Nearly quantum limited nanoSQUIDs based on cross-type Nb/AlO _x /Nb junctions

We report on the development of nearly quantum limited SQUIDs with miniature pickup loop dimensions. The implemented high quality and low capacitance cross-type Nb/AlO_x /Nb Josephson junctions offer large I_C R_N-products and therefore enable an exceptional low noise level of the SQUIDs. Devices with loop dimensions of 1 μ m exhibit white flux noise levels as low as 45 nΦ₀ Hz^−1/2 corresponding to an energy resolution ε of about 1 h at 4.2 K, with h being Planck's constant. Moreover, the large usable voltage swings of the devices of about 300 μ V allow highly sensitive and easy single-stage operation while exploring nearly the intrinsic noise of the SQUIDs, beneficial e.g. for sensor arrays in SQUID microscopy. (paper)