My question is as follows. I know that classically speaking, if you have some time varying signal $X(t)$ its power spectral density $S_X(f)$ will be symmetric around $f=0$ as the signal is real. This is why your spectrum analyzer only shows the positive part, the negative part is redundant.
But if one has a complex signal (such as often encountered in quantum mechanics) the PSD can be asymmetric around 0. This for example leads to the asymmetry between absorption and emission in atoms: atoms have spontaneous emission (emission in the absence of photons in the environment) but not spontaneous absorption.
What I am interested in is if there is a way to physically make such a complex signal with asymmetric PSD. Maybe this can be done by using two channels that output the signal with some phase difference? I've tried looking for some source material on this but I have not been successful.