JPH07174834A - Digital squid and measuring system using the squid - Google Patents

Abstract

PURPOSE:To provide a digital SQUID with which measurement at low noise and high precision can be carried out without error operation and the increase of the noise attributed to the cross-talk and the alteration ground potential and to provide a measurement system using the digital SQUID. CONSTITUTION:A measurement system is composed of at least a SQUID 1 driven by a DC power supply a comparator 2 to recognize the magnitude of the output voltage or the output current of the SQUID, and a read-out circuit 3 including a Josephson conjunction and driven by DC power supply to read out the output of the comparator 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital SQUID (SQ) capable of obtaining a signal magnitude with a digital output.
UID: Superconducting Quantum Interference Devic
e Superconducting quantum interference device) and a measurement system using the same, and more particularly to noise reduction, improvement in measurement accuracy, and prevention of malfunction.

[0002]

2. Description of the Related Art SQUID utilizes the property of superconductivity to
It is an element that can detect a minute magnetic field. SQU
The ID is originally a sensor that inputs an analog magnetic flux signal and outputs an analog voltage signal, but it is known to use this as a sensor that outputs a digital signal to improve its function, and these are digital SQUIDs.
is called. In the case of SQUIDs, as with many other sensor technologies, digitization facilitates signal processing and data transmission. Therefore, the digitization itself is not a technology unique to SQUID, and some specific methods have been disclosed so far.

One method of realizing a digital SQUID is disclosed in Japanese Patent Laid-Open No. 64-21379. A driving method of a multi-channel digital SQUID magnetometer having a plurality of digital SQUIDs is disclosed in, for example, Japanese Patent Laid-Open No. 3-197885.

FIG. 20 shows a conventional digital SQ.
It is a circuit diagram which shows UID. In the conventional digital SQUID, a superconducting quantum interference device that is a sensor is driven by an alternating current source 59, and a positive or negative pulse is output depending on the magnitude of the input magnetic flux. The magnitude of the magnetic flux to be measured can be obtained by counting the number of positive or negative pulses and feeding back a current proportional to the difference.

Next, the operation of this digital SQUID will be described with reference to FIG. Pickup coil 70
The signal magnetic flux detected at 0 is input to the SQUID by the input coil 220. SQUID is AC current source 5
It is biased by 9 and consists of a Josephson junction 110 and an inductance 221. In the SQUID, a positive or negative current pulse according to the input magnetic flux corresponding to the difference between the magnetic flux signal amount from the input coil 220 and the feedback magnetic flux amount is supplied to the AND gate 80 which is also driven by the alternating current source.
1. Switching circuit 8 composed of OR gate 800
It outputs to the feedback circuit via 10. The feedback circuit consists of a superconducting loop including the inductance 223 and the Josephson junction 111, and a write gate that converts the pulses sent through the switching circuit 810 into flux quanta.
This write gate consists of another inductance 222 that is magnetically coupled to the inductance 223. The pulse passing through the write gate is converted into a flux quantum and stored in the superconducting loop including the inductance 224.

The inductance 224 is coupled with the inductance 221 of the SQUID via the magnetic coupling 900, and the magnetic flux quantum stored in the superconducting loop including the inductance 224 is fed back to the SQUID. Therefore, the feedback circuit can measure the pulse output from the SQUID and return the magnetic flux quantum corresponding to the result to the SQUID.

Another method for implementing a digital SQUID is described in detail by Dr. Lang et al., In Cryogenics 1986, Vol. 26, pp. 623-627. Durang et al. Describe a method of using a comparator driven by an alternating current source in combination with an SQUID. However, no reference is made to the method of processing the output digital signal.

[0008]

In the digital SQUID disclosed by Durang et al., There is no mention of a technique for processing a digital signal output from a circuit in which the SQUID and a comparator are combined, and therefore, the digital signal is not described. Originally, there was no technical viewpoint necessary for achieving both processing and detection of a highly accurate magnetic flux signal. In the digital SQUID technology, the technology required to perform the digital signal processing and the magnetic flux signal detection at the same time is described in JP-A 64-213 described above.
No. 79. In this conventional technique, by using an alternating current source for driving the SQUID, the SQUID itself performs an operation as a comparator, and the SQUID is digitized. Therefore, it is essential to use an alternating current source. further,
A Josephson logic circuit driven by an alternating current source was used for processing the output digital signal or controlling by feedback. When configuring a digital SQUID using an SQUID driven by such an AC current source or a Josephson logic circuit, a high frequency AC current must be supplied to the circuit, and there are three main types described below. The problem arose.

The first is high frequency reflection. It is very difficult to completely match the impedance of the wiring from the current source with the impedance of the SQUID or Josephson logic circuit. Therefore, the high-frequency current supplied from the outside is reflected at the wiring portion immediately before entering the SQUID or Josephson logic circuit, and the input waveform is distorted.
This causes malfunction of the SQUID and the digital logic circuit.

