Fluxoid type superconducting logic element

Disclosed is a fluxoid type superconducting logic element essentially comprising a pair of SQUIDs connected with each other so as to put one of said SQUIDs into its "firing state" and the other into its "extinction state" in response to an input signal in the form of magnetic flux, thereby representing a binary digit at its output terminals.

BACKGROUND OF THE INVENTION 
The present invention relates to a superconducting logic element, and 
particularly to a superconducting logic element which is capable of 
performing, on the basis of the quantum effect, various functions as 
required for instance in an electronic computer. Such logic element is 
hereinafter referred to as "fluxoid type superconducting logic element". 
The term, "fluxoid" is an abbreviation for "magnetic flux quantum unit", 
.phi..sub.o (=2.times.10.sup.-7 gauss cm.sup.-2). 
The object of this invention is to provide a fluxoid type superconducting 
logic element using Josephson Junctions, and hence capable of operating at 
an increased speed. 
SUMMARY OF THE INVENTION 
To attain this object a fluxoid type superconducting logic element 
according to this invention comprises a pair of radio frequency 
superconducting quantum interference devices (hereinafter abbreviated "r-f 
SQUID") each essentially comprising a superconductor loop including a 
Josephson Junction; a drive means inductance-coupled with each r-f SQUID 
for applying a drive magnetic flux thereto; an input means 
inductance-coupled with each r-f SQUID for supplying thereto an input 
signal in the form of magnetic flux; and an output means 
inductance-coupled with each r-f SQUID for providing an output signal in 
the form of electric current or voltage, said r-f SQUIDs being 
inductance-coupled with each other so as to put one of the SQUIDs to its 
"firing" state and the other to its "extinction" state in response to the 
input signal

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1A shows an r-f SQUID whereas FIG. 1B shows the internal-to-external 
magnetic flux characteristics of the r-f SQUID. The r-f SQUID is shown as 
comprising a superconductor loop including a Josephson Junction J and an 
inductance L. In operation, when an external magnetic flux .phi..sub.ext 
is applied to the loop, a persistent current I will flow in the loop to 
generate a counter magnetic flux (=L I) as much as the external magnetic 
flux. The persistent current I, and hence the counter magnetic flux will 
increase with the increase of the external magnetic flux until the 
persistent current reaches its critical value Jc, thus maintaining the 
inside of the loop magnetically quiescent. When the persistent current 
increases beyond its critical value the superconducting state of the loop 
will turn to the normal state at the weak link of the Josephson Junction, 
thereby allowing magnetic flux as strong as the magnetic flux quantum unit 
.phi..sub.o to enter the loop, which magnetic flux being referred to as 
"internal magnetic flux". The internal-to-external magnetic flux 
characteristics are shown for a (=2.pi.L.multidot.Jc/.phi..sub.o).ltoreq.1 
and for a&gt;1 in FIG. 1B. FIG. 1C shows the internal-to-external magnetic 
flux characteristics of an r-f SQUID for a=.pi.. As shown, when the 
external magnetic flux .phi..sub.ext reaches the upper critical value 
.phi..sub.p, the internal magnetic flux .phi..sub.int will appear as 
indicated at Q, and when the external magnetic flux .phi..sub.ext reaches 
the lower critical value .phi..sub.R, the internal magnetic flux 
.phi..sub.int will almost disappear. This rise-and-fall variation is 
symbolically represented in FIG. 1D. The state corresponding to NINT 
(.phi..sub.int /.phi..sub.o)=1 in the hysteresis 1ooP is herein called 
"firing state" whereas the state corresponding to NINT (.phi..sub.int 
/.phi..sub.o)=0 is called "extinction state". 
