JPH0575978B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Technical Field of the Invention The present invention relates to superconductor circuits, and more particularly to a superconducting current measuring device in a Josephson integrated circuit.

(2) Background of the technology Research has been carried out on Josephson devices as superconductors with the aim of applying them to ultra-high-speed computers, etc., and known device configurations include tunnel, point contact, and bridge types. . For example, in tunnel type devices in which an oxide film is sandwiched between two superconducting films of Pb and Nb, the degree of bonding is adjusted by adjusting the thickness of the oxide film. It is well known that no voltage is generated when the current flowing through this coupling portion is less than a predetermined constant value I ₀ (critical current). It is also known that the critical current exhibits a damped oscillation as shown in FIG. 1 in conjunction with an externally applied magnetic field. Generally, when measuring the current in an integrated circuit using these Josephson devices, a magnetically coupled Josephson device has been used as a magnetic sensor.

(3) Problems with the prior art For example, in order to measure the current to be measured Ix in a loop composed of Josephson elements J ₁ and J ₂ shown in Fig. 2, one input of the Josephson element J of the magnetic field coupling type is required. The magnetic sensor measures the threshold characteristics when the current to be measured Ix flows through the signal line 2 and when the current to be measured does not flow.
Obtain the voltage Vj between the terminals of the DET's magnetically coupled Josephson element, and calculate the measured current Ix from the difference between the two threshold characteristics showing the relationship between the bias current I _B and the control current I _C shown by the solid line and the broken line in Figure 3. was looking for. The above threshold characteristics are displayed using an oscilloscope, but in order to accurately obtain the waveform shown in Figure 3, it is necessary to photograph the sweep state for about 10 seconds, so an oscilloscope or camera is used as the measurement device. Not only is this necessary, but it also has the drawback of requiring a long time for measurement because it measures two threshold characteristics, ie, when the current to be measured is flowing and when it is not flowing.

(4) Purpose of the Invention The present invention was developed in view of the above-mentioned drawbacks of the conventional art, and it smoothes the switching voltage waveform of the measurement element and provides negative feedback to the first input signal line (control line) of the Josephson element, which is the measurement element. By applying this voltage, the measuring element always switches to a voltage state at one point on the threshold characteristic, and by using the fact that the feedback current at this time is the same value as the current to be measured, it is possible to measure the superconducting current with a simple device. It is an object of the present invention to provide a current measuring device for a superconducting circuit.

(5) Structure of the Invention According to the present invention, the above object is to provide a superconductor of a Josephson element having two or more input signal lines.
A current generation source driven by an oscillator is connected to the bias current terminal of the superconductor and the first input signal line, a device for obtaining an average value of voltage is connected to both ends of the superconductor, and the output of the device is A feedback current is applied to the first input signal line through the superconductor, and negative feedback is applied so that the superconducting current to be measured flowing to the second input signal line of the superconductor is canceled by the feedback current from the device. This is achieved by measuring the superconducting current to be measured using the output of the feedback system.

(6) Embodiment of the Invention Hereinafter, an embodiment of the present invention will be described in detail with reference to FIG. Note that the same parts as in FIGS. 1 to 3 are designated by the same reference numerals. FIG. 4 shows a circuit diagram of a superconductor current measuring device according to the present invention. J is a magnetic field coupling type Josephson element, and the second input signal line 2 and first input signal line 1 (control line) of the Josephson element J as a measurement element are terminals T ₁ , T ₂ , T ₃ , T ₄
Both ends of Josephson element J are connected to output terminals T ₅ and T ₆ . 4 is a cooling device, and the Josephson element J is cooled within the cooling device.
One ends of resistors R _C and R _B are connected to the terminal T ₂ of the first input signal line 1 and one end T ₁ of the Josephson element J,
The other ends of the resistors R _B and R _C are commonly connected to the oscillator 5, and the oscillator 5 and Josephson element J
One end of is grounded. As the output of the oscillator 5, one that outputs a sawtooth waveform is suitable.

The current to be measured Ix flows through the second input signal line 2,
A first amplifier A ₁ consisting of a differential amplifier is connected to both ends of Josephson element J of output terminals T ₅ and T ₆ .
The output of the first amplifier _A1 is given to a second amplifier _A2 , and the other input terminal of the second amplifier _A2 is given a reference voltage from the reference voltage Es,
The level-shifted output of the second amplifier _A2 is applied to an integrator 6. The output of the integrator is negatively fed back to the first input signal line 1 via the measurement output terminal T ₇ and the resistor R _f , and the current to be measured Ix flowing to the second input signal line 2 is changed to the negative feedback current If. Try to cancel it. In addition, in the above embodiment, the resistances R _B , R _C ,
Rf is used for voltage-current conversion, and if the oscillator 5 and integrator 6 output current, it goes without saying that the resistors R _B , R _C and Rf are unnecessary. Further, the amplifiers A1, A2 and the integrator are an example of a device for obtaining an average voltage value, and other methods may be used.