The second is crosstalk. Since a high-frequency large current of several tens to several hundreds mA is applied to the wiring, an induction current is generated in another wiring in the vicinity or the SQUID itself by electromagnetic induction. The crosstalk generated between the wirings in this way causes noise. In particular, noise due to crosstalk generated in the SQUID causes deterioration of measurement accuracy, and noise transmitted to the measurement circuit system such as a feedback circuit causes malfunction of the circuit and eventually malfunction of the system.

Third is the fluctuation of the ground potential. Along with this, the generated noise similarly causes deterioration in measurement accuracy.

Like the SQUID, the Josephson logic circuit for digital signal processing is also driven by an alternating current source in the prior art, so that the above problems occur, which deteriorates the measurement accuracy and causes the system to malfunction. .

As described above, in the conventional digital SQUID, when the digital operation or the digital signal processing of the output signal is performed, noise reduction for measurement of a weak signal such as magnetic flux, improvement of measurement accuracy, and There was no perspective on system malfunction prevention, or sufficient attention was not paid.

The object of the present invention is to solve the above-mentioned problems of the prior art and to realize a low noise and high performance digital SQUID,
And to provide a measurement system using the same.

[0015]

SUMMARY OF THE INVENTION The above object is to provide an SQUI including two Josephson junctions and a superconducting ring arranged to connect the two Josephson junctions.
D and a SQUIS including a Josephson junction
The SQUID and the read circuit are configured to include at least a comparator that identifies the magnitude of the output voltage or the output current of D and a read circuit that includes a Josephson junction and that reads the output of the comparator. The circuit can be achieved by driving with a direct current source.

[0016]

When the SQUID is driven by the direct current source, the output is generally an analog voltage. In this case, SQ
An analog circuit that operates at room temperature, such as a lock-in amplifier, is required to control the UID, and these have large thermal noise as compared with the SQUID that operates at the liquid helium temperature, which causes a decrease in the sensitivity of the SQUID. On the other hand, if the output of the SQUID is digitized, the auxiliary circuit can be made of a superconducting circuit including a Josephson junction. Therefore, information processing can be performed on the same chip. In addition, it is not necessary to use the FLL (Flux Locked Loop) system that is generally used for signal feedback in the analog system. As a result, the band limited by the magnetic flux modulation frequency of the FLL can be widened, and the SQUID can have high performance and high functionality. These circumstances are exactly the same as when the SQUID is driven by using an alternating current source to digitize the SQUID or when the signal processing circuit is driven by using the Josephson junction.

However, the above-mentioned problems of high-frequency reflection, crosstalk, and ground potential fluctuation, which are problems that occur only when the measurement of a signal such as a magnetic flux by SQUID and digital signal processing are combined, are caused by the SQUID at the DC current source. , And by driving the signal readout circuit, or, as will be described in detail later, even if it occurs, its effect is reduced to 1/100 or less as compared with the prior art. be able to.

FIG. 1A shows a digital SQUID of the present invention.
3 is a block diagram showing the concept based on the representative embodiment of FIG. SQUID by inputting the magnetic flux itself or the magnetic flux generated by the current signal
The analog signal 1000 generated at 1 is input to the comparator 2 and converted into the digital signal 1100. The digital signal 1100 is transmitted to the feedback means 4, the output processing circuit, and the display circuit via the read circuit 3 driven by the direct current source 5. The effect of the present invention can be enhanced by using the feedback means 4, the output processing circuit, and the display circuit together.

The read circuit 3 has a function of latching the digital signal from the comparator 2 at a constant timing, a function of outputting the value to an external circuit, an output processing circuit, a display circuit, or the like, and further, a feedback to the SQUID 1. It has a function of outputting a signal corresponding to the amount of feedback to the feedback means 4 for controlling. Further, functions of integration, averaging, digital signal processing such as a digital filter can be provided as necessary. According to this configuration, it is possible to realize high performance and high functionality of the SQUID by utilizing the digital signal processing technology which could not be performed by the analog method, and the noise of the digital SQUID driven by the alternating current source has been a problem. It is possible to avoid an increase, deterioration of measurement accuracy, malfunction of the system, and the like.

The following is a detailed description of how the structure of the present invention works to solve the problems of the prior art.

The comparator 2, which is one of the constituent elements, may be driven by either an alternating current source or a direct current source. Because the comparator 2 has a one-to-one correspondence with SQUID1.
Even if a plurality of digital SQUIDs are provided, it is possible to reset the comparator by changing the power supply current after the measurement of each SQUID is completed. Therefore, even if an AC current source is used, if the measurement speed is sacrificed, the noise generated by the reset of the comparator 2 will not affect the magnetic flux itself by the SQUID 1 or the measurement result of the magnetic flux generated by the current signal. be able to. Therefore, even if only the comparator 2 is driven by the alternating current source, the digital SQ
Since it does not become noise of UID, it does not affect the measurement accuracy. In this way, with respect to the comparator 2, it is possible to drive the AC current source by devising the operation method, because the comparator 2 functions as a latch, stores information, and completely divides the measurement timing and the reset timing. Is. Of course, if the comparator 2 is also driven by a direct current source, it is not necessary to sacrifice the measurement speed, that is, the band, and there is a special advantage that the operation timing can be facilitated as a system. Therefore, it is clear that the object of the present invention can be achieved in this case as well.

