Improving chemistry calculations with virtual quantum subspace expansion

Accurate calculations in quantum chemistry require the use of large basis sets which amounts to a large number of molecular spin-orbitals. Each spin-orbital is typically mapped to a separate qubit. Many qubits are therefore necessary to achieve a desired accuracy. However, noisy intermediate-scale quantum computers have only a small number of qubits which limits the reach of quantum computing algorithms in quantum chemistry. A promising approach to overcome this problem is to use a quantum computer to solve only the classically hard part and a classical computer to solve the rest. A recently proposed virtual quantum subspace expansion (VQSE) method achieves this by modeling only the active space, that captures essential quantum effects, on a quantum computer. We report experimental results obtained using the VQSE algorithm to model small molecules. This work explores practical viability of hybrid quantum-classical methods in quantum computing.