Overlap-ADAPT-VQE: practical quantum chemistry on quantum computers via overlap-guided compact Ansätze
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Abstract
ADAPT-VQE is a robust algorithm for hybrid quantum-classical simulations of quantum chemical systems on near-term quantum computers. While its iterative process systematically reaches the ground state energy, practical implementations of ADAPT-VQE are sensitive to local energy minima, leading to over-parameterized ansätze. We introduce the Overlap-ADAPT-VQE to grow wave-functions by maximizing their overlap with any intermediate target wave-function that already captures some electronic correlation. By avoiding building the ansatz in the energy landscape strewn with local minima, the Overlap-ADAPT-VQE produces ultra-compact ansätze suitable for high-accuracy initialization of a new ADAPT procedure. Significant advantages over ADAPT-VQE are observed for strongly correlated systems including substantial savings in circuit depth. Since this compression strategy can also be initialized with accurate Selected-Configuration Interaction (SCI) classical target wave-functions, it paves the way for chemically accurate simulations of larger systems, and strengthens the promise of decisively surpassing classical quantum chemistry through the power of quantum computing. The paper explores the possibility of creating ansatz wave-functions for the variational quantum eigensolver that are much more compact than ADAPT-VQE while achieving chemical accuracy. The proposed Overlap-ADAPT-VQE combined with the classical selected-CI approach can reach chemical accuracy for a stretched linear H6 chain using an ansatz with only 40 operators compared to more than 150 for ADAPT-VQE.