Performance of a double DC SQUID magnetometer

A magnetic flux sensing device that is made by using two DC SQUIDs is presented. The DC SQUIDs are of familiar characteristics, magnetically coupled by a common coil, and are turned into the voltage state by a single DC current source. As the DC SQUIDs are magnetically coupled, the magnetic flux noise generated by one of the DC SQUIDs is sensed by the other and vice versa. By making a differential measurement across both DC SQUIDs, the magnetic flux noise detected by the DC SQUIDs can be added either constructively or destructively. This means that magnetic flux noise cancellation is possible, as demonstrated experimentally. The double DC SQUID configuration can also be used in the add flux noise mode, in which there is no loss of performance when compared with what is possible to obtain with a single DC SQUID. Another advantage of the two DC SQUID configuration is that the output impedance is twice as large as that of the DC SQUID with Josephson junctions of the same critical current and shunting resistance. In the former configuration, the matching to room temperature electronics is simplified.