In the digital SQUID of the present invention,
The components other than the comparator 2 shown in FIG. 1A, which are the SQUID 1 and the read circuit 3, cannot be operated separately from the timing of measuring the magnetic flux itself or the magnetic flux generated by the current signal. .
This is for the following reason. First, in the case of the comparator 2, it is only necessary to reset, so even if there are multiple comparators, it is sufficient to reset all the comparators at once.
The control can be easily realized. On the other hand, the read circuit 3 is a signal processing circuit using a Josephson logic circuit and has functions such as digital signal processing of measurement results and control of a feedback signal, but its operation is much more complicated than that of a comparator. Is. Therefore, when the read circuit 3 driven by the alternating current source is used, it is necessary to turn on and off the power supply several times during the measurement of the magnetic flux itself or the magnetic flux generated by the current signal. However, the generation of noise due to this is inevitable. Therefore, if the operation of the read circuit 3 is to be separated from the measurement timing, a large-scale logic circuit is additionally required to control this timing, and the system becomes bulky. Moreover, the signal processing capability of the circuit is significantly reduced.

Second, in order to realize the same function as the conventional FLL in the digital SQUID, the magnetic flux itself or the magnetic flux generated by the current signal is controlled by feedback while controlling the magnetic flux by the SQUID. It is necessary to repeat the measurement of itself or the magnetic flux generated by the current signal. Therefore, the read circuit 3 that outputs a signal corresponding to a predetermined feedback amount to the feedback unit 4 needs to operate at the same timing as the SQUID 1. Therefore, SQUID1 and read circuit 3
Must be operated at the same timing, and a circuit must be configured so that signal reflection, crosstalk, and noise due to fluctuations in the ground potential do not occur.

According to the invention, these two components,
That is, since the SQUID 1 and the read circuit 3 are composed of the Josephson logic circuit driven by the DC current source, noise due to the above problem does not occur. This is for the following reason.

If a direct current source is used, crosstalk due to electromagnetic induction will not occur. Furthermore, because it is a direct current source,
No reflection occurs at the impedance mismatch portion. In addition, Josephson logic circuits driven by direct current sources typically operate by switching the current to either one of two or more current paths. Therefore, the value of the current flowing from the power supply to the ground does not change even when the logic circuit is switched, that is, the current is switched from one path to the other, and is constant.

On the other hand, in the Josephson logic circuit driven by the AC current source, the impedance of the Josephson junction largely changes before and after the switching, so that the value of the current flowing to the ground changes with the switching. This causes noise. Moreover, since the power supply current is an alternating current, the value of the current flowing to the ground fluctuates greatly. Therefore, the ground potential is generated depending on the wiring or the value of the inductance existing in the ground plane. Therefore, in order to improve the measurement accuracy, it is necessary to lower the frequency of the alternating current source to suppress the generation of noise due to electromagnetic induction, and as a result, the measurement band is limited. The problems caused by using these Josephson logic circuits driven by an alternating current source can be easily solved by the present invention. In particular, since it is not necessary to consider the fluctuation of the ground potential in the measurement band, it is possible to raise it to about several GHz, which cannot be realized by the conventional technique.

When a plurality of digital SQUIDs are provided, a multiplexer can be used as a part of the read circuit, and one or a plurality of processing circuits are commonly used for the plurality of digital SQUIDs to configure the system. Therefore, the magnetic flux itself at a plurality of locations or the magnetic flux generated by the current signal can be measured at the same time, and the commonality can reduce the circuit scale and downsize the system.

In order to utilize the idea of the present invention, SQUI
It is most desirable to monolithically integrate the D, the comparator, and the readout circuit on the same chip, all in a cryogenic environment. As a result, external noise such as noise from a circuit placed at room temperature can be reduced and the wiring between these components can be shortened, so that the drive frequency can be increased and the measurement speed can be increased.

In order to make full use of the idea of the present invention, at least one of the comparator and the read circuit is further provided with
It is desirable to provide reset means for returning the read circuit switched according to the output signal of the SQUID to the state before the switch. Normally, the output signal of the SQUID is very weak, and the change in this value is detected by the comparator. When operating the read circuit driven by the DC power supply without using the reset means, it is necessary to perform both the switch operation and the reset operation of the read circuit by the output signal of the SQUID. The switch operation can be realized by superimposing a constant bias current on the SQUID signal, but it is difficult to reliably perform resetting because the output signal of the SQUID is weak. Therefore, it is extremely desirable to use the reset means together in order to utilize the idea of the present invention. In this way, by combining the conventional SQUID and Josephson logic circuit alone, a digital SQUID can be obtained.
D does not work perfectly. As described above, the digital SQUID realized by the present invention can provide an effect that cannot be obtained only by combining the conventional SQUID and the Josephson logic circuit. In order to make this fact clearer, a numerical value is shown how the digital SQUID of the present invention enables low noise and, as a result, highly sensitive measurement.

