Connected Correlation Function Research Articles

Monte Carlo matrix-product-state approach to the false vacuum decay in the monitored quantum Ising chain

In this work we characterize the false vacuum decay in the ferromagnetic quantum Ising chain with a weak longitudinal field subject to continuous monitoring of the local magnetization. Initializing the system in a metastable state, the false vacuum, we study the competition between coherent dynamics, which tends to create resonant bubbles of the true vacuum, and measurements which induce heating and reduce the amount of quantum correlations. To this end we exploit a numerical approach based on the combination of matrix product states with stochastic quantum trajectories which allows for the simulation of the trajectory-resolved non-equilibrium dynamics of interacting many-body systems in the presence of continuous measurements. We show how the presence of measurements affects the false vacuum decay: At short times the departure from the local minimum is accelerated while at long times the system thermalizes to an infinite-temperature incoherent mixture. For large measurement rates the system enters a quantum Zeno regime. The false vacuum decay and the thermalization physics are characterized in terms of the magnetization, connected correlation function, and the trajectory-resolved entanglement entropy.

Flocking without Alignment Interactions in Attractive Active Brownian Particles

Within a simple model of attractive active Brownian particles, we predict flocking behavior and challenge the widespread idea that alignment interactions are necessary to observe this collective phenomenon. Here, we show that even nonaligning attractive interactions can lead to a flocking state. Monitoring the velocity polarization as the order parameter, we reveal the onset of a first-order transition from a disordered phase, characterized by several small clusters, to a flocking phase, where a single flocking cluster is emerging. The scenario is confirmed by studying the spatial connected correlation function of particle velocities, which reveals scale-free behavior in flocking states and exponential-like decay for nonflocking configurations. Our predictions can be tested in microscopic and macroscopic experiments showing flocking, such as animals, migrating cells, and active colloids.

Error-mitigated simulation of quantum many-body scars on quantum computers with pulse-level control

Quantum many-body scars are an intriguing dynamical regime in which quantum systems exhibit coherent dynamics and long-range correlations when prepared in certain initial states. We use this combination of coherence and many-body correlations to benchmark the performance of present-day quantum computing devices by using them to simulate the dynamics of an antiferromagnetic initial state in mixed-field Ising chains of up to 19 sites. In addition to calculating the dynamics of local observables, we also calculate the Loschmidt echo and a nontrivial connected correlation function that witnesses long-range many-body correlations in the scarred dynamics. We find coherent dynamics to persist over up to 40 Trotter steps even in the presence of various sources of error. To obtain these results, we leverage a variety of error mitigation techniques including noise tailoring, zero-noise extrapolation, dynamical decoupling, and physically motivated postselection of measurement results. Crucially, we also find that using pulse-level control to implement the Ising interaction yields a substantial improvement over the standard CNOT-based compilation of this interaction. Our results demonstrate the power of error mitigation techniques and pulse-level control to probe many-body coherence and correlation effects on present-day quantum hardware.

Logarithmic behaviour of connected correlation function in CFT

We study the (m)-type connected correlation functions of OPE blocks with respect to one spatial region in two dimensional conformal field theory. We find a logarithmic divergence for these correlation functions. We justify the logarithmic behaviour from two different approaches: massless free scalar theory and conformal block. Cutoff independent coefficients are obtained from analytic continuation of conformal blocks. A UV/IR relation has been found in the connected correlation functions. We could derive a formal “first law of thermodynamics” for a subsystem using the deformed reduced density matrix. An area law of the connected correlation function in higher dimensions is also discussed briefly.

Area law of connected correlation function in higher dimensional conformal field theory

We present a new area law which is associated with the correlator of OPE blocks in higher dimensional conformal field theories (CFTs). The area law shows similar behaviour as black hole entropy or geometric entanglement entropy. It includes a leading term which is proportional to the area of the entanglement surface, and a logarithmic subleading term with degree q. We extract the UV cutoff independent coefficients and discuss various properties of the coefficients.

On the Derivation of the GKLS Equation for Weakly Coupled Systems

We consider the reduced dynamics of a small quantum system in interaction with a reservoir when the initial state is factorized. We present a rigorous derivation of a GKLS master equation in the weak-coupling limit for a generic bath, which is not assumed to have a bosonic or fermionic nature, and whose reference state is not necessarily thermal. The crucial assumption is a reservoir state endowed with a mixing property: the n-point connected correlation function of the interaction must be asymptotically bounded by the product of two-point functions (clustering property).

