= Aharonov-Bergmann-Lebowitz rule
{c}
{title2=$p_j=|\langle b|P_j|a\rangle|^2/\sum_k|\langle b|P_k|a\rangle|^2$}
= ABL rule
{c}
{synonym}
For an ideal <projective measurement> with the <Lüders rule>, condition on both an initial <quantum state preparation> and a final successful <postselection>. Its outcome probabilities are proportional to $|\langle b|P_j|a\rangle|^2$, where $a$ is the forward-evolved initial state and $b$ the backward-evolved final state. The normalization must be nonzero. The expression is symmetric in these two boundary states, although its derivation uses the ordinary <Born rule> and conditional state update.
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