Qu&Co comments on this publication:

Quantum computers were initially proposed to efficiently simulate quantum mechanical systems with an exponential speedup compared to classical computers. We are currently in the noisy intermediate-scale quantum (NISQ) era, which means quantum chips still have a small number of qubits. This prohibits straightforward quantum simulations of realistic molecules and materials, whose description requires hundreds of atoms and thousands to millions of degrees of freedom to represent the electronic wavefunctions. One research direction which attempts to bypass this restriction is the development of hybrid quantum-classical methods where the quantum computation is restricted to a small portion of the system.

In this paper, a quantum embedding theory is proposed for the calculation of strongly-correlated electronic states of active regions, while the rest of the system is described with density functional theory (DFT). DFT (and its various approximations) has been extremely successful in predicting numerous properties of solids, liquids and molecules, and in providing key interpretations to a variety of experimental results, but is often inadequate to describe strongly-correlated electronic state. The novel theory proposed in this paper is built on DFT and does not require the explicit evaluation of virtual electronic states, thus making the method scalable to materials with thousands of electrons. Also, it includes the effect of exchange-correlation interactions of the environment on active regions, thus going beyond commonly adopted approximations in conventional DFT.

The proposed quantum embedding theory utilizes a classical and a quantum algorithm to solve the Hamiltonian that describes the problem and yields results in good agreement with existing experimental measurements and still-tractable computations on classical computing architectures. The theory is tested in various solid-state quantum information technologies, which exhibit strongly-correlated electronic states. In this way, the authors show how the quantum-classical hybrid approach incorporating DFT enables the study of large-scale material systems while adding the strongly-correlated dynamics analysis which the quantum simulation algorithm can provide.