Jaewon Kim, Xiangyu Cao
In arXiv:1707.02197, the authors considered the Sachdev-Ye-Kitaev model with quadratic perturbation (also known as mass-deformed SYK), and claimed that the quantum Lyapunov exponent would vanish in the regime of low temperature and small quartic coupling. We calculate the exponent exactly in that regime and show that it is nonzero.
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We study the entanglement entropy of eigenstates (including the ground state) of the Sachdev-Ye-Kitaev model. We argue for a volume law, whose coefficient can be calculated analytically from the density of states. The coefficient depends on not only the energy density of the eigenstate but also the subsystem size. Very recent numerical results of Liu, Chen, and Balents confirm our analytical results.
We study the Lindbladian dynamics of the Sachdev-Ye-Kitaev (SYK) model, where the SYK model is coupled to Markovian reservoirs with jump operators that are either linear or quadratic in the Majorana fermion operators. Here, the linear jump operators are non-random while the quadratic jump operators are sampled from a Gaussian distribution. In the limit of large $N$, where $N$ is the number of Majorana fermion operators, and also in the limit of large $N$ and $M$, where $M$ is the number of jump operators, the SYK Lindbladians are analytically tractable, and we obtain their stationary Green's f…
Through a mixture of analytic and numerical techniques, we explore the optimal approximation by a free Majorana state to individual disorder realizations of the Sachdev-Ye-Kitaev model, along with a generalization of it. We elucidate the properties of the known time-reversal symmetry breaking phase in the generalized model, finding strong evidence of "spin glass" order. For the Sachdev-Ye-Kitaev model itself, our results are inconclusive but suggest a similar order may be present at zero temperature.
Deviations from maximal entanglement for eigenstates of the Sachdev-Ye-Kitaev model
针对SYK模型的谱中间本征态，研究证明尺寸占系统恒定比例的子系统的纠缠熵与最大熵的偏差至少为一个正常数，明确了其与随机态纠缠熵的差异。
The Sachdev-Ye-Kitaev model spectral form factor exhibits absence of information loss in the form of a ramp and a plateau, that are typical of random matrix theory. In a large $N$ collective fields description, the ramp was reproduced by Saad, Shenker and Stanford \cite{Saad:2018bqo}, by replica symmetry breaking saddles for a connected component of the analytically continued to real times thermal partition function two point function. We derive a two sides Schwarzians effective action for fluctuations around the ramp critical saddles, by adapting to the two replica system a method by Kitaev a…
The existence of a quantum butterfly effect in the form of exponential sensitivity to small perturbations has been under debate for a long time. Lately, this question gained increased interest due to the proposal to probe chaotic dynamics and scrambling using out-of-time-order correlators. In this work we study echo dynamics in the Sachdev-Ye-Kitaev model under effective time reversal in a semiclassical approach. We demonstrate that small imperfections introduced in the time-reversal procedure result in an exponential divergence from the perfect echo, which allows to identify a Lyapunov expone…
Disorder-free Sachdev-Ye-Kitaev models: Integrability and a precursor of chaos
本文提出两种无无序SYK模型，证明其可积性，并发现其OTOC早期指数增长，呈现混沌前兆，但无随机矩阵行为。
A long period of linear growth in the spectral form factor provides a universal diagnostic of quantum chaos at intermediate times. By contrast, the behavior of the spectral form factor in disordered integrable many-body models is not well understood. Here we study the two-body Sachdev-Ye-Kitaev model and show that the spectral form factor features an exponential ramp, in sharp contrast to the linear ramp in chaotic models. We find a novel mechanism for this exponential ramp in terms of a high-dimensional manifold of saddle points in the path integral formulation of the spectral form factor. Th…
