If you have a linearly polarized beam you can convert it into a circularly polarized beam with a polarizer and a QWP. Here's what that looks like:

[beam]
polarization = "linear"
angle = 90
angle_units = "degrees"

[[elements]]
element_type = "polarizer"
angle = 45
angle_units = "degrees"

[[elements]]
element_type = "qwp"
angle = 0
angle_units = "degrees"

The key here is that the QWP and the polarizer need to be at 45 degrees relative to on another. If this angle is off, you'll end up with elliptical polarization. Here's what the output looks like:

$ polsim circular_polarizer.toml
intensity: 5.00000e-1
x_mag: 5.00000e-1
x_phase: 0.00000e0
y_mag: 5.00000e-1
y_phase: 1.57080e0

If you go back through the previous two examples, you'll see that the phases (x_phase and y_phase) are zero for both examples. This time, however, y_phase is not zero, it's pi/2! That tells us that there is a delay between the x- and y-components.