Figure 5 shows the threshold characteristics of Josephson element J and how to apply current based on the characteristics.
SQUID (super conduicting quantum
This is the threshold characteristic of a superconducting quantum interferometer (interference devices), where the vertical axis is the bias current I _B (current flowing through the resistor R _B and Josephson element J), and the horizontal axis is the control current (current flowing through the first input signal line 1). As the output of the oscillator 5, a sawtooth wave as shown in FIG. 6a is applied to the first input signal line 1 and Josephson element J. The fifth wave is caused by the sawtooth wave.
Assume that the current is applied in the first quadrant of the figure as shown by the supply current straight line 8.

Now, suppose that the current to be measured Ix flowing through the second input signal line 2 is zero, and the reference voltage Es of the second amplifier A ₂ is selected so that the output of the integrator 6 is zero (If = 0) at this time. Then, in this state, the threshold characteristic of Josephson element J is as shown by the solid line in FIG. It becomes a voltage state. When a current to be measured indicated by Ix flows through the second input signal line 2, the threshold characteristic of Josephson element J is as shown by the broken line in FIG.
On the supply current straight line 8 at this time, the Josephson element J is in a zero voltage state from the origin 0 to a point indicated by the reference numeral 10, and then becomes a voltage state. This state is shown in FIGS. 6a and 6b.
Figure 6a shows the sawtooth wave current when the horizontal axis is time and the vertical axis is the currents I _B and I _C flowing through the Josephson element J and the resistors R _B and R _C of the first input signal line 1. 6b shows the voltage across Josephson element J.
It is a waveform in which Vj is plotted on the vertical axis and time t is plotted on the horizontal axis. In FIG. 6b, when the measured current Ix flowing through the second input signal line 2 is zero, the Josephson element J is It is switched and reset at the falling edge of the sawtooth wave, and when the current to be measured Ix is applied to the second input signal line 2, it is switched at the time indicated by 10a.

When the current to be measured Ix flows, the average level decreases by the area 11 shown by the hatching in Figure 6b, so if this is inverted and applied to the first input signal line 1 through the integrator 6, The current If flowing through the Rf portion cancels the current to be measured Ix, and the Josephson element J switches at a point 9 shown in FIG.

At this time, the relationship between the current to be measured Ix and the current If fed back to the first input signal line 1 is If = Ix, and the output of the integrator 6 is If・Rf. Therefore, from this voltage value, the current to be measured Ix can be found. i.e. resistor
By measuring the voltage across Rf, it is possible to display the superconducting current to be measured If.

According to the above measuring device, Josephson element J
Negative feedback is applied so as to switch at the same point, for example, point 9, in the threshold characteristic of , and the region 11 of the waveform written by the integrator 6 is averaged, making it possible to accurately measure the superconducting current.

FIG. 7 shows another embodiment of the present invention, and similarly to FIG. 5, it shows how to apply two biases and control currents when a symmetric two-junction superconducting quantum interferometer is used. In the case shown by 12, a constant current is passed as the control current I _C , and only the bias current I _B is made into a sawtooth wave and is applied to the Josephson element J. In this case as well, the current to be measured is measured in the same way as in Fig. 4. It can be done.

In the measuring device indicated by reference numeral 13 in FIG. 7, a constant current as shown in FIG. 8a is applied as a bias current I _B while switching, and as a control current I _C to the first input signal line 1 as shown in FIG. A sawtooth waveform as shown in b is applied intermittently, and the current to be measured Ix can also be measured in this manner.

When using the device indicated by reference numeral 12 above, the seventh
The 0th mode waveform of the threshold characteristic shown in the figure and minus 1
Next mode waveform 16 and plus first mode waveform 1
If the overlapping portions 15a and 16a with 5 are large, it is unclear whether to switch in 0-order mode or in plus or minus 1st-order mode, and the measured current range cannot be large.Those with small overlaps can be used sufficiently. It is possible to do so.

Further, the one indicated by numeral 13 requires an oscillator that supplies a bias current and an oscillator that supplies a control current.
The range of the current to be measured changes depending on the magnitude of the bias current _IB , but this value can be taken relatively large. However, according to the configuration shown in FIG. 4, a relatively large current range to be measured can be obtained with a simple configuration. FIG. 9 shows the measured current range shown in FIG. 4. When the current range to be measured is positive Ix, the current supply line 8 and the peak 17 of the zero-order mode curve 14 intersect, or until the tail 1 of the negative first-order mode curve 16.
8 intersects with the current supply line 8, and the range determined by the value that intersects first is, and when the measured current -Ix is negative, the tail 19 of the zero-order mode curve 14 intersects with the current supply line 8. The range up to is the measurement range of the current to be measured.

