则。
Starting from divisibility problem for Fibonacci numbers we introduce Fibonacci divisors, related hierarchy of Golden derivatives in powers of the Golden Ratio and develop corresponding quantum calculus. By this calculus, the infinite hierarchy of Golden quantum oscillators with integer spectrum determined by Fibonacci divisors, the hierarchy of Golden coherent states and related Fock-Bargman representations in space of complex analytic functions are derived. It is shown that Fibonacci divisors with even and odd $k$ describe Golden deformed bosonic and fermionic quantum oscillators, correspond…
A study is made of the behavior of unstable states in simple models which nevertheless are realistic representations of situations occurring in nature. It is demonstrated that a non-exponential decay pattern will ultimately dominate decay due to a lower limit to the energy. The survival rate approaches zero faster than the inverse square of the time when the time goes to infinity.
The effects of decoherence on Fermi's golden rule
本文通过非绝热分子动力学方法研究退相干对费米黄金规则的影响，发现退相干时间变短时，两种基底下的跃迁率显著偏离传统规则，并以单层WS₂为例验证了电子-声子耦合中的退相干效应。
We review the calculation of Fermi's golden rule for a system of $N$-body dipoles, magnetic or electric, weakly interacting with a blackbody radiation. By using the magnetic or electric field-field correlation function evaluated in the 1960s for the black body radiation, we deduce a general formula for the transition rates and study its limiting, fully coherent or fully incoherent, regimes.
Putting a general, physically relevant upper bound on equilibration times in closed quantum systems is a recently much pursued endeavor. In PRX, 7, 031027 (2017) Garc\'ıa-Pintos et al. suggest such a bound. We point out that the general assumptions which allow for an actual estimation of this bound are violated in cases in which Fermi's Golden Rule and related open quantum system theories apply. To probe the range of applicability of Fermi's Golden Rule for systems of the type addressed in the above work, we numerically solve the corresponding Schrödinger equation for some finite spin systems …
We shall revisit the conventional adiabatic or Markov approximation, showing its intrinsic failure in describing the proper quantum-mechanical evolution of a generic subsystem interacting with its environment. In particular, we shall show that -contrary to the semiclassical case- the Markov limit does not preserve the positive-definite character of the corresponding density matrix, thus leading to highly non-physical results. To overcome this problem, we shall propose an alternative adiabatic procedure which (i) in the semiclassical limit reduces to the standard Fermi's golden rule, and (ii) d…
Fermi's golden rule is of great importance in quantum dynamics. However, in many textbooks on quantum mechanics, its contents and limitations are obscured by the approximations and arguments in the derivation, which are inevitable because of the generic setting considered. Here we propose to introduce it by an ideal model, in which the quasi-continuum band consists of equaldistant levels extending from $-\infty $ to $+\infty $, and each of them couples to the discrete level with the same strength. For this model, the transition probability in the first order perturbation approximation can be c…
Fermi's Golden Rule (FGR) applies in the limit where an initial quantum state is weakly coupled to a {\it continuum} of other final states overlapping its energy. Here we investigate what happens away from this limit, where the set of final states is discrete, with a nonzero mean level spacing; this question arises in a number of recently investigated many-body systems. For different symmetry classes, we analytically and/or numerically calculate the universal crossovers in the average decay of the initial state as the level spacing is varied, with the Golden Rule emerging in the limit of a con…
Perspective of Fermi's golden rule and its generalizations in chemical physics
本文回顾费米黄金定则的历史、推导、假设和典型形式，综述其在化学物理中的主要应用，澄清实际应用中的模糊问题，并介绍近期推广和计算方法进展。