Sixfold Way of Traversable Wormholes in the Sachdev-Ye-Kitaev Model
该研究利用AdS2与SYK模型的对应关系，提出可穿越虫洞的对称性分类，依赖于N、q和r，并通过能级统计确认了六个普适类。
The Sachdev-Ye-Kitaev (SYK) model attracts attention in the context of information scrambling, which represents delocalization of quantum information and is quantified by the out-of-time-ordered correlators (OTOC). The SYK model contains $N$ fermions with disordered and four-body interactions. Here, we introduce a variant of the SYK model, which we refer to as the Wishart SYK model. We investigate the Wishart SYK model for complex fermions and that for hard-core bosons. We show that the ground state of the Wishart SYK model is massively degenerate and the residual entropy is extensive, and tha…
Quantum spin glasses and Sachdev-Ye-Kitaev models
本文综述了从量子Ising模型到SYK模型等无穷范围耦合随机量子模型，指出SYK模型因实现广延零温熵并为3+1维黑洞提供有效低能理论，其二维扩展2dYSYK模型则为奇异金属态提供了普适理论。
Cooling the Sachdev-Ye-Kitaev model using thermofield double states
该研究提出一种利用两个SYK模型副本局域耦合构建绝热路径的实验协议，通过平滑调节耦合强度高效冷却SYK模型至低温引力区，数值验证其有效性并基于本征态热化提供微观解释，为强相互作用量子系统冷却提供新方案。
We study chaos in a classical limit of the Sachdev-Ye-Kitaev (SYK) model obtained in a suitably defined large-S limit. The low-temperature Lyapunov exponent is found to depend linearly on temperature, with a slope that is parametrically different than in the quantum case: it is proportional to N/S. The classical dynamics can be understood as the rotation of an N-dimensional body with a random inertia tensor, corresponding to the random couplings of the SYK Hamiltonian. This allows us to find an extensive number of fixed points, corresponding to the body's principal axes of rotation. The thermo…
Continuous monitoring of an otherwise closed quantum system has been found to lead to a measurement-induced phase transition (MIPT) characterized by abrupt changes in the entanglement or purity of the many-body quantum state. For an entanglement MIPT, entangling dynamics compete with measurement dynamics, pushing the system either to a phase with extensive entanglement or to a phase with low-level entanglement. For purification MIPTs, projective measurements effectively cool and localize the system, inducing a transition from a mixed state to an uncorrelated pure state. In this work, we numeri…
Singular Value Decomposition and Its Blind Spot for Quantum Chaos in Non-Hermitian Sachdev-Ye-Kitaev Models
本文指出奇异值分解（SVD）在非厄米 SYK 模型中无法重现厄米极限统计且不能区分混沌与可积动力学，故不适用于量子混沌探测，建议改用双 Lanczos 算法。
We analytically evaluate the moments of the spectral density of the $q$-body Sachdev-Ye-Kitaev (SYK) model, and obtain order $1/N^2$ corrections for all moments, where $N$ is the total number of Majorana fermions. To order $1/N$, moments are given by those of the weight function of the Q-Hermite polynomials. Representing Wick contractions by rooted chord diagrams, we show that the $1/N^2$ correction for each chord diagram is proportional to the number of triangular loops of the corresponding intersection graph, with an extra grading factor when $q$ is odd. Therefore the problem of finding $1/N…
Entanglement production in the Sachdev-Ye-Kitaev Model and its variants
通过冯·诺依曼纠缠熵研究SYK模型及其变体在有限马约拉纳费米子数下的纠缠动力学，发现早期线性增长、后期饱和至随机矩阵理论值，但增长率不同，归因于大N效应，并指出纠缠产生率可作为更精细的混沌探针。
We compute the transport and chaos properties of lattices of quantum Sachdev-Ye-Kitaev islands coupled by single fermion hopping, and with the islands coupled to a large number of local, low energy phonons. We find two distinct regimes of linear-in-temperature ($T$) resistivity, and describe the crossover between them. When the electron-phonon coupling is weak, we obtain the `incoherent metal' regime, where there is near-maximal chaos with front propagation at a butterfly velocity $v_B$, and the associated diffusivity $D_{\rm chaos} = v_B^2/(2 πT)$ closely tracks the energy diffusivity. On the…
