  We demonstrate coherent dynamics of quantized magnetic fluxes in a
superconducting loop with a weak link - a nanobridge patterned from the same
thin NbN film as the loop. The bridge is a short rounded shape constriction,
close to 10 nm long and 20 - 30 nm wide, having minimal width at its center.
Quantum state control and coherent oscillations in the driven time evolution of
the tunnel-junctionless system are achieved. Decoherence and energy relaxation
in the system are studied using a combination of microwave spectroscopy and
direct time-domain techniques. The effective flux noise behavior suggests
inductance fluctuations as a possible cause of the decoherence.
