On-chip SQUID cascade

A superconducting circuit, comprising in combination:

• a superconducting circuit chip including a DC SQUID thereon, said SQUID being a superconducting loop having at least two Josephson devices therein,
• means for supplying a D.C. bias current to said SQUID,
• input means for coupling a signal to said SQUID,
• output means for taking a current output from said SQUID,
• resistances connected in shunt to said Josephson devices in said SQUID for rendering said devices nonhysteretic as the critical current of said Josephson devices changes depending upon the signal in said input means,
• impedance means directly connected to said shunt resistors for receiving said output current therefrom, said impedance means being comprised of a circuit providing a real impedance at frequencies up to about the frequency of Josephson oscillation of said Josephson devices, said real impedance being sufficiently large to provide a large bandwidth of transmission from said DC SQUID.

An integrated superconducting circuit comprising in combination:

• a first stage comprised of a DC SQUID, said SQUID including a superconducting loop having at least two Josephson devices therein and means for providing a DC current to said loop, there being input means for providing an input signal to said SQUID and an output terminal connected to said SQUID for delivering an output current from said SQUID,
• shunt resistors in parallel with said Josephson devices for making said Josephson devices nonhysteretic,
• an energy sensitive device located on said superconducting chip,
• coupling means connected to said output terminal of said DC SQUID for receiving said output current therefrom, said coupling means being connected to said energy sensitive device and including a transmission line connected to said shunt resistors where said coupling means includes filter means for providing a real impedance and for preventing the transmission of high frequency oscillations to said current or magnetic field sensitive device, said filter means having a real impedance matched to the impedance of said transmission line.

A superconducting circuit comprising in combination:

• a first DC SQUID stage including an input thereto and an output therefrom, a superconducting loop having at least two Josephson devices therein, shunt resistors connected in parallel with said Josephson devices for making the behavior of said Josephson devices nonhysteretic, means for applying a DC bias current to said superconducting loop, wherein said shunt resistors electrically form a superconducting loop with said Josephson devices, an output from said first stage being taken from an electrical node between said shunt resistors,
• a second DC SQUID stage comprising a superconducting loop having at least two Josephson devices therein, shunt resistors in parallel with said devices for making the behavior of said devices nonhysteretic, means for applying a DC current bias to said superconducting loop, an input to said second SQUID stage and an output therefrom,
• coupling means located between the output of said first DC SQUID stage and the input to said second DC SQUID stage for transferring current from said first SQUID stage to the input of said SQUID stage, wherein said coupling means is connected to the output of said first stage at an electrical node between said shunt resistors, said coupling means including a transmission line and an impedance matching circuit connected thereto whose impedance is substantially real up to the frequencies of Josephson oscillation of said Josephson devices,
• said coupling means receiving substantially all of the current through said shunt resistors and delivering said current to the input of said second DC SQUID stage, said coupling means including reactive means for preventing the transfer of high frequency Josephson oscillations from said first stage to said second stage, wherein the impedance of said coupling means is sufficiently high to insure a large bandwidth of said circuit.