Trimming of critical current in niobium Josephson devices by laser annealing

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The authors report a new thermal annealing technique for Nb/Al-AlOx/Nb Josephson devices based on laser heating. This technique allows to "locally" modify the Josephson critical current density. In fact, it is possible the heating of a single circuit element with a good spatial resolution using a focused Ar+ laser beam which is aligned by an optical system on a selected junction of the sample. During the procedure, performed at room temperature, a thermal camera provides a monitoring of the temperature distribution on the whole chip. A continuous reduction of the critical current density up to about 40% has been observed on high quality Josephson junction measured in liquid helium. Neighbouring junctions have not exhibited any measurable change ensuring the effective capability to locally modify the Josephson critical current density. The new technique has been employed to recover noisy dc SQUID magnetometers with non optimal critical current values obtaining a reduction of the spectrum density of magnetic field noise from about 30 to 2.5 fT/Hz1/2.