Fast multi-qubit gates through simultaneous two-qubit gates

TL;DR

This paper introduces a method to implement multi-qubit gates efficiently by applying multiple two-qubit gates simultaneously, enhancing quantum circuit depth and fidelity without hardware modifications.

Contribution

It presents a novel approach to realize multi-qubit gates through simultaneous two-qubit gates, applicable on current quantum hardware, improving speed and fidelity.

Findings

Multi-qubit gates can be realized by simultaneous two-qubit gates.

These gates are as fast or faster than individual two-qubit gates.

Numerical simulations show >99% fidelity in current hardware.

Abstract

Near-term quantum computers are limited by the decoherence of qubits to only being able to run low-depth quantum circuits with acceptable fidelity. This severely restricts what quantum algorithms can be compiled and implemented on such devices. One way to overcome these limitations is to expand the available gate set from single- and two-qubit gates to multi-qubit gates, which entangle three or more qubits in a single step. Here, we show that such multi-qubit gates can be realized by the simultaneous application of multiple two-qubit gates to a group of qubits where at least one qubit is involved in two or more of the two-qubit gates. Multi-qubit gates implemented in this way are as fast as, or sometimes even faster than, the constituent two-qubit gates. Furthermore, these multi-qubit gates do not require any modification of the quantum processor, but are ready to be used in current quantum-computing platforms. We demonstrate this idea for two specific cases: simultaneous controlled-Z gates and simultaneous iSWAP gates. We show how the resulting multi-qubit gates relate to other well-known multi-qubit gates and demonstrate through numerical simulations that they would work well in available quantum hardware, reaching gate fidelities well above 99 %. We also present schemes for using these simultaneous two-qubit gates to swiftly create large entangled states like Dicke and Greenberg-Horne-Zeilinger states.