# Theory of coupled parametric oscillators beyond coupled Ising spins

## TL;DR

This paper investigates the complex dynamics of two coupled parametric oscillators beyond the simple Ising spin analogy, revealing new phases and universal behavior through analytical and numerical methods.

## Contribution

It provides a comprehensive theoretical analysis of coupled parametric oscillators with energy-conserving interactions, extending understanding beyond traditional Ising spin models.

## Key findings

- Identification of phase boundaries and critical exponents.
- Discovery of new phases with multiple attractors.
- Universal character of the phase diagram independent of nonlinearity type.

## Abstract

Periodically driven parametric oscillators offer a convenient way to simulate classical Ising spins. When many parametric oscillators are coupled dissipatively, they can be analogous to networks of Ising spins, forming an effective coherent Ising machine (CIM) that efficiently solves computationally hard optimization problems. In the companion paper, we studied experimentally the minimal realization of a CIM, i.e. two coupled parametric oscillators [L. Bello, M. Calvanese Strinati, E. G. Dalla Torre, and A. Pe'er, Phys. Rev. Lett. 123, 083901 (2019)]. We found that the presence of an energy-conserving coupling between the oscillators can dramatically change the dynamics, leading to everlasting beats, which transcend the Ising description. Here, we analyze this effect theoretically by solving numerically and, when possible, analytically the equations of motion of two parametric oscillators. Our main tools include: (i) a Floquet analysis of the linear equations, (ii) a multi-scale analysis based on a separation of time scales between the parametric oscillations and the beats, and (iii) the numerical identification of limit cycles and attractors. Using these tools, we fully determine the phase boundaries and critical exponents of the model, as a function of the intensity and the phase of the coupling and of the pump. Our study highlights the universal character of the phase diagram and its independence on the specific type of nonlinearity present in the system. Furthermore, we identify new phases of the model with more than two attractors, possibly describing a larger spin algebra.