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Low noise SQUID readout for detection of small magnetic field signal

A low noise superconducting quantum interference device (SQUID) readout scheme is developed to detect the small magnetic field signal at frequency up to MHz. It consists of a low noise small signal amplifier and a direct-coupled flux-locked loop. The flux-locked loop keeps the working point stable by cancelling the low frequency flux interference from environment. Around the working point with the maximal flux-to-voltage transfer coefficient, the SQUID is operated as high frequency small signal flux-to-voltage converter and read out by a high frequency low noise matching amplifier using a step-up transformer and a junction field-effect transistor (JFET). Experimental tests of the readout on a conventional SQUID at 4.2 K show that the equivalent flux noise is below 5 μΦ0/√Hz with frequency up to MHz.