A SQUID fluxmeter having a SQUID chip placed on a low temperature side and an outer circuit placed on a room-temperature side, said SQUID chip comprising:
- a SQUID having a first Josephson junction, a second Josephson junction and a superconducting loop, a first end of said first Josephson junction being connected to a first end of said second Josephson junction through one part of said superconducting loop, a second end of said first Josephson junction being connected to a second end of said second Josephson junction through a remaining part of said superconducting loop;
- a superconducting logic gate, to which a voltage pulse generated between a first node in said one pan and a second node in said remaining part is input as an input voltage pulse, for converting a half-cycle of an AC bias current I.sub.C, while said input voltage pulse exists, into output voltage pulses synchronized with said AC bias current I.sub.C, said superconducting logic gate having a bias input node to which said AC bias current I.sub.C is input;
- a superconducting loop for accumulation;
- a write gate adding, in response to one pulse from said superconducting gate, a fluxoid quantum having a direction corresponding to a polarity of said pulse from said accumulation;
- and
- a feedback coil feeding a magnetic flux proportional to that accumulated in said superconducting loop back to said SQUID,
- said outer circuit comprising:
- a first AC bias current source supplying a first AC bias current I.sub.B having a frequency f between said first node in said one part and said second node in said remaining part of said superconducting loop of said SQUID;
- a second AC bias current source outputting said second AC bias current I.sub.C having a frequency equal to a product of a value of a natural number i and said frequency f, said value i being determined by a control signal CSi;
- a counter counting up said output voltage pulses of a first polarity from said superconducting logic gate :and counting down said voltage pulses of a polarity reverse to the first polarity from said superconducting logic gate, a count value counted by said counter being output as a measured magnetic flux;
- and
- a control circuit generating said control signal CSi such that said value i increases/decreases in correspondence with a time differential value of said measured magnetic flux increasing/decreasing and supplying said control signal CSi to said second AC bias current source.