The decay of a local spin excitation in an inhomogeneous spin chain is evaluated exactly: I) It starts quadratically up to a spreading time t_{S}. II) It follows an exponential behavior governed by a self-consistent Fermi Golden Rule. III) At longer times, the exponential is overrun by an inverse power law describing return processes governed by quantum diffusion. At this last transition time t_{R} a survival collapse becomes possible, bringing the polarization down by several orders of magnitude. We identify this strongly destructive interference as an antiresonance in the time domain. These …
We reconsider the problem of the spontaneous emission of light by an excited atomic state. We scrutinize the survival probability of this excited state for very short times, in the so-called Zeno regime, for which we show that the dynamics is dictated by a coherent --in phase -- response of the on-shell and off-shell vacuum modes. We also develop a perturbative approach in order to interpolate between different temporal regimes: the Zeno, golden rule (linear) and Wigner-Weisskopf (exponential) regimes. We compare results obtained with the $\hat{\mathbf{E}}\cdot\hat{\mathbf{x}}$ and $\hat{\math…
We demonstrate a fundamental breakdown of the photonic spontaneous emission (SE) formula derived from Fermi's golden rule, in absorptive and amplifying media, where one assumes the SE rate scales with the local photon density of states, an approach often used in more complex, semiclassical nanophotonics simulations. Using a rigorous quantization of the macroscopic Maxwell equations in the presence of arbitrary linear media, we derive a corrected Fermi's golden rule and master equation for a quantum two-level system (TLS) that yields a quantum pumping term and a modified decay rate that is net …
Fermi's golden rule describes the decay dynamics of unstable quantum systems coupled to a reservoir, and predicts a linear decay in time. Although it arises at relatively short times, the Fermi regime does not take hold in the earliest stages of the quantum dynamics. The standard criterion in the literature for the onset time of the Fermi regime is $t_F\sim1/Δω$, with $Δω$ the frequency interval around the resonant transition frequency $ω_0$ of the system, over which the coupling to the reservoir does not vary appreciably. In this work, this criterion is shown to be inappropriate in general fo…
Fermi's golden rule applies to a situation in which a single quantum state $|ψ\rangle$ is coupled to a near-continuum. This "quasi-continuum coupling" structure results in a rate equation for the population of $|ψ\rangle$. Here we show that the coupling of a quantum system to the standard model of a thermal environment, a bath of harmonic oscillators, can be decomposed into a "cascade" made up of the quasi-continuum coupling structures of Fermi's golden rule. This clarifies the connection between the physics of the golden rule and that of a thermal bath, and provides a non-rigorous but physica…
Many-body states described by a Schrödinger equation include states of overlapping waves of non-vanishing interaction energies. These peculiar states formed in many-body transitions remain in asymptotic regions, and lead a new component to the transition probability. The probability is computed rigorously following the von Neumann's fundamental principle of quantum mechanics with an S-matrix that is defined with normalized functions, instead of plane waves. That includes the intriguing correction term to the Fermi's golden rule, in which a visible energy is smaller than the initial energy, and…
Focusing on the $2\mathrm{p}-1\mathrm{s}$ transition in atomic Hydrogen, we investigate through first order perturbation theory the time evolution of the survival probability of an electron initially taken to be in the excited ($2\mathrm{p}$) state. We examine both the results yielded by the standard dipole approximation for the coupling between the atom and the electromagnetic field -for which we propose a cutoff-independent regularisation- and those yielded by the exact coupling function. In both cases, Fermi's golden rule is shown to be an excellent approximation for the system at hand: we …
