A qubit-ADAPT Implementation for H$_2$ Molecules using an Explicitly Correlated Basis
With the recent advances in the development of devices capable of performing quantum computations, a growing interest in finding near-term applications has emerged in many areas of science. In the era of non-fault tolerant quantum devices, algorithms that only require comparably short circuits accompanied by high repetition rates are considered to be a promising approach for assisting classical machines with finding solution on computationally hard problems. The ADAPT approach previously introduced in Nat. Commun. 10, 3007 (2019) extends the class of variational quantum eigensolver (VQE) algorithms with dynamically growing ansätze in order to find approximations to ground and excited state energies of molecules. In this work, the ADAPT algorithm has been combined with a first-quantized formulation for the hydrogen molecule in the Born-Oppenheimer approximation, employing the explicitly correlated basis functions introduced in J. Chem. Phys. 43, 2429 (1965). By the virtue of their explicit electronic correlation properties, it is shown in classically performed simulations that relatively short circuits yield chemical accuracy ($< 1.6$ mHa) for ground and excited state potential curves that can compete with second quantized approaches such as Unitary Coupled Cluster.