Closed-Form Expressions for Unitaries of Spin-Adapted Fermionic Operators

Abstract: One of the open challenges in quantum computing simulations of problems of chemical interest is the proper enforcement of spin symmetry. Efficient quantum circuits implementing unitaries generated by spin-adapted operators remain elusive, while na\"ive Trotterization schemes break spin symmetry. In this work, we analyze the mathematical structure of spin-adapted operators and derive closed-form expressions for unitaries generated by singlet spin-adapted generalized single and double excitations. These results represent significant progress toward the economical enforcement of spin symmetry in quantum simulations.