The sensitivity characteristic of the digital SQUID according to the present invention is shown in FIG. This figure shows SQUID alone, S
The magnitude of noise in the combination of the QUID and the comparator, the combination of the SQUID, the comparator, and the readout circuit is shown in comparison with the prior art. According to the present invention, the sensitivity is increased 20 times or more as compared with the prior art, and
Since it can be set to 10 ⁶ (Φ ₀ / √Hz) or less, extremely high-sensitivity measurement can be realized, for example, it becomes possible to detect a magnetic field generated by the brain, which cannot be measured by the conventional technique.

As described above, according to the configuration of the present invention, it is possible to realize a low noise and high performance digital SQUID and a measurement system using the same.

[0032]

【Example】

Example 1 An example according to the present invention will be described below. Figure 2
7 to 7 are circuit diagrams of the SQUID 1 and the comparator 2 within the dotted frame of FIG. 1 showing the principle configuration of the present invention. In this embodiment, in addition to the SQUID 1 in the configuration diagram shown in FIG. 1, the comparator 2 and the reading circuit 3 are constituted by a circuit including a Josephson junction element driven by a direct current source. The comparator 2 operates as a 1-bit analog-digital converter that digitizes the output signal from the SQUID 1. The readout circuit 3 generates a feedback signal according to the digital output from the comparator 2 and sends it to the feedback means 4. The increase / decrease in the feedback amount is determined by the read circuit 3. Also,
The reading circuit 3 can also perform digital signal processing such as signal integration and digital filter processing. When it is desired to set the timing of magnetic flux measurement by a clock or the like, this can be realized by including the latch function synchronized with the clock in the reading circuit 3. The same applies to the other examples described below.

2 to 7 are circuit configuration diagrams in the case where the output of the SQUID is digitized by using a comparator driven by a direct current source. This comparator uses a Josephson quantum interferometer and is composed of a flip-flop type Josephson circuit driven by a direct current source. FIG. 2 is a circuit diagram showing a Josephson quantum interferometer which is a constituent element of this comparator. In the circuit diagram showing the embodiment of the present invention, this Josephson quantum interferometer is shown as shown in FIG. To do.

In FIG. 4, the comparator 2000 is
Driven by the direct current source 5, at least two Josephson quantum interferometers 150 connected in series and two load resistors 401 and 402 connected in series are connected in parallel, and It is configured to include a circuit in which an intermediate contact is connected by an inductance 204, and by inputting a signal to two Josephson quantum interferometers 150,
Put one of the Josephson quantum interferometers in the voltage state,
It is a flip-flop that operates by putting the other into a superconducting state. The direct current source 52 is the comparator 2
It is provided to set 000 operating points. Further, the direct current source 51 is used as a reset means, and a current signal is caused to flow from this power source to invert the output of the comparator into a current having an opposite sign, thereby resetting the reading circuit. In this embodiment, the reset means is provided in the comparator, but it may be provided in the reading circuit.

The SQUID is driven by the direct current source 5 and is composed of at least a Josephson junction 101, an inductance 203 and a resistor 400. In the SQUID of this embodiment, the inductance 203 and the capacitance of the Josephson junction 101 are designed to be bilaterally symmetrical so that the voltage step due to LC resonance can be accurately removed by the damping resistor. Although not shown in the figure, the change in the output voltage of the SQUID caused by the change in the magnetic flux signal input to the SQUID from the magnetic flux input means becomes a change in the current via the resistor 400.
The current flowing from the signal input terminal 307 to the signal input terminal 308 changes. The change in the current is magnetically coupled to the Josephson quantum interferometer 150, which is a constituent element of the comparator, through the inductance, whereby a signal is input to the comparator, and as a result, the change in the magnetic flux signal is converted into a digital signal. It is output to the output terminal 310. Since both the SQUID and the comparator are composed of superconductors, they could be manufactured on the same chip by the usual process technology of an integrated circuit using Nb for the superconductors. Similarly, among the constituent elements shown in FIG. 1, the SQUID 1, the comparator 2, and the readout circuit 3 may be manufactured on the same chip,
Further, the feedback unit 4 and the magnetic flux input unit (not shown) may be formed on the same chip.

By thus driving the SQUID, the comparator, and the readout circuit with a direct current source, it is possible to eliminate malfunctions by reducing crosstalk as compared with a digital SQUID driven with an alternating current source. Digital SQ with similar function driven by AC current source
When realized by UID, comparator and
Assuming that the number of gates of the signal reading circuit is 40, the AC current of the power supply reaches about 20 mA. On the other hand, in the present invention, the AC current is only the clock signal required when the measurement timing is designated from the outside, and its value is as small as about 0.1 mA. Therefore, in this embodiment, the fluctuation of the ground potential caused by the inductance of the wiring to the ground via the ground terminal 309 can be suppressed to about 1/100 or less, and the noise can be reduced.

