# Simplified projection on total spin zero for state preparation on quantum computers

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Abstract

We introduce a simple algorithm for projecting on states of a many-body system by performing a series of rotations to remove states with angular momentum projections greater than zero. Existing methods rely on unitary evolution with the two-body operator , which when expressed in the computational basis contains many complicated Pauli strings requiring Trotterization and leading to very deep quantum circuits. Our approach performs the necessary projections using the one-body operators and . By leveraging the method of Cartan decomposition, the unitary transformations that perform the projection can be parameterized as a product of a small number of two-qubit rotations, with angles determined by an efficient classical optimization. Given the reduced complexity in terms of gates, this approach can be used to prepare approximate ground states of even-even nuclei by projecting onto the component of deformed Hartree-Fock states. We estimate the resource requirements in terms of the universal gate set {,,CNOT,} and briefly discuss a variant of the algorithm that projects onto states of a system with an odd number of fermions.