Josephson logic gate having a plurality of input ports and a Josephson logic circuit that uses such a Josephson logic gate

Abstract: A multiple-input Josephson AND gate having a plurality of input ports (A₁ - A₄) comprises a plurality of Josephson logic gate elements (11 - 14) provided in number corresponding to a number of said input ports and cascaded with each other from a first stage to a final stage, each of the Josephson logic gate elements of said first through final stages including: a superconducting quantum interferometer (S₁ - S₄) for producing an output signal in response to a transition from a superconducting state to a finite voltage state, an input line connected to a corresponding input port for carrying thereon an input current, the input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interferometer; and a biasing part (J₂ - J₄) for supplying a bias current to the superconducting quantum interferometer with a level such that the superconducting quantum interferometer causes the transition to the finite voltage stage in response to the input current supplied to the input line, wherein each of the biasing part of the second through final stages are supplied with the output signal of the superconducting quantum interferometer of the previous stage as a trigger signal and produces the bias current in response to the trigger signal.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to Josephson devices and more particularly to a Josephson logic gate having a multiple-input construction. The present invention is related also to a Josephson logic circuit wherein such a Josephson logic gate is used. The Josephson devices are characterized by the extremely fast operational speed and low power consumption. Thus, the device is suitable for superfast computers or processors, and intensive efforts have been made to construct various digital circuits based upon a Josephson integrated circuit. In the Josephson integrated circuits, the logic gates are generally constructed in the form of AND gate and OR gate. Conventionally, these Josephson logic gates have been constructed to have one or two input ports for the input logic signals. Hereinafter, the problem associated with the Josephson logic gates having multiple input ports will be examined briefly. FIG.1 shows an example of the simplest Josephson gate having a single input port A. Referring to FIG.1, the Josephson gate includes two Josephson junctions J11 and J12 that are arranged, together with first and second superconducting windings La and Lb, to form a closed loop 1 that acts as a superconducting interferometer. At a node where the windings La and Lb are connected, an a.c. bias current I g is supplied from a bias terminal 20 via a resistor R, and the bias current thus supplied is caused to flow to a superconducting ground plane GND via a first current path including the winding La and the Josephson junction J 1 and further via a second current path including the winding Lb and the Josephson junction J 2 . The input logic signal is supplied to an input line 21 via an input port A, wherein the input line 21 includes inductances La' and Lb' that cause a magnetic coupling respectively with the windings La and Lb of the superconducting loop 1. Thereby, the Josephson junctions J11 and J12 experience a transition to a finite voltage state according to a characteristic curve shown in FIG.2. In the device of FIG.1, the output is obtained at the node where the resistor R is connected to the superconducting loop 1. Referring to FIG.2 showing the relationship between the input current I c and the bias current I g , the Josephson junctions J11 and J12 cause the transition to the finite voltage state in the region designated in FIG.2 by hatching. Thus, when the bias current I g is held constant, the increase in the current I c causes the switching by crossing the characteristic curve, wherein the threshold value of the current I c at which the foregoing transition occurs increases with decreasing current I g . In the Josephson gate of the two-junction construction described above, however, there exists a problem in that the region of the finite voltage state is relatively limited. Associated therewith, the operational margin of the device is limited. This problem becomes particularly conspicuous when the number of the input lines is increased. FIG.3 shows another typical Josephson logic gate that uses a superconducting interferometer loop 2 wherein three Josephson junctions J11, J12 and J13 are included together with superconducting windings La and Lb. In this case, too, the input logic signal supplied to the line 21 via an input port A is transferred to the loop 2 as a result of the magnetic coupling of inductances La' and Lb' included in the line 21 with the corresponding superconducting windings La and Lb of the loop 2. In the gate of this example, the bias current I g is supplied from the bias terminal 20 to respective midpoints of the windings La and Lb via the resistor R, and the bias current thus supplied is caused to flow to the superconducting ground plane GND via the Josephson junctions J11, J12 and J13. Thereby, the device shows an operational characteristic as represented in FIG.4. There, the output of the gate is obtained at the node where the resistor R is connected to the superconducting windings La and Lb. In the characteristic curve shown in FIG.4, too, the region of the finite voltage state of the Josephson junctions J11 - J13 is indicated by the hatching. In this case, the region of the finite voltage state is expanded as compared with the characteristic curve shown in FIG.2. Thus, the device of FIG.3 shows an improved operational margin as compared with the device of FIG.1. Using the improved operational margin, the device of FIG.3 is used successfully to construct the dual input AND gate. There, another input line is provided in magnetic coupling with the interferometer loop 2, and the bias current I g is set such that the transition occurs only when the input current is supplied to both input lines simultaneously. Further, there is an attempt to construct a Josephson AND gate that has three or more input ports based upon the device of FIG.3 (Hatano, Y. et al. IEEE J. Solid-State Circuits, Vol.sc-22, No.4, August 1987). FIG.5 shows such a three-input Josephson AND gate constructed based upon the superconducting interferometer 2 of FIG.3, wherein there are provided three input lines 21 - 23 respectively connected to input ports A - C. There, the bias current I g is set to a magnitude such that the transition of the Josephson junctions J11 - J13 occurs only when the input current is caused to flow simultaneously through the input lines 21 - 23 in correspondence to the logic state wherein the logic data "1" is supplied simultaneously to the input ports A - C. In the multiple-input Josephson AND gate that is constructed based on the superconducting interferometer loop 2 of FIG.3, however, it will be easily understood that there exists a problem of operational margin that becomes increasingly smaller with increasing number of the input lines. More specifically, the tolerance in the variation of the input current becomes