= Positive-frequency part of a quantum field
{title2=$A^{(+)}$}
For a free field, its positive-frequency part consists of the annihilation terms multiplying $e^{-i\omega t}$ with $\omega>0$; its negative-frequency part consists of the creation terms. The sign refers to positive <energy>, even though the exponential has a minus sign. Imposing the <Lorenz gauge> only through $(\partial\cdot A)^{(+)}|\mathrm{phys}\rangle=0$ is the subsidiary condition in <Gupta-Bleuler quantization>, not an operator identity for the whole divergence.
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