<p>Free-space optical communication systems offer high-bandwidth, secure communication with minimal capital costs. Adaptive optics are typically added to these systems to decrease atmospheric channel losses; however, the performance of traditional adaptive optics wavefront sensors degrades in long-range, deep-turbulence conditions. Alternative wavefront sensors using phase diversity can successfully reconstruct wavefronts in deep turbulence, but current implementations require bulky setups with high latency. Here we use a nanostructured birefringent metasurface optic that enables low-latency phase diversity wavefront sensing in a compact form factor. We prove the effectiveness of this approach in mid-to-high turbulence (Rytov numbers from 0.2 to 0.6) through simulation and experimental demonstration. In both cases, an average 16-fold increase in signal from the corrected beam is obtained. We also demonstrate benefits such as noise tolerance and complex field reconstruction with high resolution. Our approach opens a pathway for compact, robust wavefront sensing that enhances range and accuracy of free-space optical communication systems.</p>