Decomposition of high-rank factorized unitary coupled-cluster operators using ancilla and multi-qubit controlled low-rank counterparts

TL;DR

This paper introduces new quantum circuit schemes that decompose high-rank unitary coupled-cluster operators into lower-depth circuits by using additional qubits, addressing hardware limitations in quantum state preparation.

Contribution

It proposes novel methods that trade qubits for reduced circuit depth to efficiently implement high-rank UCC operators in quantum algorithms.

Findings

Decomposition schemes significantly lower circuit depth for high-rank UCC operators.

Methods utilize ancilla qubits to facilitate low-depth circuit implementation.

Approaches remain effective even with fault-tolerant quantum computers.

Abstract

The factorized form of the unitary coupled-cluster approximation is one of the most promising methodologies to prepare trial states for strongly correlated systems within the variational quantum eigensolver framework. The factorized form of the UCC ansatz can be systematically applied to a reference state to generate the desired entanglement. The difficulty associated with such an approach is the requirement of simultaneously entangling a growing number of qubits, which quickly exceeds the hardware limitations of today's quantum machines. In particular, while circuits for singles and double excitations can be performed on current hardware, higher-rank excitations require too many gate operations. In this work, we propose a set of new schemes that trade off using extra qubits for a reduced gate depth to decompose high-rank UCC excitation operators into significantly lower depth circuits. These results will remain useful even when fault-tolerant machines are available to reduce the overall state-preparation circuit depth.