Josephson junction device

The present invention relates to a Josephson junction device having voltage amplification.

It is well known that a Josephson junction device displays no impedance when the current passing through the junction is at a value less than the critical current 1_0' When a value of critical current 1₀ is reached, the Josephson junction becomes resistive and switches from the zero voltage state to the voltage gap state. The gap voltage for a Josephson junction remains constant and does not vary. When a constant current source is connected to a Josephson junction device, a major portion of that current can be steered or diverted to a parallel load or loads when the Josephson junction switches. The nature of a Josephson junction is that it either displays a voltage or no voltage; thus, by definition, it does not amplify an input voltage.

Josephson junction load drivers usually employ a plurality of parallel branches and a plurality of Josephson junction devices. All of these load drivers are limited to modest gain and modest fan-out. Circuits of this type are shown and described in IEEE Transactions on Magnetics, volume 15, no. 6, November, 1979 at pages 1876-1879 and also in IEEE International Electron Devices meeting, Washington, D.C., December 3-5, 1979 at pages 482-484.

The voltages and currents which occur in Josephson junction technology are very small, typical voltage gaps being of the order of one or two millivolts, and the threshold currents being measured in micro amperes. Such magnitudes are of course quite incompatible with transistor logic circuits and problems of noise arise in attempting to amplify them.

It is an object of the present invention to provide a Josephson junction device with a high voltage gain, and in which, moreover, the amplification occurs at low temperatures within the cryogenic container so as to be substantially noise-free.

The invention will be further described with reference to the accompanying drawings, in which

• Figure 1 is a Josephson junction circuit employing two Josephson Atto-Webber Switches (JAWS) arranged in series and having an external output sensing logic circuit;
• Figure 2 is a schematic circuit showing the generic elements of the Josephson junction amplifier of the present invention;
• Figure 3 is a schematic circuit showing a modified embodiment of the Josephson junction amplifier of Figure 2;
• Figure 4 is a typical current vs. voltage characteristic curve for a Josephson junction;
• Figure 5 is a schematic waveform of current vs. time showing a typical input signal to the Josephson junction voltage amplifier;
• Figure 6 is a schematic drawing showing voltage out vs. variable voltage in for the novel Josephson junction voltage amplifier;
• Figure 7 is a schematic drawing showing a typical current vs. voltage characteristic for a Josephson junction without any amplification; and
• Figure 8 is a schematic drawing showing current vs. the voltage output from a novel Josephson junction voltage amplifier.

Figure 1 shows a voltage amplifier 10 according to the present invention in the form in which it was originally tested. In the prior art two Josephson Atto-Webber Switches (JAWS) have been coupled in series in the manner shown. According to one aspect of the invention, the sensing means 11 at the output comprises a one shot multivibrator 12, an amplifier 13 and a normally-open voltage sampling switch 14. These together with the JAWS device 15, constitute a high gain voltage amplifier. The operation of a typical JAWS switch occurs when a constant gate current I_g is being applied through a source resistor 19 (R_sl), and through a Josephson junction J1 which is initially in its zero voltage state. When an additional input current is applied at node 16, it passes through Josephson junction J₂ to node 18 and through the Josephson junction J_l to ground. The input current increases until a point is reached when a critical current I₀ is present in Josephson junction J_l which causes it to switch from its low impedance state to its high impedance state. Switching of J₁ causes a portion of the gate current I_g to flow through J₂ and through branch resistor 21 to ground, thus causing J₂ to switch, J₁ and J₂ being both in a high impedance state cause a major portion of the input current at node 16 to be diverted through resistor 21 to ground. Most of the gate current I_g appearing at node 18 now appears on input line 22 to node 23. In the Figure 1 embodiment, the resistors 24 and 25, which are labelled as R_inl and R_in2' were chosen to be equal, so that the current at node 23 was equally split into the two branches. The current I_in which flows through resistor 25 (R_in2) develops the voltage input V_in for the circuit tested as shown in Figure 1. The voltage input node 26 is initially at ground, since the Josephson junctions J₃ and J4 are in the low impedance state. The input source current I is being applied through the source resistor 27 (R_s2) to the first output node 28 and flows through the output resistor 29 to the node 31. Node 31 is also initially grounded, since the Josephson junction J₃ and the Josephson junction J₄ are in their low impedance states. When the input current I_in being applied at node 26 becomes sufficient to switch the Josephson junction J₃ to the high impedance state, the major portion of the current flowing through source resistor 27 and output resistor 29 to node 31 now flows through Josephson junction J₄ and resistor 32 to ground. Subsequently, the Josephson junction J₄ switches causing the source current I to flow through load resistor 33. An increase in voltage occurs on voltage output line 34 which is applied to the one shot multivibrator 12 and the voltage sampling switch 14. The multivibrator 12 generates an output signal on line 35 which is amplified at amplifier 13 and applied via line 36 to sampling switch 14 which has normally open contacts. The signal indicative of the switching of the junctions J₃ and J₄ causes the switch 14 to close and the voltage across output resistor 29 is coupled to the voltage amplifier output lines 37.

