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Results about induced spatial modulation of critical current density in niobium based Josephson devices by a selective radiation heating are presented. The localized heating inside a single circuit element with a good spatial resolution (less than 1 μm) is made possible by depositing a carbon film on the desired region exploiting the larger absorbance coefficient of carbon (close to 1) with respect to niobium (about 0.2). Measurements of critical current as a function of the external magnetic field on high quality Josephson junctions at T = 4.2 K evidenced that the critical current density was changed only in the absorber covered region, leaving unchanged the critical current density outside that region and ensuring the capability to locally modify the Josephson critical current density. The experimental data have been compared with theoretical predictions obtained by using a suitable junction barrier model based on step-like barrier shape. Such results are very interesting in view of applications in quantum computing, Majorana fermions detection, and superconducting magnetic sensors. [ABSTRACT FROM AUTHOR]

A thermal annealing technique for Nb/Al–AlOx/Nb Josephson devices based on laser heating is presented. This technique allows “locally” modifying the Josephson critical current density, which is not possible using standard procedures based on oven or hot plate heating. In fact, the heating of a single circuit element with a good spatial resolution is possible. At room temperature, the selected junction is exposed to a focused Ar+ laser beam aligned by an optical system. A thermographic imaging allows controlling the temperature distribution on the whole chip. Experimental results on high quality Josephson junction measured in liquid helium have shown a continuous reduction of the critical current density up to about 40%. Neighboring junctions have not exhibited any measurable change ensuring the capability to locally modify the Josephson critical current density. As first application, the present technique has been employed to recover noisy dc superconducting quantum interference device magnetometers with nonoptimal critical current values obtaining a reduction of the spectrum density of magnetic field noise from about 30 to 2.5 fT/Hz1/2. [ABSTRACT FROM AUTHOR]