arXiv subjects

Nilakash Sorokhaibam

Publications and source records attributed to Nilakash Sorokhaibam.

12 recordsLinked to original sources

arXiv2026

Monotonicity of eigenstate thermalization hypothesis in two-dimensional systems

We study numerically the enveloping $f$-function of the fluctuation term in eigenstate thermalization hypothesis (ETH) statement. We concentrate on the energy (or entropy) dependence of this function in two-dimensional systems. Our numerical results show that it is, in general, a monotonically increasing function of the entropy. This is in agreement with the general expectation that fluctuations increase with increasing entropy. We show that the $f$-function locally flattens with increasing system-size. The flattening rate is directly proportional to the system size. We also show that the flattening rate is directly proportional to the particle number for systems of same spatial size. This variation of the $f$-function is important for physics at subleading order of the system-size. So, it is relevant for intermediate-size systems (upto a few hundred qubits) which are experimentally accessible. One exception we found is that the $f$-function of the order parameter of a thermal phase transition defy the monotonic behaviour.

cond-mat.stat-mech↗

arXiv2026

ETH-monotonicity and the black hole singularity

We study the enveloping function of the fluctuation term in eigenstate thermalization hypothesis (ETH) statement for holographic conformal field theories. We use this function to identify and examine black hole microstates. We set down a set of desirable criteria for this function called ETH-monotonicity. It reinforces the Kelvin statement of the second law of thermodynamics over and above the universal entropic contribution. We show that higher-dimensional holographic conformal field theories possess ETH-monotonicity. Stronger contribution from ETH-monotonicity to the second law of thermodynamics is observed in smaller black hole microstates. It dominates other quantum fluctuations. It also measures the curvature at the horizon of the small black holes. In the smallest size limit, the black hole curvature singularity is constructed of microstates for which ETH-monotonicity starts competing with the entropic contribution. We expect that ETH-monotonicity will persist even in the ultimate quantum theory of gravity. Because it is a property of many-body quantum chaotic systems which becomes more prominent with decreasing system size, unlike other physical properties which are usually more predictable with increasing system size. Two-dimensional holographic conformal field theory does not possess all features of ETH-monotonicity which is in agreement with the absence of curvature singularity in the BTZ black hole.

hep-th↗

arXiv2025

Quantum chaos and the arrow of time

We show that quantum chaotic many-body systems possess the thermodynamic arrow of time in the thermodynamic limit. Berry's conjecture in quantum chaotic systems and equivalence of ensembles imply the Kelvin statement of the second law of thermodynamics at leading order in perturbation theory. We verify this result using numerical calculations. We also show that this gives rise to new constraints on the off-diagonal terms in eigenstate thermalization hypothesis (ETH) statement. We call the new constraints collectively as ETH-monotonicity. These constraints arise because pure entropic consideration is not enough for the emergence of the thermodynamic arrow of time.

quant-ph↗

arXiv2024

Thermalization and non-monotonic entanglement growth in an exactly solvable model

We study quantum quenches and subsequent non-equilibrium dynamics of free Dirac fermions in 1+1 spacetime dimensions using time dependent mass. The final state is a normalized boundary state which is called generalized Calabrese-Cardy (gCC) state and the system thermalizes to a generalized Gibb's Ensemble(GGE). We can also tune the initial states so that the final states are exact Calabrese-Cardy (CC) state and special gCC states. The system in the CC state thermalizes to a Gibb's ensemble. We derive closed-form analytic expressions for the growth of entanglement entropy of subsystems consisting of arbitrary number of disjoint intervals in CC state. We show that the entanglement entropy of a single interval grows monotonically before saturation. In case of certain gCC states, for particular charges, the entanglement entropy of a single interval grows non-monotonically when the effective chemical potential is increased beyond a critical value. We argue that the non-monotonic growth of entanglement entropy is due to increase in long range correlation and decrease in short range correlation at early times.

hep-th↗

arXiv2024

A measure of chaos from eigenstate thermalization hypothesis

Eigenstate thermalization hypothesis is a detailed statement of the matrix elements of few-body operators in energy eigenbasis of a chaotic Hamiltonian. Part of the statement is that the off-diagonal elements fall exponential for large energy difference. We propose that the exponent ($γ>0$) is a measure of quantum chaos. Smaller $γ$ implies more chaotic dynamics. The chaos bound is given by $γ=β/4$ where $β$ is the inverse temperature. We give analytical argument in support of this proposal. The slower exponential fall also means that the action of the operator on a state leads to higher delocalization in energy eigenbasis. Numerically we compare two chaotic Hamiltonians - SYK model and chaotic XXZ spin chain. Using the new measure, we find that the SYK model becomes maximally chaotic at low temperature which has been shown rigorously in previous works. The new measure is more readily accessible compare to other measures using numerical methods.

quant-ph↗

arXiv2021

Thermalization in different phases of charged SYK model

We study thermalization of charged SYK model in two different phases. We show that both the highly chaotic liquid phase and the dilute gas phase thermalize. Surprisingly the dilute gas state thermalizes instantaneously. We argue that this phenomenon arises because the system in this phase consists of only long-lived quasi-particles at very low density. The liquid state thermalizes exponentially fast. We also show that the additional introduction of random mass deformation (q=2 SYK term) slows down thermalization but the system thermalizes exponentially fast. This is observed despite the fact that the addition of large q=2 SYK interaction forces spectral statistics to obey Poisson statistics. An interesting new observation is that the effective temperature is non-monotonic during thermalization in the liquid state. It has a bump at relatively long time before settling down to the final value. With non-zero chemical potential, the effective temperature oscillates noticeably before settling down to the final value.