It will be understood that at the time junction J₃ and J4 switch, the current I plus the input current I_in at node 26 are equal to the critical current I₀. Further, the input current I_in at node 26 is equal to V_in/R_in2. In similar manner the source current I_s is equal to the V_out appearing across output resistor 29 divided by the value of the output resistor 29. Thus, V_out/R_out + V_in/R_in2 = I_0. Rearranging the equation and solving for V_out, it will be found that V_out = -(R_out/R_in2 V_in + R_out I₀. In this equation the latter value Rout 1₀ is a constant; thus, the amount of voltage gain which may be obtained in the novel voltage amplifier can be as much as 400 to 1 by the proper selection of the values of the resistors Rout (²⁹) and R_in2 (25).

The dash lines 38 indicate the cryogenic container in which the Josephson junction devices are placed, therefore, it will be understood that all of the elements inside the dash line 38 are maintained at cryogenic temperature. Sensing means 11 may, but need not, be inside the cryogenic container 38, and preferably are maintained outside.

Figure 2 shows the principal elements of a Josephson junction voltage amplifier according to the invention. The voltage input is applied to node 23 and develops a current across input resistor 25. Initially, node 31 is at ground potential, because the Josephson junction J₃ is in the low impedance state. The source current I is applied through the source resistor 27 and appears at the first output node 28. The current flows through the output resistor 29 and appears at the second output node 31 before flowing through Josephson junction J3 to ground. When the Josephson junction J₃ switches, it goes to its high impedance state, causing a voltage increase to appear at node 31. The voltage increase is sensed on line 34 and applied to multivibrator 12. The output of multivibrator 12 is amplified in amplifier 13 and applied via line 36 to the voltage sampling switch 14. The signal on line 36 causes the normally open contacts in switch 14 to close, and the amplified voltage output signal is coupled to output lines 37. The gain produced by the circuit shown in Figure 2 is substantially the same as that shown in Figure 1, wherein the voltage gain is equal to Rout (29)/R_in2(25).

Figure 3 shows a modified embodiment of Figure 2 for use in a sample-and-hold application. The Josephson junction device J3 inside the cryogenic container 38 is identical to that shown in Figure 2. The current source I in Figure 3 may be variable, and supplies current through the source resistor 27 and the output resistor 29 to generate voltages at the output node 28 and at node 31, which are applied to a voltage comparator 43. The voltage at node 31 on line 34 is also applied to the multivibrator 12 which senses the voltage change when J₃ switches. This generates a signal on line 39, causing the voltage reference 41 to generate output signals on lines 42, which are compared with the output voltage across resistor 29 by the voltage comparator 43. When the output voltage across resistor 29 is changing and crosses through the value of the voltage reference on output lines 42, the output voltage is generated on output lines 44. It will be understood that the voltage on lines 44 is sensed at a particular instant in time, and is not a dc voltage. If a direct voltage is desired from the output voltage comparator 43 a capacitor 45 may be placed across the lines and a dc voltage sensed at output lines 46. The formula for V ou is:-

The reference voltage on lines 42 is Rout 1₀ and this reference voltage may be eliminated from the formula, leaving -V_out as a ratio of R_out/R_in2. Voltage reference 41 cancels out the constant voltage term, R_out I_0.

