  Color centers in diamond are widely explored as qubits in quantum
technologies. However, challenges remain in the effective and efficient
integration of these diamond-hosted qubits in device heterostructures. Here,
nanoscale-thick uniform diamond membranes are synthesized via "smart-cut" and
isotopically (12C) purified overgrowth. These membranes have tunable
thicknesses (demonstrated 50 nm to 250 nm), are deterministically transferable,
have bilaterally atomically flat surfaces (Rq <= 0.3 nm), and bulk-diamond-like
crystallinity. Color centers are synthesized via both implantation and in-situ
overgrowth incorporation. Within 110 nm thick membranes, individual
germanium-vacancy (GeV-) centers exhibit stable photoluminescence at 5.4 K and
average optical transition linewidths as low as 125 MHz. The room temperature
spin coherence of individual nitrogen-vacancy (NV-) centers shows Ramsey spin
dephasing times (T2*) and Hahn echo times (T2) as long as 150 us and 400 us,
respectively. This platform enables the straightforward integration of diamond
membranes that host coherent color centers into quantum technologies.