hep-th↗

arXiv2020

Traversable wormhole without interaction

We show that strong quantum entanglement can support a stable traversable wormhole without any explicit interaction or tunnelling term between the two boundary theories of the wormhole. Specifically we work with two complex SYK models. The entangled state is prepared using a tunnelling term in imaginary time but the tunnelling term is removed from the time evolution operator so the two complex SYK models are not coupled. Low temperature states show revival dynamics which is the hallmark of a traversable wormhole geometry. To send any meaningful information from one system to the other, one only needs to turn on a very small interaction term. The technique that we are employing can be applied to other systems to study aspects of quantum entanglement.

hep-th↗

arXiv2020

Phase transition and chaos in charged SYK model

We study chaotic-integrable transition and the nature of quantum chaos in SYK model with chemical potential. We use a novel numerical technique to calculate the partition function explicitly. We show the phase transition in the presence of large chemical potential. We also show that a mass-like term consisting of two fermion random interaction (q=2 SYK term) does not give rise to a sharp transition. We find that turning on the chemical potential suppresses the Lyapunov exponent in the chaotic phase exponentially.

hep-th↗

arXiv2020

Thermalization in 2D critical quench and UV/IR mixing

We consider quantum quenches in models of free scalars and fermions with a generic time-dependent mass $m(t)$ that goes from $m_0$ to zero. We prove that, as anticipated in MSS \cite{Mandal:2015jla}, the post-quench dynamics can be described in terms of a state of the generalized Calabrese-Cardy form $|ψ\rangle$= $\exp[-κ_2 H -\sum_{n>2}^\infty κ_n W_n]| \hbox{Bd} \rangle$. The $W_n$ ($n=2,3,...$, $W_2=H$) here represent the conserved $W_\infty$ charges and $| \hbox{Bd} \rangle$ represents a conformal boundary state. Our result holds irrespective of whether the pre-quench state is a ground state or a squeezed state, and is proved without recourse to perturbation expansion in the $κ_n$'s as in MSS. We compute exact time-dependent correlators for some specific quench protocols $m(t)$. The correlators explicitly show thermalization to a generalized Gibbs ensemble (GGE), with inverse temperature $β= 4κ_2$, and chemical potentials $μ_n=4κ_n$. In case the pre-quench state is a ground state, it is possible to retrieve the exact quench protocol $m(t)$ from the final GGE, by an application of inverse scattering techniques. Another notable result, which we interpret as a UV/IR mixing, is that the long distance and long time (IR) behaviour of some correlators depends crucially on all $κ_n$'s, although they are highly irrelevant couplings in the usual RG parlance. This indicates subtleties in RG arguments when applied to non-equilibrium dynamics.

hep-th↗

arXiv2020

Quantum Quenches and Thermalization in SYK models

We study non-equilibrium dynamics in SYK models using quantum quench. We consider models with two, four, and higher fermion interactions ($q=2, 4$, and higher) and use two different types of quench protocol, which we call step and bump quenches. We analyse evolution of fermion two-point functions without long time averaging. We observe that in $q=2$ theory the two-point functions do not thermalize. We find thermalization in $q=4$ and higher theories without long time averaging. We calculate two different exponents of which one is equal to the coupling and the other is proportional to the final temperature. This result is more robust than thermalization obtained from long time averaging as proposed by the eigenstate thermalization hypothesis(ETH). Thermalization achieved without long time averaging is more akin to mixing than ergodicity.

hep-th↗

arXiv2015

Thermalization with chemical potentials, and higher spin black holes

We study the long time behaviour of local observables following a quantum quench in 1+1 dimensional conformal field theories possessing additional conserved charges besides the energy. We show that the expectation value of an arbitrary string of {\it local} observables supported on a finite interval exponentially approaches an equilibrium value. The equilibrium is characterized by a temperature and chemical potentials defined in terms of the quenched state. For an infinite number of commuting conserved charges, the equilibrium ensemble is a generalized Gibbs ensemble (GGE). We compute the thermalization rate in a systematic perturbation in the chemical potentials, using a new technique to sum over an infinite number of Feynman diagrams. The above technique also allows us to compute relaxation times for thermal Green's functions in the presence of an arbitrary number of chemical potentials. In the context of a higher spin (hs[λ]) holography, the partition function of the final equilibrium GGE is known to agree with that of a higher spin black hole. The thermalization rate from the CFT computed in our paper agrees with the quasinormal frequency of a scalar field in this black hole.

hep-th↗

arXiv2014

The inside outs of AdS(3)/CFT(2): Exact AdS wormholes with entangled CFT duals

We present the complete family of solutions of 3D gravity (Lambda<0) with two asymptotically AdS exterior regions. The solutions are constructed from data at the two boundaries, which correspond to two independent and arbitrary stress tensors T_R, \bar T_R, and T_L, \bar T_L. The two exteriors are smoothly joined on to an interior region through a regular horizon. We find CFT duals of these geometries which are entangled states of two CFT's. We compute correlators between general operators at the two boundaries and find perfect agreement between CFT and bulk calculations. We calculate and match the CFT entanglement entropy (EE) with the holographic EE which involves geodesics passing through the wormhole. We also compute a holographic, non-equilibrium entropy for the CFT using properties of the regular horizon. The construction of the bulk solutions here uses an exact version of Brown-Henneaux type diffeomorphisms which are asymptotically nontrivial and transform the CFT states by two independent unitary operators on the two sides. Our solutions provide an infinite family of explicit examples of the ER=EPR relation of Maldacena and Susskind [arXiv:1306.0533].

hep-th↗