New amplifier configuration based on the circulating current state of a DC SQUID

DC and RF SQUIDs can be biased so that they switch between clockwise and counterclockwise circulating current states under the effect of internal noise and Josephson oscillations. Under this condition, the time averaged circulating current is very susceptible to small changes in flux bias, and for the DC SQUID, changes in current bias also. Signal gains of over 25 dB have been achieved in LTS SQUIDs, and 10 dB in HTS devices when applying the signal via flux or bias current modulation. The measured gains show excellent agreement with simulation and analytic expressions. There are several advantages with the new SQUID operating mode: applying the signal via the bias current line removes the need to make multi-turn superconducting input coils, and the resulting gain is frequency independent up to the GHz regime. The resulting amplifier has significant potential benefit for very low noise amplification up to 1 GHz, and ultra wideband readout of electrically short antennas.