  An X-ray Interferometer (XRI) has recently been proposed as a theme for ESA's
Voyage 2050 planning cycle, with the eventual goal to observe the X-ray sky
with an unprecedented angular resolution better than 1 micro arcsec (5 prad)
[1]. A scientifically very interesting mission is possible on the basis of a
single spacecraft [2], owing to the compact 'telephoto' design proposed earlier
by Willingale [3]. Between the practical demonstration of X-ray interferometry
at 1 keV by Cash et al. [4] with a 1 mm baseline and 0.1 arcsec effective
resolution to a mission flying an interferometer with a baseline of one or more
meters, an effective collecting area of square meters and micro arcsec
resolution lie many milestones. The first important steps to scale up from a
laboratory experiment to a viable mission concept will have to be taken on a
scalable and flexible testbed set-up. Such a testbed cannot singularly focus on
the optical aspects, but should simultaneously address the thermal and
mechanical stability of the interferometer. A particular challenge is the
coherent X-ray source, which should provide a wavefront at the entrance of the
interferometer that is transversely coherent over a distance at least equal to
the baseline, and bright enough. In this paper, we will explore the build-up of
a testbed in several stages, with increasing requirements on optical quality
and associated thermo-mechanical control and source sophistication, with the
intent to guide the technological development of X-ray interferometry from the
lab to space in a sequence of achievable milestones.