Further, the DC current source driven comparator constituted by using the Josephson quantum interferometer has a circuit diagram shown in a broken line in FIG. 4 and circuit diagrams shown in FIGS. 5 and 6. It is also possible to use a device having a different configuration. FIG. 5 is configured to include a circuit in which two current paths are provided by connecting two series-connected bodies of the Josephson quantum interferometer 151 and the inductance 205 in parallel, and a signal input terminal 307, or , 308 from the signal input to the Josephson quantum interferometer, the comparator is configured by using a flip-flop that operates by switching the current to one of the two current paths. FIG. 6 includes a circuit in which two Josephson quantum interferometers 151 are connected in series and the series connection body is connected to a constant voltage source composed of a constant current source 5 and a Josephson junction 102. Terminal 3 for
07 or 308 Josephson quantum interferometer 1
It is a comparator configured by using a flip-flop that operates by inputting a signal to 51 to put one of the Josephson quantum interferometers in a voltage state and the other in a superconducting state. It is obvious that the same effect can be obtained by using any of these types of comparators.

The value of the threshold current at which the comparator discriminates the input current is determined by the value of the direct current supplied from the direct current sources 51 and 52. Therefore, if the value of this DC current is kept constant, the comparator operates as a 1-bit analog-digital converter as described above. On the other hand, when the above DC current is changed stepwise, the comparator discriminates the input current with respect to two or more threshold currents, and is used substantially as a 2-bit analog-digital converter. be able to. Since the comparator itself is driven by the DC current source, the DC current source corresponding to the bias is changed to change the threshold current while keeping the noise small, and one comparator is used as a multi-bit analog-digital converter. can do. This is an advantage that can be realized without being limited to this embodiment.

In addition, in FIG. 1, the read circuit 3 is configured to include a circuit for increasing the accuracy by integration in addition to the data latch function. FIG. 7 shows an example of a register circuit composed of a Josephson quantum interferometer having a latch function, which operates with a direct current source.

In the magnetic flux meter whose circuit diagram is shown in FIG. 8, the output from the SQUID is input to the comparator driven by the direct current source by magnetic coupling as in FIGS. A low pass filter composed of a resistor 400 is inserted. With this circuit configuration, the Josephson noise generated by the AC Josephson effect from the SQUID Josephson junction 101 can be cut by this low-pass filter to further reduce the noise and improve the measurement accuracy. .

(Embodiment 2) Another embodiment according to the present invention will be described below. FIG. 9 is a circuit diagram when a comparator of an alternating current source is used. Actually, as shown in FIG.
It has the configuration shown in. Output voltage of SQUID is resistance 4
00 causes a current component, and the Josephson junction 10
It is injected as a signal current into the comparator constituted by 3 and the inductance 207. SQUI
When the total value of the signal current from D and the current injected by the AC bias from the AC current source 59 is larger than the maximum superconducting current of the comparator which is determined depending on the critical current values of the four Josephson junctions 103. The comparator switches to a voltage state and, if small, maintains a superconducting state to convert the analog voltage into a digital signal. In this embodiment, an AC current source is used to drive the comparator, but if the comparator is reset after the measurement is completed, the noise associated with the reset does not affect the measurement accuracy. Therefore, also in the digital SQUID of the present invention, by driving the SQUID and the reading circuit with the DC current source, the noise of the magnetometer can be reduced.

In this embodiment, the comparator is a resistor 400.
It is a current injection type that is switched by the current injected through the. In addition to this, it goes without saying that a magnetic coupling type comparator that switches by the magnetic flux generated by the current flowing through the resistor 400 may be used. Also in this case, if the comparator is reset after the measurement is completed, the noise accompanying the reset does not affect the measurement accuracy.

Further, as shown in FIG. 10, in order to prevent the reverse current from flowing from the comparator to the SQUID by the voltage generated when the comparator is switched from the superconducting state to the voltage state, a backflow preventing means is provided. The same effect can be obtained by providing the Josephson junction 104 between the comparator and the SQUID. In this case,
There is an effect of preventing unnecessary noise from being transmitted to the feedback means magnetically coupled to the SQUID by the current flowing back from the comparator, thereby preventing normal feedback operation.

(Embodiment 3) Another embodiment of the present invention will be described below. FIG. 11 is a principle block diagram showing another embodiment of the present invention. In this embodiment, SQUID 1 driven by a direct current source
Reference numeral 1 also serves as a comparator so that a digital output can be directly obtained from the SQUID. In the SQUID of this embodiment, as indicated by point A in FIG. 12A, a current having a value slightly smaller than the critical current value of the SQUID is supplied as the bias current of the SQUID. Therefore, if magnetic flux is input to the SQUID, the superconducting state indicated by the point A in FIG.
Transition to the voltage state indicated by the dot. If the value of the bias current is reduced, the transition does not occur unless the magnetic flux signal has a large change, and thus the magnitude of the magnetic flux signal can be discriminated. It can be converted into a digital signal by associating the presence or absence of a voltage with the logic 1 and 0.