FIG. 10 shows another embodiment of the present invention, in which the method of applying the current is different from that in _FIG . The current I _C is applied. In other words, the bias current I _B or control current I _C is as shown in Figure 11a.
A sawtooth wave current that alternates between positive and negative as shown in the figure is applied. At this time, the voltage Vj across the Josephson element J has a waveform as shown in FIG. 11b.

The Josephson element has a threshold characteristic that is symmetrical to the origin 0, so when currents I _B and I _C as shown in Figs. The device switches, and the voltage Vj across Josephson device J in FIG. 11b switches at 20a, 20b. For this reason, the voltage Vj across the Josephson element J is averaged between positive and negative voltages and becomes zero. In other words, the second offset amplifier _A2 shown in FIG. 4 is not required. Next, when the current to be measured Ix flows, the threshold value moves to the position shown by the broken line in FIG. 10, and the switch points become the positions shown by 21 and 23. At this time, in FIG. 11b, when the voltage Vj across the Josephson element J is positive, the switching time changes from 20a to 21a.
When Vj is negative, it moves from 22a to 23a and the average value of Vj becomes negative, so if this component is fed back negatively, the current to be measured Ix can be determined in the same way as the measuring device shown in FIG.

In this device, when Ix=0, If=0 can be set without adding the reference voltage Es, so the circuit can be simplified.

Figure 12 shows a further development of the configuration shown in Figure 10, in which an asymmetrical element is used for Josephson element J. In Figure 10, since it is symmetrical with respect to the apex of the threshold characteristic curve, the direction of deviation can be changed in either direction. A control current I _c is required to determine whether the deviation has occurred, but in the case shown in Fig. 12, the waveform is asymmetrical with respect to the vertex 24, so the non-measurement current _I It becomes possible to find.

(7) Effects of the Invention According to the present invention, the superconducting current can be measured on the average using a feedback system, so the superconducting current can be easily measured with a voltmeter.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the relationship between magnetic flux and current of a conventional Josephson element, Figure 2 is a circuit diagram showing a current measurement circuit of a conventional Josephson element, and Figure 3 is a diagram explaining the threshold characteristics of a SQUID. 4 is a circuit diagram of a superconductivity measuring device showing an embodiment of the present invention, and FIG. 5 is a threshold value showing the relationship between bias current, control current, and non-measurement current obtained by the circuit configuration of FIG. 4. Characteristic diagram, Figure 6a is a bias current or control current waveform diagram applied to the first input signal line and Josephson element in Figure 4, Figure 6b is a voltage waveform diagram across the Josephson element shown in Figure 4, Figure 7 The figure is a threshold characteristic diagram for explaining another way of applying the current to the first input signal line or Josephson element of the present invention, and Figure 8 is the bias current and control of the current measuring device shown at 13 in Figure 7. A current waveform diagram, FIG. 9 is a diagram for explaining the current measurement range of the current measuring device to be measured shown in FIG. 4, and FIG. 10 is a current measurement diagram of a superconductor showing still another embodiment of the present invention. A threshold characteristic diagram for explaining the device, FIG. 11a is a bias current or control current waveform diagram of the current measuring device shown in FIG. 10, and FIG. 11b is a diagram of the bias current or control current waveform of the current measuring device shown in FIG. Voltage waveform diagram, 12th
This figure is a waveform diagram showing another example in which the current measuring device shown in FIG. 10 is used for measurement using an asymmetric superconducting element. J, J ₁ , J ₂ ... Josephson element, DET... magnetic sensor, 1... control line, 2... input line, Ix... current to be measured, 4... cooling device, 5... oscillator, 6... integrator,
_A1 , _A2 ...first and second amplifiers, _IB ...bias current, _IC ...control current, If...feedback current.

Claims (1)

[Claims] 1. In a superconductor of a Josephson device having two or more input signal lines, a current generation source driven by an oscillator is connected to a bias current terminal of the superconductor and a first input signal line. A device for obtaining an average value of voltage is connected to both ends of the superconductor, and a feedback current is applied to the first input signal line through the output of the device, and a second input signal of the superconductor is connected. The superconducting current flowing through the wire is negatively fed back so as to be canceled by the feedback current from the device, and the superconducting current being measured is measured using the output of the feedback system. A current measurement device for superconducting circuits. 2. A current measuring device for a superconducting circuit according to claim 1, which uses an amplifier and an integrator as a device for obtaining the average value of the voltage.