A quantum mechanical wave of a finite size moves like a classical particle and shows a unique decay probability. Because the wave function evolves according to the Schrödinger equation, it preserves the total energy but not the kinetic energy in the intermediate-time region of a decay process where those of the parent and daughters overlap. The decay rate computed with Fermi's golden rule requires corrections that vary with the distance between the initial and final states, and the energy distribution of the daughter is distorted from that of plane waves. The corrections have universal propert…
In this paper, we explore the metastable states of nonlinear Klein-Gordon equations with potentials. These states come from the instability of a bound state under a nonlinear Fermi's golden rule. In [16], Soffer and Weinstein studied the instability mechanism and obtained an anomalously slow-decaying rate $1/(1+t)^{\frac{1}{4}}$. Here we develop a new method to study the $L^2_x$ norm of solutions to Klein-Gordon equations. With this method, we prove the first $H^1$ scattering result for Klein-Gordon equations with metastable states. By exploring the oscillations, we also give another more robu…
We study heating dynamics in isolated quantum many-body systems driven periodically at high frequency and large amplitude. Combining the high-frequency expansion for the Floquet Hamiltonian with Fermi's golden rule (FGR), we develop a master equation termed the Floquet FGR. Unlike the conventional one, the Floquet FGR correctly describes heating dynamics, including the prethermalization regime, even for strong drives, under which the Floquet Hamiltonian is significantly dressed, and nontrivial Floquet engineering is present. The Floquet FGR depends on system size only weakly, enabling us to an…
We present a Gedankenexperiment that leads to a violation of detailed balance if quantum mechanical transition probabilities are treated in the usual way by applying Fermi's "golden rule". This Gedankenexperiment introduces a collection of two-level systems that absorb and emit radiation randomly through non-reciprocal coupling to a waveguide, as realized in specific chiral quantum optical systems. The non-reciprocal coupling is modeled by a hermitean Hamiltonian and is compatible with the time-reversal invariance of unitary quantum dynamics. Surprisingly, the combination of non-reciprocity wi…
Relating the Hall conductivity to the many-body Chern number using Fermi's Golden rule and Kramers-Kronig relations
本文提出一种教学性推导，仅用Kramers-Kronig关系与费米黄金规则证明关联绝缘体的量子化霍尔电导等于多体陈数，并指出可通过圆驱动下的单粒子激发率直接测量。
We describe how a structured photonic medium controls the spontaneous emission rate from an excited quantum dot in the presence of electron-phonon coupling. We analyze this problem using a polaron transformed master equation and we consider specific examples of a photonic crystal cavity and a coupled cavity waveguide. We find that when the relaxation times of the photon and phonon baths are comparable, phonons influence spontaneous emission in a non-trivial way. We demonstrate why and how the broadband frequency dependence of the local photon density of states determines the photon emission ra…
We discuss how the bath's memory affects the dynamics of a swap gate. We present an exactly solvable model that shows various dynamical transitions when treated beyond the Fermi Golden Rule. By moving continuously a single parameter, the unperturbed Rabi frequency, we sweep through different analytic properties of the density of states: I) collapsed resonances that split at an exceptional point in II) two resolved resonances ; III) out-of-band resonances; IV) virtual states; and V) pure point spectrum. We associate them with distinctive dynamical regimes: overdamped, damped oscillations, envir…
Quantum Calculus of Fibonacci Divisors and Fermion-Boson Entanglement for Infinite Hierarchy of N = 2 Supersymmetric Golden Oscillators
本文将黄金比例量子微积分推广至 N=2 超对称代数，构建了超对称黄金振子层级，并通过 concurrence 和 von Neumann 熵揭示了费米子-玻色子纠缠态的几何分类与特征。
We shall revisit the conventional adiabatic or Markov approximation, which --contrary to the semiclassical case-- does not preserve the positive-definite character of the corresponding density matrix, thus leading to highly non-physical results. To overcome this serious limitation, originally pointed out and partially solved by Davies and co-workers almost three decades ago, we shall propose an alternative more general adiabatic procedure, which (i) is physically justified under the same validity restrictions of the conventional Markov approach, (ii) in the semiclassical limit reduces to the s…