In this embodiment, the feedback means 41 using a superconducting coil is used as the feedback means. This superconducting coil is magnetically coupled to SQUID 11. Digital signal 1 corresponding to the amount of feedback output from the read circuit 3
According to 400, a current signal is sent from the digital-analog converter 6 to the superconducting coil. The magnetic flux generated by the superconducting coil is coupled to the SQUID 11 and the feedback signal 120
0 is transmitted to SQUID1. FIG. 12B is a circuit diagram of the digital-analog converter 6. The digital-analog converter 6 includes a SQUID 11 and a read circuit 3
It can be driven by a DC current source as well. Therefore, the noise can be reduced and the object of the present invention can be achieved.

(Embodiment 4) FIG. 13 shows a principle block diagram showing another embodiment of the present invention. In FIG. 13, SQUID
A magnetic coupling 900 using an inductance is used for inputting a magnetic flux from 12 to the comparator 21. FIG. 14 shows S
2 as a means to convert the output of the QUID into a digital signal
The circuit diagram at the time of using the bit analog-digital converter is shown. The voltage generated in the SQUID 12 is converted into a current by using the ladder resistor 409, and the current components in the current are input to the two comparators 21 to form a 2-bit analog-digital converter as a whole. The SQUID resistance 408 inserts a value equal to the combined resistance of the ladder resistance 409.

Also in the case of this embodiment, since the SQUID, the analog-digital converter, and the readout circuit are all driven by a direct current source, crosstalk and fluctuations in the ground potential can be prevented, and noise can be reduced. . Also in this embodiment, the feedback means 41 using the superconducting coil is used.
The configuration of the digital-analog converter 6 may be similar to that of the third embodiment. Also in this embodiment, it was possible to reduce noise and achieve the object of the present invention.

(Embodiment 5) FIG. 15 is a principle configuration diagram in a case where a digital SQUID magnetometer system having a plurality of digital SQUID magnetometers of the present invention is constituted by using a multiplexer driven by a direct current source as a read circuit. is there. One comparator 21 corresponds to each SQUID 12. The digital signal from each comparator is sequentially switched by the multiplexer 8 and converted into serial data. Since the number of signal wirings can be reduced to one pair in this part, it is possible to prevent the intrusion of heat by using the data converted into the serial signal for the signal transmission from the cryogenic environment in which the SQUID operates to the room temperature. This has the effect of reducing the burden on the refrigerator. The transmitted serial signal is returned to the parallel signal by the demultiplexer 9, and the data is transferred to the output processing and display circuit, respectively. On the other hand, the feedback signal to the SQUID is stored in the memory 7. This memory 7 determines the magnitude of the signal output from each comparator from SQUID1 to SQUID.
It is stored in association with D n. Based on the magnitude of the signal stored in the memory 7, each SQ is passed through the demultiplexer 9 via the feedback means 42 using a superconducting loop.
Returned to UID.

The superconducting loop includes a Josephson junction element or a Josephson quantum interferometer. A magnetic flux is stored in this superconducting loop by the output signal from the memory 7, and this magnetic flux is stored in SQUID1 to SQUID
Control by feedback by coupling to QUIND n. The demultiplexer 9 is a switch formed by arranging a plurality of Josephson quantum interferometers. By controlling the maximum superconducting current of the Josephson quantum interferometers, the demultiplexer 9 is fed back from only the desired SQUID. I was able to switch the signal.

FIG. 16 shows an SQUI corresponding to a 2-channel digital SQUID magnetometer system according to this embodiment.
The circuit diagram of D1, the comparator 2, and the multiplexer 8 is shown. In this circuit, all are driven by a direct current source. Although not shown in the drawing, the read circuit is also composed of a Josephson logic circuit driven by a direct current source. The output signal from the SQUID 1 is converted into a digital signal by the comparator 2 and input to the flip-flop type multiplexer 8 configured by using four Josephson quantum interferometers 152. The multiplexer 8 is driven by the DC current source 57, and can output either one of the two input data input via the signal input wirings 600 and 601 by the selection signal 55. Signal 56 is the reset means of the readout circuit. When all the circuits such as the SQUID, the comparator, and the readout circuit are driven by the direct current source as in the present embodiment, it is possible to prevent the fluctuation of the ground potential and the crosstalk between the wirings, and to reset the comparator. Since it is possible to prevent the generation of accompanying noise, SQUI
It is possible to achieve low noise and wide band of the D magnetometer.

In the above embodiment, the case where the Josephson circuit driven by the flip-flop type direct current source shown in the broken line of FIG. 4 is used, but in addition to this, FIGS. It is needless to say that the same effect can be obtained by using the Josephson circuit shown in the broken line.

