  Quantum engineering requires controllable artificial systems with quantum
coherence exceeding the device size and operation time. This can be achieved
with geometrically confined low-dimensional electronic structures embedded
within ultraclean materials, with prominent examples being artificial atoms
(quantum dots) and quantum corrals (electronic cavities). Combining the two
structures, we implement a mesoscopic coupled dot-cavity system in a
high-mobility two-dimensional electron gas, and obtain an extended spin-singlet
state in the regime of strong dot-cavity coupling. Engineering such extended
quantum states presents a viable route for nonlocal spin coupling that is
applicable for quantum information processing.
