https://arstechnica.com/gadgets/2017/11/apple-plans-to-add-a-3d-sensor-to-the-back-of-the-iphone-too/

The iPhone X's front-facing TrueDepth sensor array could be used for more than just Face ID authentication, and it fits neatly into Apple's broader march into augmented reality on the iPhone, but the iPhone X's rear camera still uses a combination of motion sensors and two rear cameras for AR. That could change in next year's iPhone; sources cited by Bloomberg claim that Apple plans to add 3D camera technology to the rear of next year's iPhone in addition to the TrueDepth array already on the iPhone X's front.

The rear camera might not use the same technology as the TrueDepth sensor array used for Face ID on the front of the iPhone X, however. Rather, the rear array might use time-of-light sensors, which would map objects in 3D space by calculating how long it takes for light from its laser to bounce off of an object in its field of view. Bloomberg's sources say that adoption of this technology is not certain, but it seems to be what Apple is testing right now. The technology is in development at Sony, Panasonic, Infineon Technologies, and STMicroelectronics.