When a measurement system having two or more channels is constructed, the multiplexers may be assembled in the required number of stages. As an example of the configuration of such a measurement system with two or more channels, FIG. 17 shows the configuration of the digital SQUID measurement system according to the present invention. In this example, the magnetic flux 3008 generated from the brain is measured. Thermal insulation container 3 made of resin containing liquid helium 3007
002 includes a detection coil 3001, a digital SQUI
A D chip 3000 is provided. Although not shown in the figure, the digital SQUID chip 3000 monolithically integrates SQUIDs, comparators, and read circuits for a plurality of channels. In this case as well, all the signals can be stored digitally and the measurement sensitivity is excellent, so there is an effect that the signals from the brain can be detected with high accuracy.

(Embodiment 6) FIG. 18 is an SQUI which is an apparatus for measuring the magnetic distribution of a weakly magnetized sample.
The structure of a D microscope is shown. In a heat insulating container 3002 made of resin containing liquid helium 3007,
Detection coil 3001, digital SQUID chip 300
0 is set. Although not shown in the figure, the digital SQUID chip 3000 monolithically integrates the SQUID, the comparator and the reading circuit. A sample 3005 magnetized with a distribution is also placed on a movable base 3006 made of resin. The movable table 3006 moves two-dimensionally in a plane.

The magnetic flux from the sample 3005 is measured while moving the movable table 3006 in small steps. The digital SQUID chip 300 is added to the relay circuit 3003 placed at room temperature.
Digital signals from 0 are accumulated and displayed as an image on the display device 3004. Thereby, the two-dimensional magnetic flux distribution of the sample 3005 can be measured. In this embodiment, the movable table 3006 is provided near the bottom surface of the heat insulating container 3002, but it goes without saying that it may be provided inside the heat insulating container 3002. The digital SQUID measurement system of this embodiment can be used, for example, for nondestructive inspection of materials, but in this case as well, since all signals can be accumulated digitally and the measurement sensitivity is excellent, material cracking, corrosion, etc. There is an effect that the defect can be detected with high accuracy.

(Embodiment 7) FIG. 19 shows the configuration of a digital SQUID measuring system in another embodiment of the present invention. Here, a window 3010 made of quartz glass is provided on a part of the wall surface of the heat insulating container 3002, and a high frequency electromagnetic field signal 3011 incident from here is received by an antenna 3012, and the signal is guided to a digital SQUID 3000 chip. Although not shown in the figure, the digital SQUID chip 3000 monolithically integrates the SQUID, the comparator and the reading circuit. The digital signal from the digital SQUID chip 3000 is stored in the relay circuit 3003 placed at room temperature and displayed on the display device 3004. As a result, the high frequency electromagnetic field signal 3011 can be detected with high sensitivity. Digital SQUID of this embodiment
The measurement system can be used, for example, to detect a high frequency weak signal. In this case as well, all the signals can be stored digitally and the measurement sensitivity is excellent, so that the signals can be detected with high accuracy. In this embodiment, the antenna 3
Although the signal is received using 012, if a bolometer is used instead of the antenna 3012, it is possible to detect a weak signal of a signal in the infrared region with high sensitivity.

[0056]

As described in detail above, the SQUID for driving the DC current source and the comparator for driving the AC or DC current source are combined to digitize the output signal, and the signal is read by the read circuit for driving the DC current source. By configuring the digital SQUID so as to output, it is possible to prevent the malfunction of the circuit due to the reflection of the high frequency, the increase of the noise due to the crosstalk and the fluctuation of the ground potential, and the low noise and high precision digital SQUID, And the measurement system using this can be provided.

[Brief description of drawings]

FIG. 1A is a principle configuration diagram of the present invention. (B) A characteristic diagram showing an improvement in sensitivity due to noise reduction in the present invention.

FIG. 2 is a circuit diagram of a Josephson quantum interferometer.

FIG. 3 is a circuit symbol diagram of a Josephson quantum interferometer.

FIG. 4 is a circuit diagram when a comparator driven by a direct current source is used.

FIG. 5 is a diagram showing a modification of a comparator driven by a direct current source.

FIG. 6 is a diagram showing a modification of a comparator driven by a direct current source.

FIG. 7 is a circuit diagram of a register for driving a direct current source.

FIG. 8 is a circuit diagram in the case of having a low-pass filter.

FIG. 9 is a circuit diagram when a comparator driven by an alternating current source is used.

FIG. 10 is a circuit diagram when a backflow prevention unit is included.

FIG. 11 is a principle configurational diagram showing another embodiment of the present invention.

12 (a) is a biasing method of the SQUID of the magnetometer shown in FIG. FIG. 3B is a circuit diagram of a digital-analog converter driven by a direct current source.

FIG. 13 is a principle configurational diagram showing another embodiment of the present invention.

FIG. 14 is a circuit diagram when a 2-bit analog-digital converter driven by a direct current source is used.

FIG. 15 is a principle configurational diagram showing another embodiment of the present invention.

FIG. 16 is a circuit diagram when a multiplexer driven by a direct current source is used in a read circuit.