increasingly stringent, as it is the sum of the input currents that cause the transition of the Josephson junctions. Thereby, the logic gate becomes increasingly vulnerable to noises and the risk of erroneous operation increases substantially. SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide a novel and useful Josephson logic circuit wherein the foregoing problems are eliminated. Another and more specific object of the present invention is to provide a Josephson AND gate that is capable of increasing the number of input ports without sacrificing the reliability of operation. Another object of the present invention is to provide a multiple-input Josephson AND gate having a plurality of input ports, comprising: a plurality of Josephson logic gate elements provided in number corresponding to a number of said input ports and cascaded with each other from a first stage to a final stage, each of said Josephson logic gate elements of said first through final stages comprising: a superconducting quantum interferometer for producing an output signal in response to a transition from a superconducting state to a finite voltage state; an input line connected to a corresponding input port for carrying thereon an input current, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interferometer; and biasing means for supplying a bias current to said superconducting quantum interferometer, said biasing means supplying the bias current with a level such that said superconducting quantum interferometer causes said transition to the finite voltage stage in response to the input current supplied to the input line, each of said biasing means of said second through final stages being supplied with the output signal of the superconducting quantum interferometer of the previous stage as a trigger signal and producing said bias current in response to said trigger signal. According to the present invention, one can increase the number of stages and hence the number of input ports of the Josephson AND gate while using the reliable single-input Josephson gate element for each stage. When the input logic signals are supplied simultaneously to the entire input ports, the transition of the Josephson gate elements occurs first at the initial stage and proceeds to the final stage consecutively. As a result of the transition, a logic signal indicative of the result of the AND operation is obtained at the Josephson logic gate element of the final stage. Another object of the present invention is to provide various Josephson logic circuits that use therein a number of Josephson AND gates each having a plurality of input ports. Another object of the present invention is to provide a Josephson decoder circuit for decoding input data represented by a plurality of bits, comprising: a plurality of multiple-input Josephson AND gates each including a plurality of Josephson logic gate elements with a number corresponding to a number of bits of the input data, said plurality of Josephson logic gate elements being cascaded, in each Josephson AND gate, with each other from a first stage to a final stage; each of said Josephson logic gate elements comprising: a superconducting quantum interferometer for producing an output signal in response to a transition from a superconducting state to a finite voltage state; an input line connected to a corresponding input port for carrying thereon an input current, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interferometer; and biasing means for supplying a bias current to said superconducting quantum interferometer, said biasing means supplying the bias current with a level such that said superconducting quantum interferometer causes said transition to the finite voltage stage in response to the input current supplied to the input line, each of said biasing means of said second through said final stages being supplied with the output signal of the superconducting quantum interferometer of the previous stage as a trigger signal and producing said bias current in response to said trigger signal; interconnection means for connecting the input lines of the Josephson logic gate elements of different Josephson AND gates with each other for each of the stages such that the input data is supplied to the Josephson AND gates according to a logic combination that is different in each Josephson AND gate. According to the present invention, one can construct the decoding circuit without complicating the circuit arrangement, by simply arranging the Josephson AND gates and connecting the input lines appropriately. Another object of the present invention is to provide a programmable logic array circuit supplied with input data for producing output data after a predetermined logic operation, comprising: a programmable Josephson AND array comprising a plurality of multiple-input Josephson AND gates each including a plurality of Josephson logic gate elements with a number corresponding to a number of bits of the input data, said plurality of Josephson logic gate elements being cascaded, in each Josephson AND gate, with each other from a first stage to a final stage; each of said Josephson logic gate elements comprising: a superconducting quantum interferometer for producing an output signal in response to a transition from a superconducting state to a finite voltage state; an input line connected to a corresponding input port for carrying thereon an input current, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interferometer; and biasing means for supplying a bias current to said superconducting quantum interferometer, said biasing means supplying the bias current with a level such that said superconducting quantum interferometer causes said transition to the finite voltage stage in response to the input current supplied to the input line, each of said biasing means of said second through said final stages being supplied with the output signal of the superconducting quantum interferometer of the previous stage as a trigger signal and producing said bias current in response to said trigger signal; interconnection means for connecting the input lines of the Josephson logic gate elements of different Josephson AND gates with each other for each of the stages such that the input data is supplied to the Josephson AND gates according to a logic combination that is different in each Josephson AND gate, said interconnection means prohibiting supply of the input data to a selected Josephson gate element of a selected Josephson AND gate; and programmable Josephson OR array supplied with the output data of the programmable Josephson AND array, said programmable Josephson OR array comprising a plurality of Josephson logic gate elements each comprising a superconducting quantum interferometer and an input line carrying thereon an input current, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interference loop, said Josephson logic OR array producing output data according to a logic sum operation of the output data of said programmable Josephson AND array, said programmable Josephson OR array including a selected Josephson logic gate element that is disabled. According to the present invention, one can construct the programmable logic array without using timed inverters or timing signals. Thereby, the circuit construction is significantly simplified. Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a circuit diagram showing the construction of a conventional Josephson logic gate that includes two Josephson junctions in a superconducting loop; FIG.2 is a diagram showing the operational characteristic of the device of FIG.1; FIG.3 is a circuit diagram showing the construction of a conventional Josephson logic gate that includes three Josephson junctions in a superconducting loop; FIG.4 is a diagram showing the operational characteristic of the device of FIG.3; FIG.5 is a circuit diagram showing the construction a three-input Josephson AND gate proposed previously; FIG.6 is a circuit diagram showing the construction of a four-input Josephson AND gate according to a first embodiment of the present invention; FIG.7 is a circuit diagram showing the construction of a four-input Josephson AND gate according to a second embodiment of the present invention; FIG.8 is a circuit diagram showing the construction of a four-input Josephson AND gate according to a third embodiment of the present invention; FIG.9 is a circuit diagram showing the construction of a four-input Josephson AND gate according to a fourth embodiment of the present invention; FIG.10 is a circuit diagram showing the construction of a four-input Josephson AND gate according to a fifth embodiment of the present invention; FIG.11 is a circuit diagram showing the construction of a four-input Josephson AND gate according to a sixth embodiment of the present invention; FIG.12 is a circuit diagram showing the construction of a Josephson decoder circuit according to a seventh embodiment of the present invention; FIG.13 is a circuit diagram showing the construction of a Josephson decoder circuit according to an eighth embodiment of the present invention; FIG.14 is a circuit diagram showing the construction of a Josephson decoder circuit according to a ninth embodiment of the present invention; FIG.15(A) is a circuit diagram showing a Josephson memory decoder according to a tenth embodiment of the present invention; FIG.15(B) is an enlarged view showing a programmable part of the circuit of FIG.15(A) in detail; FIG.16(A) is a circuit diagram showing an addressing of a redundant memory cell in the Josephson memory decoder of FIG.15(A); FIGS.16(B) and 16(C) are diagrams similar to FIG.15(B) showing the programmable part of the circuit of FIG.16(A) in an enlarged scale for two different settings; FIG.17 is a circuit diagram showing a Josephson memory decoder according to an eleventh embodiment of the present invention; FIG.18 is a circuit diagram showing a Josephson programmable logic array according to a twelfth embodiment of the present invention; FIG.19 is a circuit diagram showing a Josephson programmable logic array according to a thirteenth embodiment of the present invention; and FIG.20 is a circuit diagram showing a Josephson OR gate that is used in the circuit of FIG.19. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.6 shows a four-input AND gate according to a first embodiment of the present invention. Referring to FIG.6, the circuit includes a first input stage 11, a second input stage 12, a third input stage 13, a fourth input stage 14 and an output stage 15, wherein the first stage 11 includes a superconducting quantum interferometer S 1 that has a construction substantially identical with the Josephson gate explained with reference to FIG.3. The quantum interferometer S 1 is supplied with an a.c. bias current from a bias bus 16 via a resistor Rs 1 and operates similarly to the device of FIG.3 with the operational characteristic shown in FIG.4. It should be noted that an a.c. current source similar to the a.c. current source 160 is connected to the bias bus 16 for supplying the a.c. bias current thereto. The second stage 12 in turn includes a Josephson junction J 2 having a first end connected to the output terminal of the device S 1 via a resistor R 1 and another end connected to the superconducting ground plane. The first end of the Josephson junction J 2 is connected further to the bias bus 16 via a resistor Rs 2 and the bias current is caused to flow to the ground plane as long as the Josephson junction J 2 is in the superconducting state. Further, the second stage 12 includes a superconducting quantum interferometer S 2 that is connected to the first end of the Josephson junction J 2 via a resistor R 2 for receiving the bias current. As long as the Josephson junction J 2 is in the superconducting state, no bias current is supplied to the superconducting interferometer S 2 , and the superconducting interferometer S 2 is held in the superconducting state. The third stage 13 and the fourth stage 14 have a construction substantially identical with the construction of the second stage 12. Thus, the third stage 13 includes a Josephson junction J 3 that is biased via a resistor Rs 3 at a first end and is connected to the superconducting ground plane at the other end. Further, the first end of the Josephson junction J 3 is connected to the output terminal of the superconducting quantum interferometer S 2 of the previous stage via a resistor R 31 , while the same first end of the Josephson junction J 3 is connected to a superconducting quantum interferometer S 3 via a resistor R 32 . Similarly, the fourth stage 14 includes a Josephson junctions J 4 that is biased via a resistor Rs 4 at a first end and is connected to the superconducting ground plane at the other end. Further, the first end of the Josephson junction J 4 is connected to the output terminal of the superconducting quantum interferometer S 3 of the previous stage via a resistor R 41 , while the same first end of the Josephson junction J 4 is connected to a superconducting quantum interferometer S 4 via a resistor R 42 . In the foregoing construction, the stage 13 is connected in cascade to the stage 12 while the stage 14 is connected in cascade to the stage 13. In the output stage 15, a Josephson junction J 5 is provided such that the Josephson junction J 5 has a first end that is connected to the superconducting quantum interferometer S 4 of the previous stage via the resistor R 51 and a second end connected to the superconducting ground plane. Further, the bias current is supplied to the Josephson junction J 5 via a resistor Rs 5 . There, a load resistance R L is connected to an output terminal OUT that in turn is connected to the node where the resistors R 51 and Rs 5 are connected with each other at the first end of the Josephson junction J 5 . Here, it should be noted that the load resistance R L may represent the equivalent load resistance of the Josephson circuits that are connected after the output stage 15. In the superconducting quantum interferometer of FIG.3 that is used for the devices S 1 - S 5 of FIG.6, the Josephson junctions J 11 , J 12 and J 13 are configured such that the critical current of the Josephson junction J 12 is twice as large as the critical current of the Josephson junction J 11 or the critical current of the Josephson junction J 13 . There, the Josephson junctions J 11 and the Josephson junction J 13 have the same critical current. On the other hand, the Josephson junctions J 2 - J 5 are configured to have a same critical current represented as I m . In correspondence to the critical current I m , the values of the resistors R 22 , R 32 and R 42 are all set equal to V g /I m wherein V g represents the gap voltage of the Josephson junctions J 2 - J 5 . More specifically, the critical current I m may be set equal to 0.4mA, and the value of the resistors R 22 - R 42 is set to 8 Ω. The value of the resistors R 21 - R 51 may be set to 2 Ω. Next, the operation of the four-input AND gate of FIG.6 will be described in detail. Starting from a state wherein all the Josephson junctions in the circuit of in FIG.6 are in the superconducting state, the bias current is supplied from the bias bus 16 to each of the superconducting quantum interferometers S 1 - S 4 as well as to each of the Josephson junctions J 2 - J 5 . In this state, an input signal A supplied to the first stage causes a transition of the superconducting quantum interferometer S 1 and the bias current is supplied from the resistor Rs 1 to the Josephson junction J 2 via the resistor R 21 . There, the Josephson junction J 2 is driven with a bias current that is a sum of the bias current flowing through the resistor R 21 and the resistor Rs 2 . In response to this, the Josephson junction J 2 causes a switching to the finite voltage state. It should be noted that, before the transition of the superconducting quantum interferometer S 1 , the bias current is not supplied to the Josephson junction J 2 via the resistor R 21 because of the zero resistance of the superconducting quantum interferometer S 1 . In response to the switching of the Josephson junction J 2 , a bias current is supplied from the resistor Rs 2 to the superconducting quantum interferometer S 2 via the resistor R 22 . It should be noted that the superconducting quantum interferometer S 2 is not biased at all before the Josephson junction J 2 switches. When an input signal is supplied to the input port B under this state, the superconducting quantum interferometer S 2 causes the switching to the finite voltage state and the bias current is now supplied to the Josephson junction J 3 via the resistor R 31 . There, the Josephson junction J 3 is biased by a bias current that is equal to the sum of the bias current supplied via the Josephson junction J 3 and the bias current supplied via the resistor Rs 3 . Thereby, the Josephson junction J 3 causes a switching to the finite voltage state. This switching of the Josephson junction J 3 in turn causes the biasing of the superconducting quantum interferometer S 3 and the superconducting quantum interferometer S 3 is thereby switched to the finite voltage state when there is an input to the input port C. In this way, the switching of the superconducting interferometer S i and the Josephson junction J i proceeds consecutively from the initial stage to the final stage, provided that an input signal indicating the logic high level is supplied to all of the input ports A - D. Herein, the suffix i represents the number of the stage. When one of the input port lacks the input signal in correspondence to the logic low level state, the progressive switching to the finite voltage state is interrupted at the stage corresponding to the input port, and the output signal supplied to the load R L assumes the logic low level state. Thus, the circuit of FIG.6 operates as a multiple-input AND gate. In the foregoing operation, it is important to note that the Josephson junctions J 2 - J 5 produce a voltage corresponding to the gap voltage of the Josephson junction when switched to the finite voltage state. As the gap voltage is determined by the material that is used for the Josephson junction, this means that the bias current supplied to the superconducting quantum interferometer S i from the preceding Josephson junction J i via the resistor R i2 is determined without ambiguity by the value of the resistor R i2 . In other words, the bias current that has caused the switching of the superconducting quantum interferometer of the previous stage such as S i-1 does not affect at all the bias current that causes the switching of the superconducting quantum interferometer S i . Thus, the operation of the multiple-input AND gate of FIG.6 is reliable even when the number of stage is increased in correspondence to the increased number of input ports. Further, it should be noted that each superconducting quantum interferometer S i has only one input line. Thereby, one can obtain a sufficient operational margin for the switching operation of the superconducting quantum interferometer. See the operational characteristic of FIG.3. Because of the sufficient operational margin, it is generally not necessary supply input signal to the port A in correspondence to the progressive operation from the first state to the final stage. Even when the input signals are supplied simultaneously to the entire input ports, there is a sufficient margin of time for the foregoing progressive switching operation. FIG.7 shows a four-input AND gate according to a second embodiment of the present invention. In this embodiment, the Josephson junction J 2 and the corresponding bias resistance Rs 2 is omitted. Thereby, the superconducting quantum interferometer S 2 of the second state is supplied with the bias current from the resistors Rs 1 and R 22 when the superconducting quantum interferometer S 1 has caused a switching. Even in this case, the foregoing progressive switching occurs and the circuit operates as the multiple-input AND gate. Although the margin of the bias current may become somewhat stringent, the circuit of the present invention has an advantage of reduced number of parts. FIG.8 shows a four-input AND gate according to a third embodiment of the present invention. The circuit of FIG.8 is substantially identical with the circuit of FIG.6 except that a two-junction type superconducting quantum interferometer S 5 as shown in FIG.1 is used in place of the superconducting quantum interferometer S 1 . The superconducting quantum interferometer S 5 may be the symmetric type wherein the Josephson junctions J 11 and J 12 both have the same critical current or the asymmetric type wherein the critical current is different in the Josephson junction J 11 and J 21. As the two-junction type superconducting quantum interferometer has a limited operational margin as already explained with reference to FIG.2, it is desirable to set a timing of the input signals, in the circuit of the present invention, such that the input signals are supplied first to the input ports B - D and subsequently to the input port A. On the other hand, the circuit of FIG.8 has an advantageous feature in that the circuit construction can be simplified as a result of use of the two-junction type superconducting quantum interferometer. FIG.9 shows a four-input AND gate according to a fourth embodiment of the present invention. In this embodiment, the superconducting quantum interferometer S 1 of the circuit of FIG.7 is replaced with the two-junction