Superconductive single flux quantum logic circuit

[0002] The present invention relates to a configuration of a superconductive logic circuit using a single flux quantum as an information binary digit.

[0003] The configuration of a superconductive logic circuit using a single flux quantum as an information binary digit is described in IEEE Transactions on Applied Superconductivity vol. 1 (1991) pp. 77-89.

[0004] In a single flux quantum logic circuit, it is necessary to set the inductance of a superconductive closed loop to tens pH or smaller for circuit operation and it is also necessary to set the load inductance to tens pH or smaller in order to obtain a large output current. In order to supply an output signal of a single flux quantum logic circuit to a single flux quantum logic circuit at the next stage, a load inductance part has to be taken out of the superconductive closed loop. As shown in FIG. 1 in the above literature, the single flux quantum logic circuit generally includes only one means for applying flux to the superconductive closed loop and the load inductance part is positioned inside of the superconductive closed loop in order to reduce the inductance. When the load inductance part is taken to the outside of the superconductive closed loop, as shown in FIG. 4 of the literature, since superconductive wires cross each other, a problem such that the fabrication process becomes difficult occurs. Since the wire becomes longer, the inductance increases. Consequently, a problem such that the margin of the circuit operation decreases and the output current becomes smaller occurs.

[0005] Further, in order to construct a logic circuit, a sequential circuit is necessary. In the conventional technique, as shown in FIG. 16 of the literature, a shift register as one of sequential circuits is fabricated by using a multi-phase clock. In the multi-phase clock, however, signals are transmitted and received at overlapped parts of clocks. Consequently, a problem such that the timings of the clocks have to be accurately provided arises.

[0006] In order to solve the problems, it is an object of the invention to provide a structure which can realize a flip-flop circuit as a basic element of a sequential circuit without using a multi-phase clock and take a load inductance part out of a superconductive closed loop without increasing the inductance.

[0007] The invention is realized by a structure in which a load inductance part of the superconductive logic circuit having two superconductive closed loops each comprising a Josephson junction and one or more inductor(s) has at least an inductor and a Josephson junction.

[0008] FIG. 1 is a diagram showing an equivalent circuit of a superconductive single flux quantum logic circuit of a first embodiment of the invention.

[0009] FIG. 2 is a diagram showing an operation waveform of the superconductive single flux quantum logic circuit of the first embodiment of the invention.

[0010] FIG. 3 is a diagram showing an operating state of the superconductive single flux quantum logic circuit of the first embodiment of the invention.

[0011] FIGS. 4(a) to 4(e) E are diagrams showing a fabricating process of the superconductive single flux quantum logic circuit of the first embodiment of the invention and arrangement of circuits seen from above of the fabricated superconductive single flux quantum logic circuit.

[0012] FIGS. 5(a) and 5(b) are diagrams each showing an equivalent circuit of the superconductive single flux quantum logic circuit of a second embodiment of the invention.

[0013] FIG. 6 is a diagram showing arrangement of circuits from above of the superconductive single flux. quantum logic circuit of the second embodiment of the invention.

[0014] FIG. 7 is a diagram showing an equivalent circuit of a demultiplexer of a third embodiment of the invention.

[0015] FIG. 8 is a diagram showing an operation waveform of the demultiplexer of the third embodiment of the invention.

[0016] Embodiments of the invention will be described hereinbelow. A first embodiment will be described by using FIGS. 1, 2, 3, and 4(a) to 4(e). The embodiment relates to an example of fabricating a flip-flop circuit as a basic element of a sequential circuit by using a QFP (Quantum Flux Parametron) as a single flux quantum logic circuit. As a superconductor, an oxide superconductor is employed.

[0017] The circuit configuration is shown in FIG. 1. A QFP comprises a superconductive closed loop constructed by a first Josephson junction 1, a first inductor 3, and a load inductance part 8, a superconductive closed loop constructed by a second Josephson junction 2, a second inductor 4, and the load inductance part 8, and an inductor 7 as means for applying flux to the superconductive loops via the inductors 3 and 4. The load inductance part 8 is shared by the two superconductive closed loops.

