Quantum State Preparation Of Multiconfigurational States For Quantum Chemistry

TL;DR

This paper compares two quantum circuit methods for preparing multiconfigurational states in quantum chemistry, highlighting the efficiency of exploiting state sparsity over controlled Givens rotations.

Contribution

It introduces an automated method for external controls in Givens rotations and demonstrates that sparse state techniques can outperform traditional controlled rotations.

Findings

Sparse state techniques lead to highly reduced quantum circuits.

Exploiting chemical wavefunction sparsity improves state preparation efficiency.

Methods are effective for ground and excited state applications.

Abstract

The ability to prepare states for quantum chemistry is a promising feature of quantum computers, and efficient techniques for chemical state preparation is an active area of research. In this paper, we implement and investigate two methods of quantum circuit preparation for multiconfigurational states for quantum chemical applications. It has previously been shown that controlled Givens rotations are universal for quantum chemistry. To prepare a selected linear combination of Slater determinants (represented as occupation number configurations) using Givens rotations, the gates that rotate between the reference and excited determinants need to be controlled on qubits outside the excitation (external controls), in general. We implement a method to automatically find the external controls required for utilizing Givens rotations to prepare multiconfigurational states on a quantum circuit. We compare this approach to an alternative technique that exploits the sparsity of the chemical state vector and find that the latter can outperform the method of externally controlled Givens rotations; highly reduced circuits can be obtained by taking advantage of the sparse nature (where the number of basis states is significantly less than 2$^{n_q}$ for $n_q$ qubits) of chemical wavefunctions. We demonstrate the benefits of these techniques in a range of applications, including the ground states of a strongly correlated molecule, matrix elements of the Q-SCEOM algorithm for excited states, as well as correlated initial states for a quantum subspace method based on quantum computed moments and quantum phase estimation.