1 pA/$\sqrt{\text{Hz}}$ Amplifier Using Superconducting Quantum Interference Device Array

An ultrahigh current resolution of 1 pA/√Hz was achieved by a superconducting quantum interference device (SQUID) amplifier, which consisted of 128 repeated dc-SQUID elements. Each element had a 7-turn input coil. The SQUID amplifier was fabricated using a Nb/AlOx/Nb fabrication process. To evaluate current resolution, flux noise and flux-voltage (Φ-V) characteristics were measured at 4.2 K. The Φ-V characteristics showed a large voltage swing of more than 4 mV. The SQUID amplifier was operated at a smooth slope in the Φ-V characteristics, while asymmetric behavior appeared in the same Φ-V characteristics.