Kicked Transverse Field Ising Model

• Kicked Transverse Field Ising Model is a driven quantum many-body system characterized by periodic transverse kicks in a spin chain, producing chaotic evolution and high entanglement.
• It employs Floquet dynamics with non-integrable interactions and resonance conditions to reveal detailed quantum chaos and entanglement growth mechanisms.
• The model underpins applications in quantum simulators, enabling benchmarks for random state generation and controlled investigations of quantum heating.

The Kicked Transverse Field Ising Model (kTFIM) refers to a class of quantum many-body spin systems in which a transverse field (typically along the $x$ axis) is applied in a time-periodic, “kicked” (delta-function pulsed) manner, often complemented by a continuous or ramped longitudinal field and/or Ising exchange interactions. These models represent an intersection of quantum Floquet dynamics, quantum chaos, non-integrable many-body evolution, and the paper of entanglement and statistical properties characteristic of random pure states.

1. Model Definition and Quantum Map Structure

The Hamiltonian of the kTFIM is typically formulated as

$$\mathcal{H}(t) = J_z \sum_{j=1}^L \sigma_j^z \sigma_{j+1}^z + h_z(t) \sum_{j=1}^L \sigma_j^z + \sum_{k=-\infty}^{\infty} \delta\left(k - \frac{t}{\tau}\right) h_x(t) \sum_{j=1}^L \sigma_j^x,$$

where:

• $J_z$ is the Ising interaction (often set to unity),
• $h_z(t)$ is a longitudinal field, commonly ramped in time, e.g., $h_{z0}\sin(\alpha t)$,
• $h_x(t)$ is the (pulsed or kicked) transverse field, e.g., $h_{x0}\cos(\alpha t)$,
• $\delta$ is the Dirac delta, generating kicks at intervals $\tau$,
• $\sigma_j^{x,z}$ are Pauli operators at site $j$.

Between kicks, the system evolves under $\mathcal{H}_\text{Ising}+h_z(t)\sum_j\sigma_j^z$. At kick times, all spins experience a non-commuting rotation from the transverse field. The stroboscopic evolution is described by the Floquet (quantum map) sequence: $U(k) = U_x(k) U_z(k),$ with

$$U_z(k) = \prod_{j} \exp\left[-i\tau \sigma_j^z \sigma_{j+1}^z - i\left(\int_{k\tau}^{(k+1)\tau} h_z(t) dt\right)\sigma_j^z\right],$$

$$U_x(k) = \prod_{j} \exp\left[-i\tau h_x(k\tau)\sigma_j^x\right].$$

This checkerboard of unitary operations reflects the interplay between non-commuting dynamical generators and defines a non-integrable, time-crystalline structure even for fixed $h_x$ and $h_z$.

The state at stroboscopic times $t = N\tau$ is recursively,

$$|\psi(N\tau)\rangle = \prod_{k=0}^{N-1} U(k) |\psi(0)\rangle.$$

2. Dynamical Protocols, Entanglement Growth, and Random State Statistics

The kTFIM supports distinct regimes depending on field protocols:

Quench Protocol (Pulsed Fields):

• $h_x(t)$ is swept from a maximal value to zero, while $h_z(t)$ ramps from zero to a peak—typically $h_x(t) = h_{x0}\cos(\alpha t)$, $h_z(t) = h_{z0}\sin(\alpha t)$, $\alpha=\pi/(2t_\text{max})$.
• Initial state is usually the product eigenstate of $\sigma^x$ (all spins polarized along $x$), closely aligned with the maximal transverse field ground state.
• Observables:
  • Nearest-neighbor concurrence $C(i,j)$: Two-qubit entanglement measure; vanishes as longitudinal field increases.
  • Q-measure: $$Q = 2\left[1-\frac{1}{L}\sum_{k=1}^L\text{Tr}(\rho_k^2)\right]$$, quantifies single-site purity loss, rising as multipartite entanglement spreads.
  • Block Entropy ($S_{L/2}$): $$-\text{Tr}[\rho_{1,\dots,L/2}\log_2 \rho_{1,\dots,L/2}]$$, measures half-chain entanglement; saturates near the random state value, as given by Page's formula $S_{\text{avg}} \approx \ln m - m/(2n)$ for $m$-dimensional subsystems [$1305.5129$].

Time-evolved states rapidly converge to statistical properties mirroring Haar-random pure states: (i) Block entropy approaches Page’s value, (ii) reduced density matrix eigenvalue distributions match Marchenko–Pastur law, and (iii) wavefunction intensities follow exponential statistics.

Steady Protocol (Fixed Fields):

• Both $h_x$ and $h_z$ are time-independent.
• Time evolution is purely by repeated application of a Floquet operator $U = U_x U_z$.
• Observed entanglement is non-monotonic (“U”-shaped in kick period $\tau$) and statistical properties can deviate from random-matrix predictions, especially with periodic boundary conditions.