FIG. 2 shows schematically the structure of a fluxoid type superconducting 
logic element according to this invention. It is shown as essentially 
comprising a pair of r-f SQUIDs SQ1 and SQ2 including Josephson Junctions 
J1 and J2 respectively. These r-f SQUIDs are inductance-coupled with each 
other so as to put one of the SQUIDs in its "firing state" and put the 
other in its "extinction state" in response to an input signal in the form 
of magnetic flux. As shown an inductance means DL is provided to apply a 
drive of external magnetic flux .phi..sub.d to each SQUID. The SQUIDs SQ1 
and SQ2 are magnetically coupled with each other by a mutual inductance M 
with such a coupling polarity that development of one of the SQUIDs into 
its "firing state" may drive the other SQUID into its "extinction state". 
The strength of the drive magnetic flux is just below the upper critical 
value .phi..sub.p, and when a very small input magnetic flux 
.epsilon..phi. is applied to one of the SQUIDs in addition to the drive 
magnetic flux, the SQUID will be put in its "firing state", and as a 
counter action, the other SQUID will be put in its "extinction state". For 
the purpose of applying an input magnetic flux to the SQUIDs a 
superconducting inductance means IDL is used to carry an input current 
.epsilon.I thereby generating and applying input magnetic fluxes of 
opposite polarities to the SQUIDs SQ1 and SQ2 respectively to assure of 
the positive appearance of the opposite states in the two SQUIDs. It, 
however, should be noted that it suffices that a single superconducting 
inductance means is provided to magnetically couple with one of the 
SQUIDs. For the purpose of providing an output signal another 
superconducting inductance means AL is used to magnetically couple with 
the SQUIDs to detect the appearance of the "firing state" in one of the 
SQUIDs. In short, a fluxoid type superconducting logic element might be 
said to be a fluxoid type binary amplifier which is designed to be 
magnetically energized and to be responsive to a very small amount of 
input magnetic flux .epsilon..phi. for providing an amplified amount of 
output magnetic flux .+-..phi.. 
FIG. 3 shows a modification of the embodiment of FIG. 2. As shown, it is 
constructed so as to apply an external magnetic flux to each SQUID by 
supplying a dc electric current Id to the inductance L of each SQUID. 
Also, it includes an inductance M' and an associated output circuit AL in 
which electric currents from SQ1 and SQ2 flow, so as to put one of the 
SQUIDs in its "firing state" and the other in its "extinction state" in 
response to an input signal from IDL, thus attaining the same result as in 
FIG. 2. 
FIG. 4 shows another modification of the embodiment of FIG. 2. As shown, it 
has the same configuation as FIG. 2 in applying an external magnetic flux 
to each SQUID. However, an inductance M" is used to couple the two SQUIDs 
with each other. In operation the logic element of FIG. 4 responds to an 
input magnetic flux from IDL for putting one of the SQUIDs to its "firing 
state" and the other to its "extinction state". 
FIG. 5 shows a threshold logic circuit, specifically a three-input majority 
logic circuit, f=maj (x, y, z), composed of a plurality of logic elements 
FF1-FF4 according to the embodiment of FIG. 3. Three logic elements 
FF1-FF3 are coupled to a logic element FF4 as indicated at LL. Assume that 
a drive current Idl flows to apply an external magnetic flux to each of 
the logic elements FF1, FF2 ad FF3 and that these logic elements represent 
different binary variables x, y and z respectively. Then, a drive current 
Id2 is supplied to the logic element FF4 to represent the majority logic 
result at its output terminal AL. 
A "NOT" circuit can be easily constructed simply by reversing the polarity 
of either of the primary or secondary windings of a superconducting flux 
transformer for applying an input magnetic flux to a logic element. The 
superconducting flux transformer may be used as an input inductance means 
IDL, and therefore no extra circuit element is required for constructing a 
"NOT" circuit. 
