Axion Polarimetric Experiment (APE)

We present the Axion Polarimetric Experiment (APE), a cavity-enhanced polarimeter designed to search for ultralight axion and axion-like-particle dark matter through a time-dependent rotation of the linear polarization of laser light.

In cavity-based schemes, intracavity quarter-wave plates can restore coherent buildup of the axion-induced orthogonal polarization, but their transmissive loss limits the achievable finesse.

To avoid transmissive intracavity optics, we propose a folded Fabry-Perot cavity that employs dielectric phase-shifting mirrors.

At an incidence angle near $45^\circ$, these mirrors provide a reflection-phase difference $\Delta\phi \equiv \phi_s-\phi_p \simeq \pi/2$ between $s$ and $p$ polarizations and therefore act as reflective quarter-wave plates.

We present the coating design, thickness optimization, and measurements of the phase shift and optical loss of the phase-shifting mirrors.

Using a heterodyne polarimetric readout and an explicitly stated noise model, we derive design-level sensitivity projections for the axion-photon coupling $g_{a\gamma\gamma}$.

These projections should be interpreted as target sensitivities for the proposed cavity configuration, since the full-system birefringence noise and angular-jitter coupling remain to be measured.