Figure 4 is a current vs. voltage characteristic curve for a typical Josephson junction. When the current is increased from the zero point 47 toward the critical current point 48, there is no voltage across the Josephson junction. When the junction reaches its critical current state I₀, the voltage increases abruptly from zero to a gap voltage, shown at point 49. If the current is subsequently decreased, the voltage across the Josephson junction follows the sub gap resistance path shown as curve 51; however, if the current in the Josephson junction is subsequently increased from point 49 it follows a normal resistance path shown as curve 52. The arrows on the curves 51 and 52 indicate the direction in which the current in the junction is changing.

Figure 5 shows an input current waveform which may be used to explain the current vs. voltage characteristic curve of Figure 4. Assuming that the source current I in Figure 2 is constant and shown at line 50, the input current I_in, shown at curve 53, is also applied to node 31, and adds to the source current, so that I_in + I_s eventually equals I₀, which occurs at point 54. It is possible for I_in to increase past the critical current point 1₀ which would explain the normal resistance part 52 of the waveform shown in Figure 4. It will be understood that in the Josephson junction amplifier circuit of this invention, the maximum voltage amplification is sensed at critical time TO when the current I_in2 plus Is is just equal to I₀.

Refer now to Figure 6, which will assist in explaining the voltage gain achieved by the Josephson junction amplifiers of Figures 1 through 3. The variable input to Figure 6 is shown as voltage in V_in on the abscissa. The amplified output voltage V_out is shown on the ordinate. The ratio of voltage out to voltage in is defined as the voltage amplification gain of the Josephson junction device and it is to be understood that the gain ratio is defined by the slope of the curve 55 and is equal to -R_out/R_in2. The Josephson junction voltage amplifier may be used as an amplifier or an amplifier inverter.

Figure 7 is a schematic drawing for a current vs. voltage characteristic of a typical Josephson junction device under test. Waveform 56 shows that for a single Josephson junction device that an input current of 200 micro amperes will typically produce an output voltage of approximately 2.5 millivolts. The point 57 on curve 56 is at the critical current 1₀ level.

Figure. 8 is a schematic characteristic curve of current vs. voltage of a similar Josephson junction device as shown in Figure 7 after it has been embodied into the novel Josephson junction amplifier circuits of figures 1 to 3. Curve 58 is substantially identical in shape to curve 56 except that the voltage output has been amplified by a factor of 10. Thus, at point 59 an input current of 200 micro amps is shown producing a negative output voltage of 25 millivolts when the constant voltage term Rout 10 is cancelled out. It will be understood that the ratio of the output resistor Rout to the input resistor R_in2 defines the gain of the Josephson junction voltage amplifier and may be raised to provide gains of approximately 400 to 1. Accordingly, it is now possible to amplify the low voltage portion of curve 56 where voltages of only a few microvolts are present and to detect and resolve accurately this region.

Having explained a preferred embodiment and a modification of the present invention, it will now be understood that the voltage amplifier may be embodied into numerous circuits without modification. For example, prior art logic gates employing Josephson junction devices have required room temperature amplification devices to achieve voltage levels which are compatible with the prior art utilisation and sensing devices. Employing the present Josephson junction voltage amplifier, no external level shifting or amplification circuit is required.

As a second example, it is now possible to employ a single Josephson junction device embodied into a voltage regulator. A typical prior art Josephson junction voltage regulator consists of a string of Josephson junction devices in series with the voltage across the entire string being used as the regulated voltage. It is now possible using the present invention Josephson junction voltage amplifier to employ a single Josephson junction and multiply the input voltage to any reasonable multiple at least 400 times.

As a further example, the present invention Josephson junction voltage amplifier may be employed as a microwave power output device. Power translation is possible at the frequency of resonant portion of the device characteristic curve which suggests that precision oscillators with high power are now possible employing the present invention.

The present invention Josephson junction voltage amplifier may be employed with prior art magnetometer circuits to increase their sensitivity through voltage amplification. It would appear to be possible to multiply microvolt inputs up to millivolt signals with extreme accuracy. Since the amplification is occurring at cryogenic temperatures, the final stage amplification would be more accurate by approximately two orders of magnitude than amplifiers operating at room temperature because the thermal noise is proportional to the absolute temperature.

Employing the present invention Josephson junction voltage amplifier, it is now possible to perform basic computer logic operations in a cryogenic environment and to use the output from the cryogenic amplifier to drive room temperature peripheral devices directly without level converters and amplifiers.