New regime in the stochastic resonance dynamics of SQUIDs

We have measured the response of DC SQUIDs to various levels of internal and external noise. By varying external experimental parameters, the internal potential can be modified to allow the SQUID to be operated in modes in which it switches states due to internal thermal noise alone. With appropriate selection of bias conditions, a highly non-linear transfer function between output and input flux can be established. Five SQUIDs with different junction and loop parameters have been studied. When biased into the thermally switching regime, the output signal and signal to noise ratio are found to go through maxima as a function of both applied magnetic flux and bias current. Typical enhancements of order 30 dB and 10 dB respectively are observed for all devices. When driven by external noise, we find that it is possible to fully recover the input SNR while producing amplification.