# Generation of Thermofield Double States and Critical Ground States with a Quantum Computer

## TL;DR

This paper demonstrates the generation of thermofield double and critical ground states of the transverse-field Ising model using a quantum computer, advancing the simulation of complex quantum states relevant to physics.

## Contribution

It introduces protocols inspired by QAOA to prepare thermal and critical states on a quantum computer, including noise threshold analysis.

## Key findings

- Successfully prepared thermofield double states at various temperatures.
- Prepared the critical ground state of the TFIM using hybrid optimization.
- Identified noise thresholds where shallow QAOA circuits outperform deeper ones.

## Abstract

Finite-temperature phases of many-body quantum systems are fundamental to phenomena ranging from condensed-matter physics to cosmology, yet they are generally difficult to simulate. Using an ion trap quantum computer and protocols motivated by the Quantum Approximate Optimization Algorithm (QAOA), we generate nontrivial thermal quantum states of the transverse-field Ising model (TFIM) by preparing thermofield double states at a variety of temperatures. We also prepare the critical state of the TFIM at zero temperature using quantum-classical hybrid optimization. The entanglement structure of thermofield double and critical states plays a key role in the study of black holes, and our work simulates such nontrivial structures on a quantum computer. Moreover, we find that the variational quantum circuits exhibit noise thresholds above which the lowest depth QAOA circuits provide the best results.