Modified Fermi's golden rule rate expressions
本文提出修正的费米黄金规则速率表达式，以解决FGR在特定情况下评估的歧义问题，并提供更可靠的速率建模方法。
We discuss the decay of unstable states into a quasicontinuum using models of the effective Hamiltonian type. The goal is to show that exponential decay and the golden rule are exact in a suitable scaling limit, and that there is an associated renormalization group (RG) with these properties as a fixed point. The method is inspired by a limit theorem for infinitely divisible distributions in probability theory, where there is a RG with a Cauchy distribution, i.e. a Lorentz line shape, as a fixed point. Our method of solving for the spectrum is well known; it does not involve a perturbation exp…
Fermi's golden rule in tunneling models with quantum waveguides perturbed by Kato class measures
本文研究二维量子波导中由Kato测度扰动引起的共振，推导出共振能量的渐近表达式，并证明其虚部满足Fermi黄金规则。
We study the prethermalization and thermalization dynamics of local observables in weakly perturbed nonintegrable systems, with Hamiltonians of the form $\hat{H}_0+g\hat{V}$, where $\hat{H}_0$ is nonintegrable and $g\hat{V}$ is a perturbation. We explore the dynamics of far from equilibrium initial states in the thermodynamic limit using a numerical linked cluster expansion (NLCE), and in finite systems with periodic boundaries using exact diagonalization. We argue that generic observables exhibit a two-step relaxation process, with a fast prethermal dynamics followed by a slow thermalizing on…
In this article, we present a unified reciprocal quantum electrodynamics (QED) formulation of quantum light-matter interaction. For electron-light interactions, we bridge the underlying theories of Photon-Induced Near-field Electron Microscopy (PINEM), Laser-induced Particle Accelerators and radiation sources, such as Free Electron Laser (FEL), transition radiation and Smith-Purcell effect. We demonstrate an electron-photon spectral reciprocity relation between the electron energy loss/gain and the radiation spectra. This "Acceleration/Radiation Correspondence" (ARC) conserves the electron-ene…
A novel and readily understandable derivation of the Golden Rule of time dependent perturbation theory is presented. The derivation is based on adiabatic turning on of the perturbation as used, for instance, in some formal developments of scattering theory. Energy conservation is expressed in terms of an intuitively and physically appealing Lorentzian line shape rather than the artificial, oscillatory sin(x)=x type line shape that appears in conventional derivations. The conditions for the derivation's validity are compactly and conveniently expressed in the frequency/energy domain rather than…
Beyond Fermi's golden rule with the statistical Jacobi approximation
本文通过统计雅可比近似推导出量子淬火后保真度的解析表达式，适用于弱和强淬火，并揭示了强淬火下衰减率偏离费米黄金规则的现象。
Steady-state observables, such as occupation numbers and currents, are crucial experimental signatures in open quantum systems. The time-convolutionless (TCL) master equation, which is both exact and time-local, is an ideal candidate for the perturbative computation of such observables. We develop a diagrammatic approach to evaluate the steady-state TCL generator based on operators rather than superoperators. We obtain the steady-state occupation numbers, extend our formulation to the calculation of currents, and provide a simple physical interpretation of the diagrams. We further benchmark ou…
Cavity-Modified Nonequilibrium Fermi's Golden Rule Rate Coefficients from Cavity-Free Inputs
本文提出一种利用无腔输入估算电磁微腔中弱耦合分子系统的非平衡费米黄金定则（NE-FGR）电荷转移速率的框架，并引入腔修饰瞬时Marcus理论近似，通过分子三联体等模型验证了其有效性。
Spin-lattice relaxation with non-linear couplings: Comparison between Fermi's golden rule and extended dissipaton equation of motion
本文通过扩展耗散子运动方程数值评估自旋-晶格弛豫率核，发现非线性耦合下费米黄金规则因忽略非马尔可夫性而偏离精确结果，为分子量子比特$T_1$温度依赖性提供新见解。
