  We study the oscillatory flux dependence of the supercurrent in a thin
superconducting loop, closed by a Josephson junction. Quantum fluctuations of
the order parameter in the loop affect the shape and renormalize the amplitude
of the supercurrent oscillations. In a short loop, the amplitude of the
sinusoidal flux dependence is suppressed. In a large loop, the supercurrent
shows a saw-tooth dependence on flux in the classical limit. Quantum
fluctuations not only suppress the amplitude of the oscillations, but also
smear the cusps of the saw-tooth dependence. The oscillations approach a
sinusoidal form with increasing fluctuation strength. At any finite length of
the loop, the renormalized current amplitude is finite. This amplitude shows a
power-law dependence on the junction conductance, with an exponent depending on
the low-frequency impedance of the loop.
