3D nanoSQUID based on tunnel nano-junctions with an energy sensitivity of 1.3 h at 4.2 K

We report the performance of a reliable three-dimensional nanometer-sized Superconducting QUantum Interference Device (SQUID). The use of superconductor-isolator-superconductor Nb/Al-AlOx/Nb Josephson tunnel junctions together with small SQUID loop dimensions permits a high modulation depth of the SQUID's critical current and thus leads to very low intrinsic flux noise of the device. In particular, we present electrical characterization including detailed noise investigations. At 4.2 K, two-stage noise measurements with a SQUID as a low noise preamplifier result in a white flux noise of 51 nU0/Hz1/2, which is equivalent to an energy resolution of 1.3 h, with h being Planck's constant. Simulation of spin sensitivities results in about 1 lB/Hz1/2 for an electron spin positioned directly above the SQUID ring.