  The boundary between the classical and quantum worlds has been intensely
studied. It remains fascinating to explore how far the quantum concept can
reach with use of specially fabricated elements. Here we employ a tunable flux
qubit with basis states having persistent currents of 1$ \mu$A carried by a
billion electrons. By tuning the tunnel barrier between these states we see a
cross-over from quantum to classical. Released from non-equilibrium, the system
exhibits spontaneous coherent oscillations. For high barriers the lifetime of
the states increases dramatically while the tunneling period approaches the
phase coherence time and the classical regime is reached.