type superconducting quantum interferometer S 5 . As other features about the construction and operation are obvious from the description of the previous embodiments, further description of the circuit of FIG.9 will be omitted. FIG.10 shows a four-input AND gate according to a fifth embodiment of the present invention. In this embodiment, the superconducting quantum interferometers S 1 - S 4 of FIG.6 are replaced with two-junction type superconducting quantum interferometers S 5 - S 8 , respectively. With the use of the two-junction type superconducting quantum interferomters, it is possible to simplify the circuit construction and increase the integration density. As other features about the construction and operation are obvious from the description of the previous embodiments, further description of the circuit of FIG.10 will be omitted. FIG.11 shows a four-input AND gate according to a sixth embodiment of the present invention. In the present embodiment, the circuit has a construction substantially identical with the circuit of FIG.7 except that the superconducting quantum interferometers S 1 - S 4 are replaced with the two-junction type superconducting quantum interferometers S 5 - S 8 . As other features about the construction and operation of the present embodiment are obvious from the description of the previous embodiments, further description of the circuit of FIG.11 will be omitted. FIG.12 shows a Josephson decoder circuit according to a seventh embodiment of the present invention wherein the foregoing multiple-input Josephson AND gate is used. Referring to FIG.12, a plurality of four-input Josephson AND gates G 1 - G 16 each having a construction shown in FIG.6, are provided with a number corresponding to the number of outputs of the Josephson decoder circuit. There, each Josephson AND gate G i (i = 1 - 16) has multiple input ports IN i0 , IN i2 , IN i3 and IN i4 (i = 1 - 16), wherein the Josephson AND gates G 1 - G 16 are connected with each other by a superconducting interconnection wiring fixture 30 such that: either of complementary input signals A 0 and A 0b is supplied to the input port IN i0 (i = 1 - 16), either of complementary input signals A 1 and A 1b is supplied to the input port IN i1 , either of complementary input signals A 2 and A 2b is supplied to the input port IN i2 , either of complementary input signals A 3 and A 3b is supplied to the input port IN i3 , and either of complementary input signals A 4 and A 4b is supplied to the input port IN i4 , wherein the suffix i covers all the Josephson AND gates G 1 - G 16 . It should be noted that the interconnection wiring fixture 30 includes a number of signal lines l provided in correspondence to each input signals A 0 , . . ., wherein each signal line l establishes a magnetic coupling with the corresponding superconducting quantum interferomters of the Josephson AND gates G i (i = 1 - 16) consecutively. As usual in the decoders, the combination of the true input signals A 0 - A 3 and the complementary input signals A 0b - A 3b is changed in each Josephson AND gate G i . Thereby, each Josephson AND gate G i produces an output at an output terminal OUT i in corresponding to a combination of input signals that is different in each Josephson AND gate as indicated in FIG.12. In the Josephson decoder circuit of FIG.12, it should be noted that the circuit construction can be simplified significantly over the conventional decoder circuit wherein the multiple-input AND gate is constructed by using a number of Josephson AND gates and Josephson OR gates. FIG.13 shows a Josephson decoder circuit according to an eighth embodiment of the present invention, wherein the Josephson decoder circuit has a construction similar to the circuit of FIG.12 except that there are provided voltage boosters DG 0 - DG 3 and DG 0b - DG 3b in correspondence to each signal line l of the interconnection wiring fixture 30 for boosting the voltage level of the input signals A 0 - A 3 and A 0b -A 3b . The voltage boosters may have a construction proposed previously by the inventor of the present invention and disclosed in the Laid-open European Patent Application EP 0 441 299, which is incorporated herein as reference. By using the voltage booster, one can obtain a sharp rising of the leading edge of the input logic signals and the operational speed of the decoder is improved. Further, by providing a terminal resistor such as a resistor R o in correspondence to each signal line l of the interconnection wiring fixture 30, and by setting the value of the terminal resistors appropriately, one can obtain an ideal transfer characteristic of the input signals. It should be noted that FIG.13 shows the case wherein the Josephson decoder circuit is used for a memory decoder that addresses a memory cell. FIG.14 shows a Josephson decoder circuit according to ninth embodiment of the present invention, wherein the decoder circuit of the present invention has a construction substantially identical with the construction of the decoder of FIG.13 except that voltage boosters EG 0 - EG 3 and EG 0b - EG 3b are provided in place of the voltage boosters DG 0 - EG 3 and DG 0b - DG 3b . The construction of each voltage booster EG is represented also in FIG.14. More detailed description of the voltage booster EG can be found in the aforementioned Laid-open European Patent Application EP 0 411 299. Thereby, one can improve the operational speed of the decoding operation similarly to the previous embodiment. Next, a Josephson memory decoder according to a tenth embodiment of the present invention will be described with reference to FIG.15(A). Referring to FIG.15(A), the Josephson memory decoder of the present embodiment has a construction similar to the Josephson decoder of FIG.12 except that each output of the Josephson AND gate G i is provided with a fuse f i that is selectively blown up by irradiation of a laser beam. In addition, the Josephson memory decoder of FIG.15(A) includes a redundant gate G red having a similar construction as other Josephson AND gates, for addressing a redundant memory cell. There, the redundant gate G red is connected to the interconnection wiring fixture 30 via programmable wiring elements 31, 32 of which construction is shown in derail in FIG.15(B). Referring to FIG.15(B), the programmable wiring element includes conductor strips 30a and 30b that extend as a part of the signal line l of the interconnection wiring fixture 30. In addition, there are provided crossing conductor strips 30c that cross with each other between the conductor strips 30a and 30b. When there is no defective memory cell in the memory cell array, the crossing conductor strips 30c are disconnected by irradiation of the laser beam. Further, the fuse f r at the output of the redundant gate G red is disconnected simultaneously. Thereby, the addressing of the memory cell by the redundant gate G red is prohibited and the Josephson memory decoder of FIG.15(A) operates similarly to the Josephson decoder of FIG.12. When there is a defective memory cell included in the memory cell array, the fuse for the Josephson AND gate that addresses the defective memory cell such as the gate G 4 