For iOS: Experience with ARKit and Metal.
For Android: Experience with ARCore and Vulkan.
For cross-platform solutions: Consider Unity for mobile AR with experience in developing native plugins, ARKit and/or ARCore, and optionally also 3rd party solutions like Vuforia to overlay digital content on physical objects.

In general: Some degree of experience with 3D production pipelines (Blender, modo, 3DS Max, etc), and if it's innovative, some level of higher math skills; In particular 3D transforms, 3D geometry (skills along the lines of those outlined in the book Multiple View Geometry for Computer Vision); Some general computer vision experience may be beneficiary as well, but not necessary as long as the goal isn't to innovate on AR technology itself.

Happy to tell you more in a phone call.