We study a two-site Sachdev-Ye-Kitaev model with complex couplings and a weak inter-site interaction. At low temperatures, the system is dual to a Euclidean wormhole in Jackiw-Teitelboim gravity plus matter. Interestingly, the energy spectrum becomes real for sufficiently strong inter-site coupling despite the Hamiltonian being non-Hermitian. In gravity, this complex-to-real transition corresponds to a Euclidean-to-Lorentzian transition: a dynamical restoration of the gravitational SL(2,R) symmetry of the Lorentzian wormhole, broken to U(1) in the Euclidean wormhole. We show this by identifyin…
Lévy Sachdev-Ye-Kitaev Model
本文研究具有 Lévy 稳定重尾相互作用的 Sachdev-Ye-Kitaev 模型，通过特征值谱分层分析揭示其从混沌到可积行为的转变，并将其解释为区别于 Lévy 随机矩阵迁移率边相变的多体效应。
The eigenstate thermalization hypothesis (ETH) is a conjecture on the nature of isolated quantum systems that guarantees the thermal behavior of subsystems when it is satisfied. ETH has been tested in various forms on a number of local many-body interacting systems. Here we examine the validity of ETH in a class of nonlocal disordered many-body interacting systems --- the Sachdev-Ye-Kitaev Majorana (SYK) models --- that may be tuned from chaotic behavior to integrability. Our analysis shows that SYK$_4$ (with quartic couplings), which is maximally chaotic in the large system size limit, satisf…
Periodically driven quantum matter can realize exotic dynamical phases. In order to understand how ubiquitous and robust these phases are, it is pertinent to investigate the heating dynamics of generic interacting quantum systems. Here we study the thermalization in a periodically-driven generalized Sachdev-Ye-Kitaev (SYK)-model, which realizes a crossover from a heavy Fermi liquid (FL) to a non-Fermi liquid (NFL) at a tunable energy scale. Developing an exact field theoretic approach, we determine two distinct regimes in the heating dynamics. While the NFL heats exponentially and thermalizes …
Exact spectrum and anomalous relaxation in the open disorder-free Sachdev-Ye-Kitaev system
利用无无序SYK模型的可积性，在Gorini-Kossakowski-Sudarshan-Lindblad形式下精确求解了Liouvillian谱，发现热力学极限下能隙在无耗散极限不消失，导致反常弛豫，并观察到谱特征如特征值实复转变。
Defect conformal field theory from Sachdev-Ye-Kitaev interactions
本研究通过将 $N$ 条 Majorana 链与缺陷处的 SYK$_q$ 相互作用耦合，提出了一族新的边界与界面缺陷共形场论，并利用鞍点方法计算了其纠缠特征与透射系数，发现该理论超越了已知的高斯缺陷 CFT 实例。
Precise Low-Temperature Expansions for the Sachdev-Ye-Kitaev model
该研究通过勒让德多项式分解高精度求解SYK模型的大N Dyson-Schwinger方程，得到其低温能量展开至$T^{7.54}$的结果，修正了此前的幂次预测并验证了共形微扰论的适用性。
The Sachdev-Ye-Kitaev (SYK) model is a quantum mechanical model of fermions interacting with $q$-body random couplings. For $q=2$, it describes free particles, and is non-chaotic in the many-body sense, while for $q>2$ it is strongly interacting and exhibits many-body chaos. In this work we study the entanglement entropy (EE) of the SYK$q$ models, for a bipartition of $N$ real or complex fermions into subsystems containing $2m$ real/$m$ complex fermions and $N-2m$/$N-m$ fermions in the remainder. For the free model SYK$2$, we obtain an analytic expression for the EE, derived from the $β$-Jacob…
Spectral correlations are a powerful tool to study the dynamics of quantum many-body systems. For Hermitian Hamiltonians, quantum chaotic motion is related to random matrix theory spectral correlations. Based on recent progress in the application of spectral analysis to non-Hermitian quantum systems, we show that local level statistics, which probe the dynamics around the Heisenberg time, of a non-Hermitian $q$-body Sachdev-Ye-Kitev (nHSYK) model with $N$ Majorana fermions, and its chiral and complex-fermion extensions, are also well described by random matrix theory for $q > 2$, while for $q …
Keldysh Wormholes and Anomalous Relaxation in the Dissipative Sachdev-Ye-Kitaev Model
该研究发现耦合马尔可夫环境的耗散SYK模型实时动力学等价于双位点非厄米SYK的欧几里得近零温动力学，提出其引力对偶为Keldysh虫洞构型，可在无环境耦合时产生有限衰减率。
Stabilizer Rényi Entropy and its Transition in the Coupled Sachdev-Ye-Kitaev Model
本文研究了耦合SYK模型中退相干瑞尼熵的转变，揭示了SRE的一级相变及作为序参量的新类别。
