# Variational Consistent Histories as a Hybrid Algorithm for Quantum Foundations

## TL;DR

This paper introduces a hybrid quantum-classical algorithm leveraging quantum computers to find consistent histories in the Consistent Histories formalism, enabling complex foundational quantum calculations.

## Contribution

It proposes a novel variational hybrid algorithm for consistent histories, combining quantum evaluation of the decoherence functional with classical optimization.

## Key findings

- Successful implementation on a cloud quantum computer for a spin in a magnetic field.
- Simulation demonstrating emergence of classicality in a chiral molecule.
- Exponential speedup in evaluating the decoherence functional.

## Abstract

Although quantum computers are predicted to have many commercial applications, less attention has been given to their potential for resolving foundational issues in quantum mechanics. Here we focus on quantum computers' utility for the Consistent Histories formalism, which has previously been employed to study quantum cosmology, quantum paradoxes, and the quantum-to-classical transition. We present a variational hybrid quantum-classical algorithm for finding consistent histories, which should revitalize interest in this formalism by allowing classically impossible calculations to be performed. In our algorithm, the quantum computer evaluates the decoherence functional (with exponential speedup in both the number of qubits and the number of times in the history), and a classical optimizer adjusts the history parameters to improve consistency. We implement our algorithm on a cloud quantum computer to find consistent histories for a spin in a magnetic field, and on a simulator to observe the emergence of classicality for a chiral molecule.