  We present a high precision, self-referencing, common path XUV interferometer
setup to produce pairs of spatially separated and independently controllable
XUV pulses that are locked in phase and time. The spatial separation is created
by introducing two equal but opposite wavefront tilts or using superpositions
of orbital angular momentum. In our approach, we can independently control the
relative phase/delay of the two optical beams with a resolution of 52 zs (zs =
zeptoseconds). In order to explore the level of entanglement between the
non-local photons, we compare three different beam modes: Bessel-like, and
Gaussian with or without added orbital angular momentum. By reconstructing
interference patterns one or two photons at a time we conclude that the beams
are not entangled, yet each photon in the attosecond pulse train contains
information about the entire spectrum. Our technique generates non-local,
quantum equivalent XUV photons with a temporal jitter of 3 zs, just below the
Compton unit of time of 8 zs. We argue that this new level of temporal
precision will open the door for new dynamical QED tests. We also discuss the
potential impact on other areas, such as imaging, measurements of non-locality,
and molecular quantum tomography.
