Variational Quantum Circuits for Multi-Qubit Gate Automata

TL;DR

This paper introduces a variational quantum algorithm for designing time-independent Hamiltonians that implement multi-qubit gates, demonstrating high-fidelity Toffoli gate simulation on NISQ devices.

Contribution

It presents a novel NISQ-compatible hybrid method for Hamiltonian design to realize specific multi-qubit unitaries, including the Toffoli gate, with high fidelity.

Findings

Successfully simulated Toffoli gate with high fidelity

Demonstrated robustness against stochastic noise

Proposed a scalable approach for multi-qubit gate implementation

Abstract

Implementing quantum operations in the form of natural Hamiltonian dynamics is desirable, since they almost require no external control or feedback. In this work, we propose a NISQ-friendly quantum-classical hybrid approach to designing a time-independent Hamiltonian that generates a given multi-qubit unitary. In particular, we execute a Variational Quantum Algorithm, whose ansatz is carefully chosen to be a sequence of appropriately parametrized unitaries describing at most two-qubit nearest neighbour interactions, dictating the target unitary. Subsequently, we apply our approach to simulate multi-qubit target gates, with and without stochastic noise. We demonstrate that our strategy allows us to implement a Toffoli gate with sufficiently high fidelity, as compared to the other similar techniques. Our approach is an example of the usage of quantum computing for the design of quantum computers.