QUANTUM FLUCTUATIONS IN THE EQUILIBRIUM STATE OF A THIN SUPERCONDUCTING LOOP

We study the oscillatory flux dependence of the supercurrent in a thin superconducting loop, closed by a Josephson junction. Quantum fluctua tions of the order parameter in the loop affect the shape and renormal ize the amplitude of the supercurrent oscillations. In a short loop, t he amplitude of the sinusoidal flux dependence is suppressed. In a lar ge loop, the supercurrent shows a sawtooth dependence on flux in the c lassical limit. Quantum fluctuations not only suppress the amplitude o f the oscillations, but also smear the cusps of the sawtooth dependenc e. The oscillations approach a sinusoidal form with increasing fluctua tion strength. At any finite length of the loop, the renormalized curr ent amplitude is finite. This amplitude shows a power-law dependence o n the junction conductance, with an exponent depending on the low-freq uency impedance of the loop.