= Lamb shift quenching readout
{c}
{title2=$Q=(I_{\rm off}-I_{\rm on})/(I_{\rm off}-I_{\rm bg})$}
For a simple original detector model, let $\Phi_S$ be the incident rate of surviving $2S$ atoms, $e$ the positive <elementary charge>, $\eta$ the mean number of collected <Electrons> per incident $2S$ atom, and $I_{\rm bg}$ a stable background current. Take current magnitudes as positive. If <microwaves> remove a fraction $p$ before the target,
$$
I_{\rm off}=I_{\rm bg}+e\eta\Phi_S,\qquad
I_{\rm on}=I_{\rm bg}+e\eta\Phi_S(1-p),\qquad Q=p.
$$
The equality needs the same beam flux, background and collection efficiency in both measurements. It is an ideal interpretation of a normalized current loss, not a claim that an actual galvanometer directly counts individual transitions. Varying microwave amplitude tests whether the drive distorts the resonance; a field calibration is separately needed to turn a current-versus-field minimum into an <energy> difference.
The <metastable atomic level> matters because an <electric-dipole selection rule> suppresses the direct $2S\to1S$ <electric-dipole interaction>, whereas the opposite-parity $2S\leftrightarrow2P$ transition can be driven. A long lifetime can store the prepared population until interrogation. After transfer to $2P$, a short lifetime supplies the loss mechanism and a broad <natural linewidth>. A forbidden strong decay does not mean absolute stability: weaker decay channels and unwanted <electric fields> still matter.
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