is disconnected by the laser beam irradiation, and the programmable wiring elements 31 and 32 are modified as shown in FIGS.16(B) and 16(C) also by the laser beam irradiation. There, the elements designated by Y is modified as shown in FIG.16(B) by blowing up the conductor strips 30a and 30b. Thereby, the true input signal A 0 and the complementary input signal A 0b are exchanged. In the gate G 4 , a similar exchange of the true input signal and the complementary input signal is achieved also for the signals A 1 and A 1b . In the element designated by Z, on the other hand, the crossing conductor strips 30c are blown up by the irradiation of the laser beam. Thereby, the true input signal and the complementary input signal are passed through the programmable wiring element 31 as they are, without being exchanged. As a result of the exchange of the true and complementary input signals caused by the programmable wiring element 31, the redundant Josephson AND gate G red is activated by the combination of the logic state of the input signals that is identical with the combination that activates the Josephson AND gate G 4 . Thereby, the redundant memory cell is addressed by the redundant Josephson AND gate G red . On the other hand, the selection of the defective memory cell is prohibited by disconnecting the fuse f 4 simultaneously. At the downstream side of the redundant Josephson AND gate G red , there is provided another programmable wiring element 32 that is programmed such that the exchange of the true input signal and the complementary input signal caused by the programmable wiring element 31 is canceled upon passage of the input signals therethrough. Thus, when the input signals A 0 and A 0b are exchanged in the programmable wiring element 31, the programmable wiring element 32 for the input signals A 0 and A 0b are programmed identically with the element 31 as shown in FIG.16(B) such that the input signals A 0 and A 0b are exchanged again. A similar programming is made also for the programmable wiring element 32 for the input signals A 1 and A 1b . On the other hand, the programmable wiring element 32 for the input signals A 2 and A 2b , A 3 and A 3b are programmed as shown in FIG.16(C) in correspondence to the programming of the corresponding element 31. By providing the programmable wiring elements 31 and 32 as a pair in each signal line l of the interconnection wiring fixture 30, one can achieve the supply of the input signals to each of the normal Josephson AND gates G 1 - G n as if there is no redundant Josephson AND gate G red . FIG.17 shows the case wherein two redundant Josephson AND gates G red1 and G red2 are provided. In this case, the programmable wiring elements 31, 32 and 33 are provided as indicated in the drawing. As the construction and operation of the present embodiment is obvious from the description of the previous embodiment, further description of the present embodiment will be omitted. It will be understood that one can increase the number of redundant Josephson AND gates by providing the programmable wiring elements at the upstream side and the downstream side of each redundant Josephson AND gate along the signal lines l of the interconnection wiring fixture 30. FIG.18 shows a twelfth embodiment of the present invention. Referring to FIG.18, the circuit of the present invention forms a programmable logic array and includes an AND array 41 and an OR array 42, wherein the AND array 41 has a construction similar to the decoder of FIG.12 in that there are provided a plurality of Josephson AND gates G 1 - G n connected with each other by the interconnection wiring fixture 30. On the other hand, the AND array 41 is different from the decoder of FIG.12 in that some of the superconducting quantum interferometers in the Josephson AND gate represented as S x are de-coupled from the signal line l of the interconnection wiring fixture 30. Such a decoupling can be achieved by providing a conductor path such that the signal line l circumvents some of the superconducting quantum interferometers. Thereby, the AND array 41 produces output signals O 1 - O n with a number corresponding to the number of the Josephson AND gates arranged in the AND array 41. In the AND array 41 thus formed, the logic product operation is achieved based upon the input signals A 0 . . . except for those input signals that are supplied without magnetic coupling with the superconducting quantum interferomters. The OR array 42 includes a number of OR gates OR 1 - OR m , wherein each OR gate includes a number of superconducting quantum interferometers S connected in series for receiving the a.c. bias current from the a.c. current source 160. It should be noted that the a.c. current source 160 also drives the superconducting quantum interferometers of the Josephson AND gate G 1 - G n . As the superconducting quantum interferometers S are connected in series between the a.c. current source 160 and the superconducting ground plane GND, the switching of any one of the superconducting quantum interferometer S causes a transition of the output voltage obtained at an output terminal X 0 - X m . In other words, the gate OR 1 - OR m achieves the logic OR operation. When the output signals O 1 - O n of the AND array 41 are supplied to the OR array 42, the signals OR 1 - OR m are transferred from one OR gate to the next OR gate along the signal lines 35 1 - 35 n of an interconnection wiring fixture 35 that is similar to the interconnection wiring fixture 30. Thereby, the signals O 1 - O n cause the switching of the OR array 42. There, some of the signal lines 35 1 - 35 n are provided to bypass the superconducting quantum interferometer of the OR gates such that the signal that is supplied on the signal line does not cause the switching of the OR gate. By combining the logic AND operation thus achieved in the AND array 41 and the logic OR operation achieved in the OR array 42, one can achieve any logic operation. It should be noted that the selective bypassing of the signal lines 35 1 - 35 n can be achieved by providing a bypassing conductor pattern in addition to the signal lines that establish the magnetic coupling. FIG.19 shows a programmable logic array according to a thirteenth embodiment of the present invention. The programmable logic array includes an AND array 51 substantially identical in construction with the AND array 41 of the previous embodiment and an OR array 52 to be described below. Thus, the AND array 51 produces the output signals O 1 - O n in response to the input signals A 0 , A 0b , . . . and supplies the same to the OR array 52. The OR array 52 includes a number of Josephson OR gates each having a construction shown in FIG.20 and achieves the logic OR operation similarly to the OR array 42. Thereby, the result of the desired logic operation is obtained at output terminals X 0 - X m . FIG.20 shows the construction of the Josephson OR gate used in the OR array 52. Referring to FIG.20, the Josephson OR gate has a construction similar to the Josephson AND gate described with reference to FIG.6, except that there is provided an additional resistance Rs i ' in correspondence to each superconducting