We modify the theory of the Quantum Zeno Effect to make it consistent with the postulates of quantum mechanics. This modification allows one, throughout a sequence of observations of an excited system, to address the nature of the observable and thereby to distinguish survival from non-decay, which is necessary whenever excited states are degenerate. As a consequence, one can determine which types of measurements can possibly inhibit the exponential decay of the system. We find that continuous monitoring taken as the limit of a sequence of ideal measurements will only inhibit decay in special …
Breakdown of Fermi's Golden Rule in 1d systems at non-zero temperature
针对非零温度一维晶格费米子中费米黄金定则计算发散、准粒子寿命标度关系不明的问题，结合DAOE数值模拟与非微扰图重求和，提出准粒子衰变速率存在对数增强的结论，且该效应适用于多种量子及经典系统。
In certain regimes, the fidelity of quantum states will decay at a rate set by the classical Lyapunov exponent. This serves both as one of the most important examples of the quantum-classical correspondence principle and as an accurate test for the presence of chaos. While detecting this phenomenon is one of the first useful calculations that noisy quantum computers without error correction can perform [G. Benenti et al., Phys. Rev. E 65, 066205 (2001)], a thorough study of the quantum sawtooth map reveals that observing the Lyapunov regime is just beyond the reach of present-day devices. We p…
Decay rates of almost strong modes in Floquet spin chains beyond Fermi's Golden Rule
本文通过微扰理论研究了弱不可积Floquet自旋链中近强零模和π模的衰减率，发现其寿命由准能量守恒条件决定并正比于1/Jz^{2n}，同时给出了与精确对角化吻合的解析表达式。
Emergence of Fermi's Golden Rule in the Probing of a Quantum Many-Body System
本研究利用强相互作用均匀自旋-1/2费米气体与射频场耦合的系统，观测费米黄金定则（FGR）的涌现与失效，明确了弱驱动下的三个动力学区域及拉比振荡阈值，为量子多体系统光谱的线性响应理论应用提供了参考。
We study fidelity decay by a uniform semiclassical approach, in the three perturbation regimes, namely, the perturbative regime, the Fermi-golden-rule (FGR) regime, and the Lyapunov regime. A semiclassical expression is derived for fidelity of initial Gaussian wave packets with width of the order $\sqrt{\hbar}$ ($\hbar$ being the effective Planck constant). Short time decay of fidelity of initial Gaussian wave packets is also studied, with respect to two time scales introduced in the semiclassical approach. In the perturbative regime, it is confirmed numerically that fidelity has the FGR decay…
Fermi's golden rule rate expression for transitions due to nonadiabatic derivative couplings in the adiabatic basis
本文推导了在绝热基矢中非绝热跃迁的费米黄金规则速率表达式，考虑了非Condon效应及振动位移耦合，适用于核振动模型的闭合形式速率表达式。
The Loschmidt echo (LE) measures the ability of a system to return to the initial state after a forward quantum evolution followed by a backward perturbed one. It has been conjectured that the echo of a classically chaotic system decays exponentially, with a decay rate given by the minimum between the width $Γ$ of the local density of states and the Lyapunov exponent. As the perturbation strength is increased one obtains a cross-over between both regimes. These predictions are based on situations where the Fermi Golden Rule (FGR) is valid. By considering a paradigmatic fully chaotic system, th…
Beyond The Fermi's Golden Rule: Discrete-Time Decoherence Of Quantum Mesoscopic Devices Due To Bandlimited Quantum Noise
本文利用量子噪声的带限特性，基于Kotelnikov采样定理将其建模为离散时间过程，从而为介观量子器件在费米黄金规则之外的实时耗散动力学提供了微观推导的碰撞模型和带限输入-输出形式。
The probability of the events that the final states are detected with or interact with the nucleus in a finite time interval T was found to be, $P=\text T Γ_0 +P^{(d)}$. $Γ_0$ is computed with Fermi's golden rule, and does not depend on the nuclear wave functions. $P^{(d)}$ is not given by Fermi's golden rule, and depends on the nuclear wave functions. In the electron mode of pion decays, $Γ_0$ is proportional to $m_e^2$ but $P^{(d)}$ for the event that the neutrino is detected is proportional to $m_ν^{-2}$. $P^{(d)}$ does not hold the helicity suppression satisfied in $Γ_0$ and is inevitable …
The decay dynamics of a local excitation interacting with a non-Markovian environment, modeled by a semi-infinite tight-binding chain, is exactly evaluated. We identify distinctive regimes for the dynamics. Sequentially: (i) early quadratic decay of the initial-state survival probability, up to a spreading time $t_{S}$, (ii) exponential decay described by a self-consistent Fermi Golden Rule, and (iii) asymptotic behavior governed by quantum diffusion through the return processes and leading to an inverse power law decay. At this last cross-over time $t_{R}$ a survival collapse becomes possible…