Remarks on correlators of Polyakov loops

Polyakov loop eigenvalues and their $N$ dependence are studied in two- and four-dimensional $SU(N)$ Yang-Mills (YM) theory. The connected correlation function of the single-eigenvalue distributions of two separated Polyakov loops in two-dinemsional YM is calculated and is found to have a structure differing from the one of corresponding Hermitian random matrix ensembles. No large-$N$ nonanalyticities are found for two-point functions in the confining regime. Suggestions are made for situations in which large-$N$ phase transitions involving Polyakov loops might occur.

Matrix Theory, AdS/CFT, and Gauge/Gravity Correspondence

WithN→∞ being fixed,R→∞, the free energy of the Matrix theory on a supergravity backgroundFis a functional ofF,W=W(R,F). We try to relate this functional withSeff(R,F), the effective action ofF, whereFis translation invariant alongx-. The vertex function is then associated with the connected correlation function of the current densities. FromW(R,F), one can construct an effective actionΓ(Y)for the arbitrary matrix configurationY.Γ(Y)isR,and thusp+independent. IfW(R,F)=Seff(R,F),Γ(Y)will give the supergravity interactions amongMtheory objects with no light-cone momentum exchange. We then discuss the Matrix theory dual of the11dbackground generated by branes with the definitep+as well as the gauge theory dual of the10dbackground arising from thex-reduction. Finally, for SYM4with the4dbackground fieldF0, we give a possible way to induce the radial dependent5dfieldF(σ).

An extension of Pohlmeyer's theorem

Applying the exact renormalization group to scalar field theory in Euclidean space of general (not necessarily integer) dimension, it is proven that the only fixed point with vanishing anomalous dimension is the Gaussian one. The proof requires positivity of the two-point connected correlation function together with a technical assumption concerning solutions of the flow equation. The method, in which the representation of the flow equation as a heat equation plays a central role, extends directly to non-gauge theories with arbitrary matter content (though nonlinear sigma models are beyond the scope of the current method).

Quench dynamics near a quantum critical point: Application to the sine-Gordon model

We discuss the quench dynamics near a quantum critical point focusing on the sine-Gordon model as a primary example. We suggest a unified approach to sudden and slow quenches, where the tuning parameter $\lambda(t)$ changes in time as $\lambda(t)\sim \upsilon t^r$, based on the adiabatic expansion of the excitation probability in powers of $\upsilon$. We show that the universal scaling of the excitation probability can be understood through the singularity of the generalized adiabatic susceptibility $\chi_{2r+2}(\lambda)$, which for sudden quenches ($r=0$) reduces to the fidelity susceptibility. In turn this class of susceptibilities is expressed through the moments of the connected correlation function of the quench operator. We analyze the excitations created after a sudden quench of the cosine potential using a combined approach of form-factors expansion and conformal perturbation theory for the low-energy and high-energy sector respectively. We find the general scaling laws for the probability of exciting the system, the density of excited quasiparticles, the entropy and the heat generated after the quench. In the two limits where the sine-Gordon model maps to hard core bosons and free massive fermions we provide the exact solutions for the quench dynamics and discuss the finite temperature generalizations.

ON THE NON-TRIVIALITY OF THE λφ4 MODEL

The λφ4 model is conventionally used to explain the origin of mass of elementary particles through the Spontaneous Symmetry Breaking (SSB) phenomena. The triviality status of the λφ4 model in 4-dimensional spacetime remains an open question despite attempts by several authors. This study establishes a new approach to determine the triviality status of the λφ4 model based on an unpublished note by Professor Bryce DeWitt. We adopted the DeWitt's Ansatz for the 2-point connected correlation function on the lattice [Formula: see text] where α is a parameter that measures the departure from triviality. Calling α's continuum counterpart as β, then a non-zero value of β signifies non-triviality of the λφ4 model. The 2-point connected correlation function, given in terms of an Euclidean functional integral, is computed numerically via Monte Carlo methods. Our analysis, based on β, is different from the traditional analysis based on the renormalized coupling constant λR. To test the new approach, we performed the simulation in 2 dimensions and obtained results that are consistent with previous findings: 2-dimensional λφ4 model is non-trivial. Finally, for the case in 4 dimensions, our results show that the model is non-trivial.

