  The ability to gain insights into the 3D properties of artificial or
biological systems is often critical. However, 3D structures are difficult to
retrieve at low dose and with extremely fast processing, as most techniques are
based on acquiring and computing hundreds of 2D angular projections. This is
even more challenging with ultrashort X-rays which allow realizing nanometre
scale studies and ultrafast time resolved 2D movies. Here we show that computer
stereo vision concepts can be transposed to X-rays. We demonstrate nanoscale
three-dimensional reconstruction from a single ultrafast acquisition. Two
diffraction patterns are recorded simultaneously on a single CCD camera and
after phase retrieval two stereo images are reconstructed. A 3D representation
of the sample is then computed from quantitative disparity maps with about
130x130x380nm3 voxel resolution in a snapshot of 20 femtoseconds. We extend our
demonstration to phase contrast X-ray stereo imaging and reveal hidden 3D
features of a sample. Computed phase stereo imaging will find scientific
applications at X-ray free electron lasers, synchrotrons and laser-based
sources, but also in fast industrial and medical 3D diagnostics.
