Enabling Sub‐150 Picosecond TOF‐PET Imaging with Ultrafast‐Efficient Oxide Scintillators by Synergistic Defect and Energy Transfer Engineering

High‐energy physics and nuclear medicine urgently demand ultrafast and efficient scintillators. Compared to organic and halide scintillators, cerium‐doped oxide scintillators stand out for their superior overall performance (e.g., chemical stability, radiation hardness), yet their slow decay dynamics (tens of nanoseconds) severely limit applications in ultrafast detection. Here, a novel strategy is proposed by creating Yb3+‐mediated non‐radiative energy transfer pathways and synergistically introducing multi‐site cation codoping (Li+, Ca2+, and Al3+) to suppress electron traps. The resulting Li(LuYCaYb)2(SiAl)O5:Ce scintillators overcome the intrinsic lifetime barrier, achieving an ultrafast decay time of 7.3 ns (30.6% contribution), along with a high light yield (25 000 photons MeV−1) and minimal afterglow (0.02% at 40 ms). These advancements enable 149 ps coincidence time resolution in a clinically‐sized (3 × 3 × 18 mm3) crystal for time‐of‐flight positron emission tomography, outperforming commercial LYSO:Ce systems (214 ps) by 30%. This work provides a universal strategy for designing ultrafast and efficient oxide scintillators, advancing applications from particle detection to nuclear medical imaging.