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In the 17th century Huyghens observed that two coupled pendula may synchronize their motion. Since that time much progress has been achieved to describe phase locking of oscillators1, 2. One of the most perfect manifestation of phase locking happens whenever two superconductors are weakly coupled through a so-called Josephson junction. Constant voltage to frequency (Shapiro) steps observed at multiple of the flux quantum h/2e (∼2 × 10−15 V. s) in the voltage-intensity characteristic have been used3 to realize a voltage standard with a relative uncertainly of 10−7.