Josephson signal detector

Disclosed is a Josephson signal detector having (1) a sensor with at least one Josephson junction (e.g., a SQUID) and (2) a comparator (e.g., an analog-to-digital converter); a measurement system using such detector; and a method of using such detector and measurement system. The detector can be, e.g., a digital SQUID. The detector can also include a readout circuit for reading out the output of the comparator. In one aspect of the invention, the sensor, or both the sensor and readout circuit, are powered by a DC current source. Through use of the DC current source, it is possible to provide, e.g., a digital SQUID, having reduced crosstalk and reduced fluctuation of ground potential, and which is kept from an increase in noise or a miss operation. In another aspect of the present invention, the comparator, which includes at least one Josephson junction, is provided with a capacitance such that Josephson oscillation generated by the Josephson junction of the sensor during operation of the sensor has a higher frequency than that corresponding to the turn-on delay of the Josephson junction of the comparator. Due to this latter aspect, current noise due to Josephson oscillation can be reduced, to provide a detector with high accuracy and which can be kept from miss operation.