[0018] The logic value of the QFP is determined by the number of flux quanta stored in the superconductive closed loops. When it is assumed that the number of flux quanta stored in the superconductive closed loop constructed by the first Josephson junction 1, first inductor 3 and load inductance part 8 is (m) and the number of flux quanta stored in the superconductive closed loop constructed by the load inductance part 8, second inductor 4, and second Josephson junction 2 is (n), (m, n)=(1, 0) corresponds to the logic value 1 of the QFP and (m, n)=(0, 1) corresponds to the logic value 0.

[0019] In the conventional QFP, the load inductance part 8 is formed only by inductors and an activation current as a clock is supplied to the inductor 7 and an input current is supplied to a connection point 5. When the activation current is set to zero, the output current becomes almost zero, so that information of an input signal cannot be held and the QFP cannot be used as a sequential circuit.

[0020] On the contrary, in the embodiment, the load inductance part 8 is constructed by a series circuit of inductors 10 and 11 and a third Josephson junction 9 and circuit means for supplying a first signal current I₁, and a second signal current I₂to the inductors 10 and 11, respectively. Consequently, information of an input signal can be held and a flip-flop circuit as a basic element of the sequential circuit can be realized.

[0021] The circuit operation will be described with reference to FIGS. 2 and 3. Circuit parameters of the embodiment are as follows. The critical current I_cof the Josephson junctions 1 and 2 is 0.05 mA, an inductance L of each of the inductors 3 and 4 is 5 pH, the critical current I_cof the Josephson junction 9 is 0.1 mA, and inductances L of the inductors 10 and 11 are 1 pH and 9 pH, respectively. A constant bias current is applied to the inductor 7. FIG. 2 shows an operation waveform and FIG. 3 shows an operating state of the QFP. The operating state of the QFP can be expressed by the numbers (m, n) of flux quanta stored in the two superconductive closed loops. FIG. 3 shows the dependency on the first signal current I₁and the second signal current I₂of (m, n).

[0022] In FIG. 2, I₀₁represents a current passing through the inductor 11 and V_J3denotes a voltage applied to the Josephson junction 9. The current I₀₁passing through the inductor 11 and the voltage V_J3applied to the Josephson junction 9 are shown. When the signal currents Iand I₂are increased little by little from 0 to 30×10⁻⁶(A) in a period from 0 to 0.05×10⁻⁹seconds, only the signal current I₁is once increased to 100×10⁻⁶(A) and then returned to the original value in a period from 0.1×10⁻⁹to 0.6 ×10⁻⁹seconds. Similarly, only the signal current I₂is once increased and then returned to the original value in a period from 0.6×10⁻⁹seconds to 1.1×10⁻⁹seconds. After that, the signal currents I₁and I₂are maintained at 30×10⁻⁶(A) which is the same as that before increase.

[0023] It is understood from FIGS. 2 and 3 that when the first and second signal currents I₁and I₂are small, I₀₁is a positive value and the QFP is in the (1, 0) state. When I₁increases and exceeds a threshold value, I₀₁rapidly becomes a negative value and the QFP is transitioned to the (0, 1) state. At this time, V_J3has a sharp pulse waveform which corresponds to a change in the numbers (m, n) of the flux quanta. The reason why the state transition occurs in the QFP is because there is the third Josephson junction 9 in the load inductance part 8. Specifically, a current (I₀₂) which is the sum of I₁and I₀₁is passed to the Josephson junction 9. When the current exceeds the critical current, the junction is transitioned to a voltage state and the state transition in the QFP occurs.

[0024] After that, even when I₁decreases and is returned to the original value, I₀₁is still a negative value and the state remains at (0, 1) and is held. Subsequently, when I₂increases and exceeds a threshold value, I₀₁becomes positive and the QFP is shifted to the (1, 0) state. At this time as well, V_J3has a sharp pulse waveform which corresponds to the change in the numbers (m, n) of the flux quanta. The state transition of the QFP occurs due to the first and second Josephson junctions 1 and 2. Specifically, a current obtained by subtracting I₀₁from I₂is passed to the Josephson junctions 1 and 2. When the current exceeds the critical current, the junction shifts to a voltage state and the state transition in the QFP occurs. After that, when I₂decreases and is returned to the original value, I₀₁is still a positive value and the state remains at (1, 0). Similarly, the state is held.

