Calculation of nucleon-pair transition intensities on a quantum computer

We simulated the calculation of nucleon-pair transition intensities between systems with Ω+1 and Ω+2 nucleon pairs around the critical point of the transition between vibrational and rotational phases on an ideal quantum computer. To this end we mapped the bosonic two-level Hamiltonian, describing the motion of nucleon pairs in two j-shells with identical pair degeneracy Ω, into the Hilbert space of the quantum device. The unitary matrices transforming the initial state of the quantum computer into the ground states of the systems involved in the nucleon-pair transition were obtained using the Variational Quantum Eigensolver algorithm with simple quantum circuits consisting of two layers of RY rotations, and a fully connecting layer of CZ gates in between them. Combining these matrices with the transition operator defined in the two-level model yielded a chain of Pauli strings that was measured on the quantum computer's initial state to calculate the two-nucleon transition intensity. Initial results are in good agreement with classical ones up to a pair degeneracy Ω=15. Beyond this degeneracy, the transition intensities agree with classical results qualitatively.