NOISE-REDUCTION IN LOW-FREQUENCY SQUID MEASUREMENTS WITH LASER-DRIVENSWITCHING

We have developed a technique to modulate the input to a superconducti ng quantum interference device (SQUID) using a very low noise double-p ole, double-throw switching network. This network is composed of four niobium microbridges that are driven normal in pairs by pulses of lase r light. An input signal is modulated at a frequency above the 1/f noi se regime, amplified by the SQUID, and subsequently demodulated to obt ain an output in which the excess low-frequency SQUID noise has been r emoved. With this technique, we have reduced the low-frequency noise b y an order of magnitude in energy over measurements made without this technique.