[0025] By making Iand I₂correspond to a setting signal and a resetting signal, respectively, a flip flop can be realized. In the embodiment, a constant bias current is just applied to the inductor 7 and a multi-phase clock is not used. A flip flop can be therefore realized without considering the problem of the timing of the clock.

[0026] The conventional QFP has a configuration that the Josephson junction is not included in the load inductance part 8. The state of the QFP does not therefore transition between (1, 0) and (0, 1) in accordance with the values I₁and I₂, so that a flip flop operation with respect to I₁and I₂cannot be realized.

[0027] A process of fabricating a flip flop circuit of the embodiment will be described with reference to cross sections of FIGS. 4(a) to 4(d). As shown in FIG. 4(a), an oxide superconductor YBa₂Cu₃O₇film 200 is deposited by laser deposition on the top face of an SrTiO₃bicrystal substrate 100 having a junction face CS. As shown in FIG. 4(b), a predetermined pattern is formed in the YBa₂Cu₃O₇film 200 by electron beam lithography and Ar ion etching. Further, as shown in FIG. 4(c), an SrTiO₃film 300 is deposited as an insulating interlayer by laser deposition. Further, as illustrated in FIG. 4(d), an Au film 400 is deposited by a resistance heating evaporation method and formed in a predetermined pattern by electron beam lithography and Ar ion etching, thereby fabricating a desired QFP.

[0028] FIG. 4(e) shows an arrangement of circuits seen from above of the fabricated QFP. The Josephson junctions 1 and 2, the inductors 3 and 4, and the load inductance part 8 comprised of the inductors 10 and 11 and the third Josephson junction 9 are formed at the time of patterning the YBa₂Cu₃O₇film 200 in FIG. 4(b). At this time, a Josephson junction is formed in a position corresponding to the junction face CS of the SrTiO₃bicrystal substrate 100 shown by a broken line. The two signal currents I₁and I₂are passed as shown by arrows in the diagram. The inductor 7 is formed at the time of patterning the Au film 400 in FIG. 4(d). In the diagram, the inductor 7 is shown by broken lines so as to be easily distinguished from the other circuit part. In order to apply flux to the superconductive closed loop comprised of the Josephson junctions 1 and 2 and the inductors 3 and 4, the inductor 7 is formed in the position corresponding to the inductors 3 and 4. As shown in the equivalent circuit of FIG. 1, the common part of the circuit at the left end in FIG. 4(e) is connected to the ground. The wiring for this arrangement is omitted in the diagram.

[0029] A second embodiment of the invention in which the load inductance part is taken out of the superconductive closed loop without increasing the inductance in the QFP as a single flux quantum logic circuit by paying attention to the configuration that the load inductance part has at least the inductor and the Josephson junction will be described by using FIGS. 5(a) and 5(b) showing the equivalent circuit and FIG. 6 showing the circuit arrangement.

[0030] The embodiment of FIGS. 5(a) and 5(b) comprises a superconductive closed loop constructed by the first Josephson junction 1, the first inductor 3, and the second Josephson junction 2, the superconductive closed loop constructed by the second Josephson junction 2, the second inductor 4, the load inductance part 8, and the third Josephson junction 9, and two inductors 7₁and 7₂as means for applying flux to the superconductive closed loops via the inductors 3 and 4. The second Josephson junction 2 is shared by the two superconductive closed loops. In the embodiment, it is unnecessary to divide the load inductance part 8 into the inductors 10 and 11.

[0031] Elements designated by the same reference numerals as those in FIG. 1 are equivalents. Although the same activation current is supplied to the inductors 3 and 4 in FIG. 1, activation currents I_f1and I_f2are supplied to the inductors 7₁and 7₂, respectively, and it is set so that external flux Φ₀is applied to the first superconductive closed loop and external flux −Φ₀/2 is applied to the second superconductive closed loop. As a result, the flux quanta of the loops become as shown in the diagram in accordance with the polarity of the input signal current I_dand the polarity of the output current I_outpassing through the load inductance part 8 becomes as shown in the diagram.