We compute the Lyapunov exponent characterizing quantum scrambling in a family of generalized Sachdev-Ye-Kitaev models, which can be tuned between different low temperature states from Fermi liquids, through non-Fermi liquids to fast scramblers. The analytic calculation, controlled by a small coupling constant and large $N$, allows us to clarify the relations between the quasi-particle relaxation rate $1/τ$ and the Lyapunov exponent $λ_L$ characterizing scrambling. In the Fermi liquid states we find that the quasi-particle relaxation rate dictates the Lyapunov exponent. In non-Fermi liquids, w…
With the assistance of a pair of Majorana fermions, we propose schemes to entangle two quantum dots by Lyapunov control in the charge and spin degrees of freedom. Four different schemes are considered, i.e., the teleportation scheme, the crossed Andreev reflection scheme, the intradot spin flip scheme, and the scheme beyond the intradot spin flip. We demonstrate that the entanglement can be generated by modulating the chemical potential of quantum dots with square pulses, which is easily realized in practice. In contrast to Lyapunov control, the preparation of entangled states by adiabatic pas…
Lyapunov exponents in a Sachdev-Ye-Kitaev-type model with population imbalance in the conformal limit and beyond
本文数值研究了一个由两组仅限组间相互作用的 Majorana 费米子构成的 SYK 类型模型的四点函数与 Lyapunov 指数，发现强耦合下指数仍为最大，且不随粒子数比例和耦合强度变化。
A simple quantum mechanical model consisting of a discrete level resonantly coupled to a continuum of finite width, where the coupling can be varied from perturbative to strong (Fano-Anderson model), is considered. The particle is initially localized at the discrete level, and the time dependence of the amplitude to find the particle at the discrete level is calculated without resorting to perturbation theory. The deviations from the exponential decay law, predicted by the Fermi's Golden Rule, are discussed. We also study analytic structure of the Green's function (GF) for the model. We analyz…
The Loschmidt echo (LE) is a magnitude that measures the sensitivity of quantum dynamics to perturbations in the Hamiltonian. For a certain regime of the parameters, the LE decays exponentially with a rate given by the Lyapunov exponent of the underlying classically chaotic system. We develop a semiclassical theory, supported by numerical results in a Lorentz gas model, which allows us to establish and characterize the universality of this Lyapunov regime. In particular, the universality is evidenced by the semiclassical limit of the Fermi wavelength going to zero, the behavior for times longe…
We study the multi-periodic oscillations in the spontaneous emission rate of an atom in a medium with refractive index n sandwiched between two parallel mirrors. The oscillations are not obvious in the analytical formula for the rate derived based on Fermi's golden rule but can be extracted using Fourier transforms by varying the system scale while holding the configuration. The oscillations are interpreted as interferences and correspond to various closed-orbits of the emitted photon going away from and returning to the atom. This system provides a rare example that the oscillations can be ex…
Signatures of Chaos in a Quasiperiodically Driven Quantum Impact Oscillator
本研究证明准周期驱动碰撞振子在擦边条件下产生量子混沌，通过多种诊断确认混沌特征，并揭示频率比无理程度影响全局纠缠扩散。
Enhancement of superconducting pairing via quantum Lyapunov control
本文提出基于反馈的量子 Lyapunov 控制方案，可在 Fermi-Hubbard 模型中稳健增强超导配对，并支持双向抑制与渐近控制实现。
We compute parameters characterizing many-body quantum chaos for a critical Fermi surface without quasiparticle excitations. We examine a theory of $N$ species of fermions at non-zero density coupled to a $U(1)$ gauge field in two spatial dimensions, and determine the Lyapunov rate and the butterfly velocity in an extended random-phase approximation. The thermal diffusivity is found to be universally related to these chaos parameters i.e. the relationship is independent of $N$, the gauge coupling constant, the Fermi velocity, the Fermi surface curvature, and high energy details.
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