Quantum dynamics in sine-square deformed conformal field theory: Quench from uniform to non-uniform CFTs

TL;DR

This paper investigates the non-equilibrium quantum dynamics of sine-square deformed conformal field theories, revealing a crossover in entanglement entropy growth and confirming findings through numerical simulations on a critical fermion chain.

Contribution

It provides the first detailed analysis of entanglement dynamics in SSD CFTs, including a crossover time and a universal logarithmic growth, supported by numerical validation.

Findings

Entanglement entropy remains constant before crossover time and grows as log t afterward.

The growth rate is independent of subsystem and total system sizes.

Numerical results on a critical fermion chain confirm the theoretical predictions.

Abstract

In this work, motivated by the sine-square deformation (SSD) for (1+1)-dimensional quantum critical systems, we study the non-equilibrium quantum dynamics of a conformal field theory (CFT) with SSD, which was recently proposed to have continuous energy spectrum and continuous Virasoro algebra. In particular, we study the time evolution of entanglement entropy after a quantum quench from a uniform CFT, which is defined on a finite space of length $L$, to a sine-square deformed CFT. We find there is a crossover time $t^{\ast}$ that divides the entanglement evolution into two interesting regions. For $t\ll t^{\ast}$, the entanglement entropy does not evolve in time; for $t\gg t^{\ast}$, the entanglement entropy grows as $S_A(t)\simeq \frac{c}{3}\log t$, which is independent of the lengths of the subsystem and the total system. This $\log t$ growth with no revival indicates that a sine-square deformed CFT effectively has an infinite length, in agreement with previous studies based on the energy spectrum analysis. Furthermore, we study the quench dynamics for a CFT with M$\ddot{\text{o}}$bius deformation, which interpolates between a uniform CFT and a sine-square deformed CFT. The entanglement entropy oscillates in time with period $L_{\text{eff}}=L\cosh(2\theta)$, with $\theta=0$ corresponding to the uniform case and $\theta\to \infty$ corresponding to the SSD limit. Our field theory calculation is confirmed by a numerical study on a (1+1)-d critical fermion chain.