3. Quantum Resonance and Kicking Interval Effects

A remarkable non-ergodic effect emerges at specific $\tau$: $$U_z^I(\tau) = \prod_{j=1}^L \exp\left(-i\tau \sigma_j^z \sigma_{j+1}^z\right)$$ At $\tau = \pi/2$, with periodic boundaries,

$U_z^I(\pi/2) = (-i)^L I_{2^L},$

rendering the interaction trivial (up to a phase). Here, entanglement generation is "frozen," even in a system that is non-integrable away from these points. Such resonance results in stroboscopic "pauses" or suppression of scrambling, paralleling quantum resonances in the kicked rotor.

This freezing is robust even when the full time evolution away from resonance is highly entangling and drives the system to nearly maximal multipartite entanglement.

4. Role of Integrability Breaking and Nonintegrable Dynamics

The addition of a longitudinal field $h_z(t)\ne0$ breaks the integrability of the free-fermion solution (contrasting the exactly solvable static TFIM). This promotes nontrivial entanglement growth and state complexity. The combined effect of non-commuting pulses (transverse field kicks) and a ramping $h_z(t)$ creates a regime in which the conventional mapping to quadratic fermions fails, and eigenstate statistics exhibit Wigner-Dyson level statistics and random matrix universality.

Numerically, as multipartite entanglement grows, the concurrence drops to zero, and the system's density matrices yield Marchenko–Pastur eigenvalue distributions, providing evidence for the generation of random-like states [$1305.5129$].

5. Impact of Boundary Conditions and Spectral Statistics

Recent spectral analysis of related kicked Ising models at self-dual points reveals a strong sensitivity to boundary conditions [$2506.15816$]:

• Periodic Boundaries: The trace of the Floquet propagator behaves as a real Gaussian (higher moments enhanced: $M_{2\ell}\approx (2\ell-1)!!(K(t))^\ell$), due to extra conjugate symmetries.
• Open Boundaries: Trivializes extra symmetry, so the trace is complex Gaussian ($M_{2\ell}\approx\ell!(K(t))^\ell$), the standard COE prediction.

This boundary-driven crossover affects the interpretation of spectral fluctuations, relevant in assessing quantum chaos.

6. Connections to Quantum Chaos, Ergodicity, and Statistical Mechanics

The kTFIM serves as a paradigmatic system for studying the emergence of quantum chaos in Floquet systems:

• Level Statistics: As in higher-dimensional kicked models [$1409.2915$], level spacing distributions closely follow the Wigner surmise (COE predictions) except at integrable or resonant points.
• Dynamical Susceptibility: Time-averaged autocorrelation functions of local observables (e.g., magnetization) can exhibit sharp transitions between ergodic (decaying) and non-ergodic (persisting) dynamics, sometimes decoupled from global spectral transitions (flatness of spectral density).
• Out-of-time-order correlators (OTOCs) and Lyapunov Exponents: In generalizations with all-to-all or $p$-body interactions, short-time OTOC growth rates can be extracted from classical Lyapunov exponents, confirming a semiclassical-to-quantum chaos correspondence [$2103.00748$].

Observable	Quench (Pulsed)	Steady (Fixed)
Block Entropy, Q-measure	Grows to Page/random state	Non-monotonic, “U”-shaped
State Intensity Statistics	Exponential law (random state)	Possible deviations
Reduced Density Eigenvalues	Marchenko–Pastur law	Deviations (periodic BCs)
Two-site concurrence	Drops to zero (large $h_z$)	Oscillatory, dips at resonance

7. Applications, Experimental Realizations, and Extensions

• Quantum Simulators: The protocols described—especially quenching and kicking—map directly onto cold atom, trapped ion, and superconducting qubit platforms where time-dependent control of transverse and longitudinal fields is routine.
• Benchmarking and Random State Generation: The kTFIM dynamically generates multipartite random states, critical for benchmarking quantum devices, testing volume-law entanglement scaling, and as state-initialization for quantum information tasks.
• Probing Quantum Phase Transitions: Through tuning pulse sequences and field ramps, the kTFIM provides a testbed for investigating nonequilibrium Kibble-Zurek scalings and symmetry breaking gap detection protocols.
• Control of Quantum Heating: The identification of resonance points and corresponding dynamical freezing suggests mechanisms for mitigating Floquet heating or for engineering “prethermal” plateaus in driven quantum many-body systems.

8. Summary and Outlook

The kicked transverse field Ising model embodies a tightly controlled avenue for exploring the boundary between fully quantum chaotic, highly entangling evolution and the existence of precise, dynamically stabilized or “frozen” regimes. Its paper highlights:

• The role of time-dependent protocols (quench versus steady),
• The importance of competing non-commuting terms,
• The emergence of quantum resonance phenomena and their analytic characterization,
• The dynamical generation of nearly-random quantum states exhibiting maximal multipartite entanglement,
• The sensitivity of dynamical and statistical observables—especially spectral moments and entanglement—to boundary conditions and protocol details.

Ongoing research explores further generalizations (e.g., $p$-body interactions, monitored nonunitary evolution), connections to quantum thermodynamic universality, and implications for the stability of quantum simulation and quantum information processing in kicked many-body systems [$1305.5129$, $1409.2915$, $2506.15816$].