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$F_Q$ is the quantum Fisher information of the radiation’s reduced state with respect to g ; it carries units of $g^{-2}$ , i.e. inverse radians squared, and since g is already dimensionless, $F_Q$ is a pure number. $\delta$ is the alignment tolerance the observer has decided to demand, also in radians, and $t_G(\delta)$ has units of time [ 12 ] . This bound is achievable asymptotically by the symmetric logarithmic derivative measurement; nothing here claims it is achieved by any specific realizable circuit, only that it is the correct floor no circuit can beat.

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