# Increasing the representation accuracy of quantum simulations of chemistry without extra quantum resources

## TL;DR

This paper presents a measurement-based technique that enhances the accuracy of quantum chemistry simulations on quantum computers without requiring extra qubits or circuit depth, improving the efficiency of near-term quantum experiments.

## Contribution

The authors introduce a novel measurement-based method to improve the accuracy of active space quantum simulations without additional quantum resources.

## Key findings

- Achieves 20-qubit accuracy with only 4 qubits for a hydrogen molecule.
- Requires only a modest increase in measurements, not circuit complexity.
- Theoretically analyzed and demonstrated with proof-of-concept calculations.

## Abstract

Proposals for near-term experiments in quantum chemistry on quantum computers leverage the ability to target a subset of degrees of freedom containing the essential quantum behavior, sometimes called the active space. This approximation allows one to treat more difficult problems using fewer qubits and lower gate depths than would otherwise be possible. However, while this approximation captures many important qualitative features, it may leave the results wanting in terms of absolute accuracy (basis error) of the representation. In traditional approaches, increasing this accuracy requires increasing the number of qubits and an appropriate increase in circuit depth as well. Here we introduce a technique requiring no additional qubits or circuit depth that is able to remove much of this approximation in favor of additional measurements. The technique is constructed and analyzed theoretically, and some numerical proof of concept calculations are shown. As an example, we show how to achieve the accuracy of a 20 qubit representation using only 4 qubits and a modest number of additional measurements for a simple hydrogen molecule. We close with an outlook on the impact this technique may have on both near-term and fault-tolerant quantum simulations.