Logarithmic Depth Decomposition of Approximate Multi-Controlled Single-Qubit Gates Without Ancilla Qubits

TL;DR

This paper introduces a novel, resource-efficient method for decomposing multi-controlled quantum gates with logarithmic depth and no ancilla qubits, enhancing scalability for quantum computing.

Contribution

It presents new decompositions of multi-controlled gates that reduce circuit depth and ancilla requirements, including a relative-phase approach and optimized approximations.

Findings

Logarithmic depth decomposition with a single ancilla qubit.

Ancilla-free relative-phase multi-controlled NOT gate.

Reduced circuit depth and CNOT count for large controlled-unitary gates.

Abstract

The synthesis of quantum operators involves decomposing general quantum gates into the gate set supported by a given quantum device. Multi-controlled gates are essential components in this process. In this work, we present an improved decomposition of multi-controlled NOT gates with logarithmic depth using a single ancilla qubit while reducing the ancillary resource requirements compared to previous work. We further introduce a relative-phase multi-controlled NOT gate that eliminates the need for ancillas. Building on these results, we optimize a previously proposed decomposition of multi-target, multi-controlled special unitary SU(2) gates by identifying the presence of a conditionally clean qubit. Additionally, we introduce the best-known decomposition of multi-controlled approximate unitary U(2) gates, which do not require ancilla qubits. This approach significantly reduces the overall circuit depth and CNOT count while preserving an adjustable error parameter, yielding a more efficient and scalable solution for synthesizing large controlled-unitary gates. Our method is particularly suitable for both NISQ and fault-tolerant quantum architectures. All software developed in this project is freely available.