quantum interferometer S i of the i-th stage for supplying thereto an a.c. bias current from the bias bus 16. There, each superconducting quantum interferometer S i is biased by the resistor Rs i ' to a level slightly below a level above which the transition to the finite voltage state occurs. When biased as such, the input signal to the superconducting quantum interferometer of any stage causes the switching of the superconducting quantum interferometer to the finite voltage stage, and such a transition to the finite voltage state triggers the progressive transition of the superconducting quantum interferomters of the subsequent stages as described already with reference to the Josephson AND gate. Thereby, the circuit of FIG.20 acts as the logic OR gate. Further, the present invention is not limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims: A multiple-input Josephson AND gate having a plurality of input ports (A - D), comprising: a plurality of Josephson logic gate elements (11 - 14) provided in number corresponding to a number of said input ports and cascaded with each other from a first stage to a final stage, each of said Josephson logic gate elements of said first through final stages comprising: a superconducting quantum interferometer (S 1 - S 4 ) for producing an output signal in response to a transition from a superconducting state to a finite voltage state; an input line connected to a corresponding input port (A - D) for carrying thereon an input current, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interferometer; and biasing means (J 2 - J 4 , Rs 1 - Rs 4 , R 21 - R 41 , R 22 - R 42 ) for supplying a bias current to said superconducting quantum interferometer, said biasing means supplying the bias current with a level such that said superconducting quantum interferometer causes said transition to the finite voltage stage in response to the input current supplied to the input line, each of said biasing means of said second through final stages being supplied with the output signal of the superconducting quantum interferometer of the previous stage as a trigger signal and producing said bias current in response to said trigger signal. A multiple-input Josephson AND gate as claimed in claim 1 characterized in that said biasing means of said second through final stages comprises a first bias resistor (Rs 2 - Rs 4 ) having a first end connected to a bias current source (16) and having a second end, a Josephson junction (J 2 - J 4 ) having a first end connected to said second end of said first bias resistor and a second end connected to a superconducting ground plane (GND), an input resistor (R 21 - R 41 ) having a first end connected to the superconducting quantum interferometer of the previous stage for receiving the output signal therefrom as said trigger signal and a second end connected to the first end of the Josephson junction, and a second bias resistor (R 22 - R 42 ) having a first end connected to the first end of the Josephson junction and a second end connected to a bias terminal of the superconducting quantum interferometer of the current stage. A multiple-input Josephson AND gate as claimed in claim 2 characterized in that said second bias resistor (R 22 - R 42 ) has a resistance set substantially equal to a gap voltage (V g ) of the Josephson junction divided by a critical current (I m ) of the Josephson junction. A multiple-input Josephson AND gate as claimed in claim 2 characterized in that said multiple-input Josephson AND gate further comprises an output circuit (15), said output circuit comprising a bias resistor (R 5 ) having a first end connected to the bias current source (16) and a second end, a Josephson junction (J 5 ) having a first end connected to said second end of said bias resistor and a second end connected to a superconducting ground plane, and an input resistance (R 51 ) having a first end connected to the superconducting quantum interferometer of the previous stage for receiving the output signal therefrom and a second end connected to said first end of the Josephson junction of the output circuit. A multiple-input Josephson AND gate as claimed in claim 1 characterized in that said biasing means of said third through final stages comprises a first bias resistor (Rs 3 - Rs 4 ) having a first end connected to a bias current source (16) and having a second end, a Josephson junction (J 3 - J 4 ) having a first end connected to said second end of said first bias resistor and a second end connected to a superconducting ground plane, an input resistor (R 31 -R 41 ) having a first end connected to the superconducting quantum interferometer of the previous stage for receiving the output signal therefrom as said trigger signal and a second end connected to the first end of the Josephson junction, and a second bias resistor (R 32 - R 42 ) having a first end connected to the first end of the Josephson junction and a second end connected to a bias terminal of the superconducting quantum interferometer of the current stage; said biasing means of said second stage essentially consists of a bias resistor (R 22 ) having a first end connected to the superconducting quantum interferometer of said initial stage for receiving the output signal therefrom and a second end connected to a bias terminal of the superconducting quantum interferometer of said second stage. A Josephson decoder circuit for decoding input data represented by a plurality of bits, comprising: a plurality of multiple-input Josephson AND gates (G 1 - G 16 ) each including a plurality of Josephson logic gate elements (11 - 14) with a number corresponding to a number of bits of the input data, said plurality of Josephson logic gate elements being cascaded, in each Josephson AND gate, with each other from a first stage to a final stage; each of said Josephson logic gate elements comprising: a superconducting quantum interferometer (S 1 - S 4 ) for producing an output signal in response to a transition from a superconducting state to a finite voltage state; an input line connected to a corresponding input port (A - D) for carrying thereon an input current, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interferometer; and biasing means (Rs 1 - Rs 4 , R 21 - R 41 , R 22 - R 42 ) for supplying a bias current to said superconducting quantum interferometer, said biasing means supplying the bias current with a level such that said superconducting quantum interferometer causes said transition to the finite voltage stage in response to the input current supplied to the input line, each of said biasing means of said second through said final stages being supplied with the output signal of the superconducting quantum interferometer of the previous stage as a trigger signal and producing said bias current in response to said trigger signal; interconnection means (30) for connecting the input lines of the Josephson logic gate elements of different Josephson AND gates with each other for each of the stages such that the input data is supplied to the Josephson AND gates according to a logic combination that is different in each Josephson AND gate. A Josephson decoder circuit as claimed in claim 