Thermalization of a Closed Sachdev-Ye-Kitaev System in the Thermodynamic Limit
本文研究封闭耦合马约拉纳SYK系统在热力学极限下的热化，通过淬火引入随机跳跃项，发现最终稳态在格林函数和能量上达到热平衡，并计算了最终温度和热化速率。
Sachdev-Ye-Kitaev Model in a Quantum Glassy Landscape
本文研究了将Majorana费米子与量子p-自旋模型耦合的扩展Yukawa模型，探讨了其在低温下表现出的量子玻璃相及对费米子和玻色子动力学的影响。
We study the Wishart-Sachdev-Ye-Kitaev (WSYK) model consisting of two $\hat{q}$-body Sachdev-Ye-Kitaev (SYK) models with general complex couplings, one the Hermitian conjugate of the other, living in off-diagonal blocks of a larger WSYK Hamiltonian. The spectrum is positive with a hard edge at zero energy. We employ diagrammatic and combinatorial techniques to compute analytically the low-order moments of the Hamiltonian. In the limit of large number $N$ of Majoranas, we have found striking similarities with the moments of the weight function of the Al-Salam-Chihara $Q$-Laguerre polynomials. F…
Non-Stabilizerness of Sachdev-Ye-Kitaev Model
研究Sachdev-Ye-Kitaev模型的非稳定性，发现其Majorana谱呈高斯分布，具有更高的魔性。
We present a detailed quantitative analysis of spectral correlations in the Sachdev-Ye-Kitaev (SYK) model. We find that the deviations from universal Random Matrix Theory (RMT) behavior are due to a small number of long-wavelength fluctuations from one realization of the ensemble to the next one. These modes can be parameterized effectively in terms of Q-Hermite orthogonal polynomials, the main contribution being the scale fluctuations for which we give a simple estimate. Our numerical results for $N=32$ show that only the lowest eight polynomials are needed to eliminate the nonuniversal part …
We present a review of the Sachdev-Ye-Kitaev (SYK) model of compressible quantum many-body systems without quasiparticle excitations, and its connections to various theoretical studies of non-Fermi liquids in condensed matter physics. The review is placed in the context of numerous experimental observations on correlated electron materials. Strong correlations in metals are often associated with their proximity to a Mott transition to an insulator created by the local Coulomb repulsion between the electrons. We explore the phase diagrams of a number of models of such local electronic correlati…
Characterizing Fermionic Non-Gaussianity in the Sachdev-Ye-Kitaev Model via Replica Twist Entropy
通过复制扭曲熵作为探针，在SYK模型中研究费米子非高斯性，揭示动态相图及对称性破缺，为强关联系统非高斯性研究开辟新途径。
Superconductivity results from an instability of the Fermi surface -- contour of \textit{poles} of the single particle propagator -- to an infinitesimally small attraction between electrons. Here, we instead discuss the analogous problem on a model \textit{Luttinger} surface, or contour of \textit{zeros} of the Green function. At zero temperature ($β\rightarrow \infty$) and a critical interaction strength ($u_{c\infty}$) characterized by the residue of self-energy pole, we find that the pair susceptibility diverges leading to a superconducting instability. We evaluate the pair fluctuation part…
Universality and its limits in non-Hermitian many-body quantum chaos using the Sachdev-Ye-Kitaev model
本研究以nHSYK模型和非厄米随机矩阵为对象，分析非厄米多体量子混沌的长程谱关联，揭示谱刚性弱化等普适性偏离的原因，提出局域谱形状因子作为有效诊断工具。
Chaotic-Integrable Transition for Disordered Orbital Hatsugai-Kohmoto Model
本文通过谱形因子和相邻间距比研究无序多轨道 Hatsugai-Kohmoto 模型的混沌—可积转变，并指出 OTOC 平台值无法可靠区分多体混沌与可积相。
In this letter, we use the exactly solvable Sachdev-Ye-Kitaev model to address the issue of entropy dynamics when an interacting quantum system is coupled to a non-Markovian environment. We find that at the initial stage, the entropy always increases linearly matching the Markovian result. When the system thermalizes with the environment at a sufficiently long time, if the environment temperature is low and the coupling between system and environment is weak, then the total thermal entropy is low and the entanglement between system and environment is also weak, which yields a small system entr…
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