  We examine the experimental requirements for realizing a high-gain Quantum
free-electron laser (Quantum FEL). Beyond fundamental constraints on electron
beam and undulator, we discuss optimized interaction geometries, include
coherence properties along with the impact of diffraction, space-charge and
spontaneous emission. Based on desired Quantum FEL properties, as well as
current experimental capabilities, we provide a procedure for determining a
corresponding set of experimental parameters. Even for an idealized situation,
the combined constraints on space-charge and spontaneous emission put strong
limits on sustaining the quantum regime over several gain lengths. Guided by
these results we propose to shift the focus towards seeded Quantum FELs instead
of continuing to aim for self-amplified spontaneous emission (SASE). Moreover,
we point out the necessity of a rigorous quantum theory for spontaneous
emission as well as for space-charge in order to identify possible loopholes in
our line of argument.