[0032] The fabricating process of the embodiment shown in FIGS. 5(a) and 5(b) is similar to the first embodiment shown in FIGS. 4(a) to 4(d) as easily understood from the comparison between FIGS. 1 and 5(a) and 5(b). FIG. 6 is a diagram showing the arrangement of circuits seen from above of the fabricated QFP. The superconductive closed loop is formed by the Josephson junctions 1 and 2 and the inductor 3 and the superconductive closed loop is formed by the Josephson junctions 2 and 9 and the inductors 4 and 8. The inductors 7₁and 7₂are magnetically connected to the inductors 3 and 4 and apply flux to the superconductive closed loops. Since the load inductance part 8 is one of elements of the superconductor closed loop and is taken out of the superconductive closed loop, an output signal of the QFP can be easily supplied to the QFP at the next stage. A signal current I_dis supplied from the connection point 5. The inductors 7₁and 7₂are magnetically connected to the inductors 3 and 4 and apply the flux. By passing the activation currents to the inductors 7₁and 7₂so that the flux applied by the inductor 7₁via the inductor 3 is a flux quantum and the flux applied by the inductor 7₂via the inductor 4 is −1/2 flux quantum, the QFP operation can be obtained.

[0033] As described above, in the embodiment, the load inductance part can be taken out of the superconductive closed loop without crossing the superconductive wires or making the wiring longer. Thus, the QFP capable of outputting a large current and easily supplying an output signal to the next stage can be realized. In order to supply the output signal to the next stage, it is sufficient to place an input inductor at the next stage which is magnetically connected to the load inductance part 8 by the side.

[0034] A third embodiment of the invention will now be described with reference to FIGS. 7 and 8. The embodiment relates to a case of fabricating a 1:2 demultiplexer (DMUX) by using the QFP of the second embodiment. An equivalent circuit is shown in FIG. 7. Flux is applied by an inductor 641 through which a current I_act1is passed to a superconductive closed loop comprised of Josephson junctions 611 and 613 and inductors 621, 631, and 623. Flux is applied by an inductor 642 through which a current I_act2is passed to a superconductive closed loop comprised of Josephson junctions 613 and 612 and inductors 623, 632, and 622. A current I_Lpassing through the inductor 621 is detected by a superconducting quantum interference device 651 and a current I_Rpassing through the inductor 622 is detected by a superconducting quantum interference device 652. Since the inductors 621 and 622 are formed on the outer sides of the superconductive closed loops, the superconducting quantum interference devices 651 and 652 can be arranged so as to be magnetically connected to the inductors 621 and 622 without crossing wires.

[0035] FIG. 8 shows the operational waveform. When the currents I_act1and I_act2are passed to the inductors 641 and 642 so that one flux quantum is applied by the inductor 641 and the −1/2 flux quantum is applied by the inductor 642, the inductors 622 and 632 and the Josephson junction 612 function as a load inductance part and the output current corresponding to the signal current I_dflows in the inductor 622. Similarly, when the currents I_act1and I_act2are passed to the inductors 641 and 642 so that one flux quantum is applied by the inductor 642 and the −1/2 flux quantum is applied by the inductor 641, the inductors 621 and 631 and the Josephson junction 611 function as a load inductance part and the output current corresponding to the signal current I_dflows in the inductor 621. In practice, as shown in FIG. 8, in the case where the signal current I_dis positive, that is, the logic value is 1, when I_act1=2.07 mA and I_act2=−1.035 mA, I_Rbecomes a sufficiently large positive value, that is, the logic value of 1. When I_act1=−1.035 mA and I_act2=2.07 mA, I_Lbecomes a sufficiently large positive value, that is, a logic value of 1. In the case where the signal current I_dis negative, that is, the logic value is 0, when I_act1=2.07 mA and I_act2=−1.035 mA, I_Rbecomes a sufficiently large negative value, that is, the logic value of 0. When I_act1=−1.035 mA and I_act2=2.07 mA, I_Lbecomes a sufficiently large negative value, that is, a logic value of 0. By the above, the correct operation can be confirmed. By using the currents I_act1and I_act2passing through the inductors 641 and 642 as selection signals, the logic value of the signal current can be outputted to the superconducting quantum interference device 651 or 652, thereby enabling a 1:2 demultiplexer to be obtained. In FIG. 8, the currents I_act1and I_act2are shown on the scale of 1/20.

[0036] As described above, the superconductive single flux quantum logic circuit of the invention has the configuration such that the load inductance part is comprised of at least the inductor and the Josephson junction, thereby enabling various circuits to be easily constructed.