In Chapter 13 of "Quantum Computing - From Linear Algebra to Physical Realizations" by Nakahara and Ohmi, there is a discussion about initializing qubits in a trapped Ca$^+$ qubit. The relevant states are $4S_{1/2}$, which is used as the $|0\rangle$ state, $3D_{5/2}$, which is used as the $|1\rangle$ state and the auxiliary $4P_{3/2}$ state. The book goes on to discuss how to initialize the ion by an auxilary transition between from $3D_{5/2}$ to $4P_{3/2}$ which will spontaneously decay to $4S_{1/2}$. This is done because the $|0\rangle \leftrightarrow |1\rangle$ transition is dipole forbidden.

The electron configuration of Ca$^+$ is $1s^2 2s^2 2p^6 3s^2 3p^6 4s$, which makes the $4S_{1/2}$ the last occupied state in the ion ground state. Why is it needed to excite the electron from $3D_{5/2}$ to $4P_{3/2}$? Wouldn't the qubit already be initialized to begin with since $4S_{1/2}$ is the last occupied state?



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