The linear optical response of the quantum vacuum

We show that the interpretation of $\mathbf{D}=\varepsilon_{0} \mathbf{E}$ as vacuum polarization is consistent with quantum electrodynamics. A free electromagnetic field polarizes the vacuum but the magnetization and polarization currents cancel giving zero source current. The speed of light is a universal constant while the fine structure constant that couples the EM field to matter runs. In that sense, the quantum vacuum can be understood as a modern Lorentz invariant ether.