A magnetically coupled configuration of logic elements as shown in FIG. 5 
has no directionality in conveying pieces of information. To assure that 
pieces of information are conveyed in a predetermined direction three or 
more phase clock signal generator may be used to supply drive magnetic 
fluxes (on electric currents), as is the case with a parametron. FIG. 6 
shows the waveforms of such three-phase clock signal. Phase 1 shows a 
drive magnetic flux (or current) .PHI.dl (Idl) to be applied to FF1 FF2 
and FF3; phase 2 shows a drive magnetic flux (or current) .PHI.d2 (Id2) to 
be applied to FF4; and phase 3 shows a drive magnetic flux (or current) 
.PHI.d3 (Id3) to be applied to a subsequent circuit (not shown). As shown, 
every waveform varies sinusoidally with its maximum above the external 
upper threshold magnetic flux and its minimum below the external lower 
threshold magnetic flux. When the majority logic circuit of FIG. 5 is 
controlled with the clock signal of FIG. 6, a group of logic elements FFl 
to FF3 and the final logic element FF4 will be put in standby in the order 
named, but will never be put in standby in the opposite way, thus assuring 
of unidirectional transmission. As an actual example fluxoid type 
superconducting logic elements including "weak link" or bridge type 
Josephson Junctions made of a superconducting niobium were used at 
"a"=.pi., and then their switching time was below 1 picosecond (10.sup.-12 
seconds). Therefore, these logic elements can operate with clock signal of 
ten or more gigahertz. Clock signals other than sinusoidal waves are 
difficult to use. In FIG. 6 each clock signal is formed by superposing a 
sinusoidal wave on a d c bias corresponding to the intermediate value 
between .PHI.P and .PHI.R. 
FIG. 7 shows another clock signal waveform which, if applied to the 
majority logic circuit of FIG. 5, can assure that pieces of information 
are conveyed in a predetermined direction. Assume that the logic elements 
FF1, FF2 and FF3 have same upper and lower threshold fluxes .PHI.P1 and 
.PHI.R1 and that the logic element FF4 has upper and lower threshold 
fluxes .PHI.P2 and .PHI.R2, the absolute values of which are larger than 
.PHI.P1 and .PHI.R1 respectively. The clock flux .PHI.d is shown as 
varying sinusoidally with its maximum above .PHI.P2 and its minimum below 
.PHI.R2 in FIG. 7. As is apparent from the drawing, the majority logic 
circuit of FIG. 5 is controlled with the clock flux .PHI.d, the logic 
elements FF1, FF2 and FF3 are assured to establish their binary states 
before the logic element FF4, as indicated at FF1, 2, 3 and FF4. 
As for the amplitude of the sinusoidal clock signals of FIGS. 6 and 7, the 
maximum of the sinusoidal wave must be determined so as to be above the 
upper threshold value .PHI.P of the external magnetic flux but below a 
certain upper limit, for instance, 7/3 times as much as the upper 
threshold value .PHI.P for a=.pi.. The allowance of amplitude is 
advantageously great, and this is attributable to the differential 
combination of two SQUIDs according to this invention. 
Niobium may be preferably used in producing a fluxoid type superconducting 
logic element according to this invention, and a configuration of 
inductances and Josephson Junctions as required can be easily formed, for 
instance, according to the lithography or sputtering technique. 
As for the energy required for driving a single logic element according to 
this invention, assume that the device has an inductance L of 10pH. (this 
much inductance can be easily produced by the photolithography technique 
in the form of a single circular loop about 10 .mu.m across or of a single 
square about 10 .mu.m side.) Then, the required energy E is given by: 
EQU E=.phi..sub.o.sup.2 /2L=2.times.10.sup.-19 joules 
If this logic element is driven by clock signal of 10GHz, the required 
energy is equal to 2.times.10.sup.-9 watts. This figure is very small 
compared with the energy (2.times.10.sup.-6 watts) required for a 
conventional dc SQUID circuit capable of representing a binary digit in 
the form of voltage state. 
One tenth of reduction of the logic element in size will multiply the 
required energy ten times. This is because the inductance of the reduced 
logic element decreases ten times as a result of physical size reduction. 
The required energy, however, can be reduced N.sup.2 times by increasing 
the number of turns N times. Thus, the required energy can be reduced to a 
desired value.