Control of the polarization state of light is essential for many technologies, but is often limited by weak light-matter interactions that necessitate long device path lengths or significantly reduce the signal intensity. Here, we investigate a nanoscale plasmonic aperture capable of modifying the polarization state of far-field transmitted light without loss in the probe signal. The aperture is a coaxial resonator consisting of a dielectric ring embedded within a metallic film; parity-time (PT) symmetric inclusions of loss and gain within the dielectric ring enable polarization control. Since the coaxial aperture enables near-thresholdless PT symmetry breaking, polarization control is achieved with realistic levels of loss and gain. Exploiting this sensitivity, we show that the aperture can function as a tunable waveplate, with the transmitted ellipticity of circularly polarized incident light changing continuously with the dissipation coefficient from pi/2 to 0 (i.e. linear polarization). Rotation of linearly polarized light with unity efficiency is also possible, with a continuously-tunable degree of rotation. This compact, low-threshold, and reconfigurable polarizer may enable nextgeneration, high-efficiency displays, routers, modulators, and metasurfaces.Recently, the concept of PT symmetry has been extended to the polarization degree of freedom. Employing a periodic array of specially designed subwavelength resonators, or meta-atoms, distinct amplification or dissipation rates can be achieved for spatially co-located but orthogonally polarized light. 37,42-44 These highly anisotropic systems have been shown to exhibit polarization phase transitions and polarization exceptional arXiv:1712.05383v2 [physics.optics]