# In situ upgrade of quantum simulators to universal computers

## TL;DR

This paper introduces a method for transforming quantum simulators into universal quantum computers by using optimal control techniques that learn to implement logic gates directly on the simulator, bypassing classical modeling limitations.

## Contribution

It proposes an in situ learning approach to efficiently construct quantum logic gates, enabling quantum simulators to be upgraded to universal quantum computers.

## Key findings

- The scheme scales polynomially with the number of qubits.
- Numerical simulations up to 9 qubits show promising results.
- The method effectively bypasses classical modeling challenges.

## Abstract

Quantum simulators, machines that can replicate the dynamics of quantum systems, are being built as useful devices and are seen as a stepping stone to universal quantum computers. A key difference between the two is that computers have the ability to perform the logic gates that make up algorithms. We propose a method for learning how to construct these gates efficiently by using the simulator to perform optimal control on itself. This bypasses two major problems of purely classical approaches to the control problem: the need to have an accurate model of the system, and a classical computer more powerful than the quantum one to carry out the required simulations. Strong evidence that the scheme scales polynomially in the number of qubits, for systems of up to 9 qubits with Ising interactions, is presented from numerical simulations carried out in different topologies. This suggests that this in situ approach is a practical way of upgrading quantum simulators to computers.