Fiducial state is a quantum state that can be reliably reproduced with low variability.
With Ion Trapping, the state preparation and initial initialization of qubits are performed by optically pumping the trapped ion population to a desired qubit state. Individual-ion readout can be performed using a laser beam to selectively couple one of the two-qubit states to a third energy level via a cycling transition.
When the readout laser is pulsed many photons can be scattered; an ion will fluoresce if it is in the state which couples to the cycling transition but will appear dark if it is in the other qubit state. This state-dependent fluorescence display can be collected with a special camera.
A string of trapped ions can be spatially resolved on a camera to be individually read out the state of each ion.
In Superconducting quantum computers, the initialization of qubits is similar but using a different technique that I will cover in the following posts.
Trapped-Ion Quantum Computing: Progress and Challenges:
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