CMB temperature anisotropies from third-order gravitational perturbations

In this paper we present a complete computation of the cosmic microwave background(CMB) anisotropies up to third order from gravitational perturbations accounting forscalar, vector and tensor perturbations. We then specify our results to the large-scale limit,providing the evolution of the gravitational potentials in a flat universe filled with matterand cosmological constant which characterizes the integrated Sachs–Wolfe effect. As aby-product in the large scale approximation we are able to give non-perturbative solutionsfor the photon geodesic equations. Our results are the first step towards providing acomplete theoretical prediction for cubic nonlinearities which are particularly relevant forcharacterizing the level of non-Gaussianity in the CMB through the detection of thefour-point angular connected correlation function (trispectrum). For this purposewe also allow for generic initial conditions due to primordial non-Gaussianity.

Spectral Gap and Exponential Decay of Correlations

We study the relation between the spectral gap above the ground state and the decay of the correlations in the ground state in quantum spin and fermion systems with short-range interactions on a wide class of lattices. We prove that, if two observables anticommute with each other at large distance, then the nonvanishing spectral gap implies exponential decay of the corresponding correlation. When two observables commute with each other at large distance, the connected correlation function decays exponentially under the gap assumption. If the observables behave as a vector under the U(1) rotation of a global symmetry of the system, we use previous results on the large distance decay of the correlation function to show the stronger statement that the correlation function itself, rather than just the connected correlation function, decays exponentially under the gap assumption on a lattice with a certain self-similarity in (fractal) dimensions D<2. In particular, if the system is translationally invariant in one of the spatial directions, then this self-similarity condition is automatically satisfied. We also treat systems with long-range, power-law decaying interactions.

WAVY WILSON LINE AND AdS/CFT

Wilson loops which are small deviations from straight, infinite lines, called wavy lines, are considered in the context of the AdS/CFT correspondence. A single wavy line and the connected correlation function of a straight and wavy line are considered. It is argued that, to leading order in "waviness," the functional form of the loop is universal and the coefficient, which is a function of the 't Hooft coupling, is found in weak coupling perturbation theory and the strong coupling limit using the AdS/CFT correspondence. Supersymmetric arguments are used to simplify the computation and to show that the straight line obeys the Migdal–Makeenko loop equation.

Breakdown of universality in random matrix models

We calculate smoothed cotrelators for a large random matrix model with a potential containing products of two traces tr W 1 ( M) · tr W 2 ( M) in addition to a single trace tr V ( M). Connected correlation function of density eigenvalues receives corrections besides the universal part derived by Brézin and Zee and it is no longer universal in a strong sense.

C-periodicity and the physical mass in the 3-state Potts model

The standard infinite-volume definition of connected correlation function and particle mass in the 3-state Potts model can be implemented in Monte Carlo simulations by using C-periodic spatial boundary conditions. This avoids both the breaking of translation invariance (cold wall b.c.) and the phase-dependent and thus possibly biased evaluation of data (periodic b.c.). The numerical feasibility of the standard definitions is demonstrated by sample computations on a 24 × 24 × 48 lattice.

Correlation function of random-field Ising chains: is it Lorentzian or not?

The two-point connected correlation function, or wavevector-dependent susceptibility, of the ferromagnetic Ising chain in a random field is calculated exactly at any temperature, for a two-parameter family of diluted symmetric exponential distributions of the magnetic fields. Thermodynamic properties of this model have been derived in a previous work by the authors. Besides the correlation function itself, the solution provides exact results for the (usual) susceptibility, the correlation length and the Edwards-Anderson parameter. The low-temperature regime is examined in full detail: the authors obtain in closed form the limit value at zero temperature of various quantities, and the first correction to this limit, which behaves linearly with temperature. The correlation length is discontinuous at zero temperature. They also derive the scaling form of the correlation function for small p, where the dilution p is the probability for a spin to be subjected to a non-zero random field. Even in this limit, the correlation function is more complicated than the simple Lorentzian predicted, e.g., by mean-field theory.