6 characterized in that said plurality of Josephson AND gates (G 1 - G 16 ) are provided in number corresponding to the number of logic combination of the input data. A Josephson decoder circuit as claimed in claim 6 characterized in that said plurality of Josephson AND gates (G 1 - G n ) are provided in number exceeding the number of logic combination of the input data, said interconnection means comprising programmable interconnection means (31, 32) for connecting said Josephson AND gates to said interconnection means such that more than one Josephson AND gates produce an output signal in response to an identical input data. A programmable logic array circuit supplied with input data for producing output data after a predetermined logic operation, comprising: a programmable Josephson AND array (41) comprising a plurality of multiple-input Josephson AND gates (G 1 - G n ) each including a plurality of Josephson logic gate elements (11 - 14) with a number corresponding to a number of bits of the input data, said plurality of Josephson logic gate elements being cascaded, in each Josephson AND gate, with each other from a first stage to a final stage; each of said Josephson logic gate elements comprising: a superconducting quantum interferometer (S 1 - S 4 ) for producing an output signal in response to a transition from a superconducting state to a finite voltage state; an input line connected to a corresponding input port (A - D) for carrying thereon an input current, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interferometer; and biasing means (J 2 - J 4 , Rs 1 - Rs 4 , R 21 - R 41 , R 22 - R 42 ) for supplying a bias current to said superconducting quantum interferometer, said biasing means supplying the bias current with a level such that said superconducting quantum interferometer causes said transition to the finite voltage stage in response to the input current supplied to the input line, each of said biasing means of said second through said final stages being supplied with the output signal of the superconducting quantum interferometer of the previous stage as a trigger signal and producing said bias current in response to said trigger signal; interconnection means (30) for connecting the input lines of the Josephson logic gate elements of different Josephson AND gates with each other for each of the stages such that the input data is supplied to the Josephson AND gates according to a logic combination that is different in each Josephson AND gate, said interconnection means prohibiting supply of the input data to a selected Josephson gate element of a selected Josephson AND gate; and programmable Josephson OR array (42) supplied with the output data of the programmable Josephson AND array, said programmable Josephson OR array comprising a plurality of Josephson logic gate elements (OR 1 - OR m ) each comprising a superconducting quantum interferometer (S) and an input line (35 1 - 35 n ) carrying thereon an input current, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interferometer, said Josephson logic OR array producing output data according to a logic sum operation of the output data of said programmable Josephson AND array, said programmable Josephson OR array including a selected Josephson logic gate element that is disabled. A programmable logic array circuit as claimed in claim 9 characterized in that said programmable Josephson OR array comprises a plurality of multiple-input Josephson OR gates (OR 1 - OR m ), each of said programmable Josephson OR array comprising: a plurality of Josephson logic gate elements cascaded with each other with a number corresponding to a number of output signals of said programmable Josephson AND array; each of said Josephson logic gate elements comprising: a superconducting quantum interferometer (S 1 ) for producing an output signal in response to a transition from a superconducting state to a finite voltage state; an input line (O 1 - O n ) connected to said programmable Josephson AND array for carrying thereon an output current of said programmable Josephson AND array, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the output current to said superconducting quantum interferometer; first biasing means (Rs 2 ' - Rs 4 ') for supplying a first bias current to said superconducting quantum interferometer with a level such that said superconducting quantum interferometer causes said transition to the finite voltage state in response to the input current supplied to the input line; and second biasing means (J 2 - J 4 , Rs 1 - Rs 4 , R 21 - R 41 , R 22 - R 42 ) for supplying a second bias current to said superconducting quantum interferometer, said second biasing means supplying the second bias current with a level such that said superconducting quantum interferometer causes said transition to the finite voltage stage irrespective of the input current supplied to the input line provided that said superconducting quantum interferometer is supplied with said first bias current, each of said second first biasing means of said second through final stages being supplied with the output signal of the superconducting quantum interferometer of the previous stage as a trigger signal and producing said second bias current in response to said trigger signal. A multiple-input Josephson OR gate having a plurality of input ports (O 1 - O n ), comprising: a plurality of Josephson logic gate elements cascaded with each other with a number corresponding to a number of said input ports, each of said Josephson logic gate elements comprising: a superconducting quantum interferometer (S 1 - S 4 ) for producing an output signal in response to a transition from a superconducting state to a finite voltage state; an input line connected to said input port for carrying thereon an input current, said input line being coupled magnetically to the superconducting quantum interferometer for transferring the input current to said superconducting quantum interferometer; first biasing means (Rs 2 ' - Rs 4 ') for supplying a first bias current to said superconducting quantum interferometer with a level such that said superconducting quantum interferometer causes said transition to the finite voltage state in response to the input current supplied to the input line; and second biasing means (J 2 - J 4 , Rs 1 - Rs 4 , R 21 - R 41 , R 22 - R 42 ) for supplying a second bias current to said superconducting quantum interferometer, said second biasing means supplying the second bias current with a level such that said superconducting quantum interferometer causes said transition to the finite voltage stage irrespective of the input current supplied to the input line provided that the superconducting quantum interferometer is supplied with said first bias current, each of said second first biasing means of said second through final stages being supplied with the output signal of the superconducting quantum interferometer of the previous stage as a trigger signal and producing said second bias current in response to said trigger signal.

Inventors: SUZUKI HIDEO, SUZUKI, HIDEO, Suzuki, Hideo, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H03K19
IPC codes: H03K19, H03M7