FIG. 17 is a configuration diagram of a brain magnetic field measuring apparatus according to the present invention.

FIG. 18 is a block diagram of an SQUID microscope according to the present invention.

FIG. 19 is a configuration diagram of a high-frequency electromagnetic field detection device according to the present invention.

FIG. 20: Conventional AC current source driven digital SQUID
It is a circuit diagram of.

[Explanation of symbols]

1, 11, 12 ... SQUID, 2, 21 ... Comparator, 3 ... Read-out circuit, 4 ... Feedback means, 41 ... Feedback means using superconducting coil, 42 ... Feedback means using superconducting loop, 5, 51, 52, 53, 54, 55, 56, 57 ... DC current source, 59 ... AC current source, 6 ... Digital-analog converter, 7 ... Memory, 8 ... Readout circuit using multiplexer, 9 ... Readout circuit using demultiplexer, 10 , 320 ... Output processing circuit, terminals to display circuit, 100, 101, 102, 103, 104, 110, 1
11 ... Josephson junction, 200, 201, 202, 203, 204, 205, 2
06,207,221,222,223,224 ... Inductance, 220 ... Input coil, 150, 151, 152 ... Josephson quantum interferometer (J
I), 303, 304, 307, 308 ... Signal input terminal, 301, 302, 305, 306 ... Power supply current terminal, 309 ... Ground terminal, 310 ... Signal output terminal, 400, 401, 402, 403, 404, 405, 4
06, 407, 408 ... Resistance, 409 ... Ladder resistance, 500 ... Capacitance, 600, 601 ... Wiring for signal input, 700 ... Pickup coil, 800 ... OR gate, 801, AND gate, 810 ... Switching circuit, 900 ... Magnetic coupling, 1000 ... Analog signal input from SQUID to comparator, 1100 ... Digital signal input to read circuit from comparator, 1200 ... Feedback signal, 1300 ... Digital signal input to read circuit from SQUID, 1400 ... Digital signals input from the readout circuit to the digital-analog converter, 2000, 2001, 2002 ... Comparator using Josephson quantum interferometer, 3000 ... Digital SQUID chip, 3001 ... Detection coil, 3002 ... Adiabatic container, 3003 Relay circuit, 3004 ... display device, 3005 ... Sample, 3006 ... movable table, 3007 ... liquid helium, the magnetic flux generated from the 3008 ... brain, windows made of 3010 ... quartz glass, 3011 ... high-frequency electromagnetic field signals, 3012 ... antenna.

Claims (58)

[Claims] 1. An SQUID including two Josephson junctions and a superconducting ring arranged by connecting the two Josephson junctions, and an output voltage or output of the SQUID including the Josephson junctions. At least a comparator for identifying the magnitude of current and a read circuit configured to include a Josephson junction for reading the output of the comparator are included, and the SQUID and the read circuit are DC current sources. Digital SQUI characterized by being driven
D. 2. The digital SQUID according to claim 1, wherein the digital SQUID is configured to include a magnetic flux input means to the SQUID. 3. A feedback means for feeding back a control signal to the SQUID according to the output signal of the read circuit or the output signal of the comparator, according to claim 1 or 2. A digital SQUID. 4. The digital SQUID according to claim 1, wherein the SQUID, the comparator, and the readout circuit are integrated on the same chip. 5. The reset means according to claim 1, wherein at least one of the comparator and the read circuit returns the read circuit switched in accordance with the output signal of the SQUID to a state before switching. Digital S characterized by having
QUID. 6. The reset means according to claim 5,
A digital S, which is a signal generating means for shifting the output signal of the SQUID by a constant value.
QUID. 7. The signal generating means according to claim 6,
A digital SQUID, which is a wiring provided in the comparator, in which a current inputted through the wiring is added or subtracted from an output current of the SQUID. 8. The signal generating means according to claim 6,
The magnetic flux signal generated by the current input through the wiring provided in the comparator is the SQUID.
The digital SQUID is characterized in that the output current is generated and added to or subtracted from the magnetic flux signal applied to the comparator. 9. The reset means according to claim 5,
A digital SQUID, which is a signal generating means for causing a current having a sign opposite to that of the output current of the SQUID when the comparator is switched by the output signal of the SQUID to flow to the comparator. 10. The digital SQUID according to claim 5, wherein the reset means is signal generation means for shifting the output signal of the comparator by a constant value. 11. The signal generating means according to claim 10, wherein the signal generating means is a wiring provided in the reading circuit, and the current input through the wiring is added or subtracted from the output current of the comparator. A digital SQUID characterized by that. 12. The signal generating means according to claim 10, wherein the signal generating means is a wiring provided in the reading circuit, and a magnetic flux signal applied to the reading circuit due to a current input through the wiring is generated by the comparator. A digital SQUID characterized by being wired so as to be added to or subtracted from a magnetic flux signal generated by an output current and applied to the read circuit. 13. The resetting means according to claim 5, wherein the reset means is a signal generating means for causing a current having a sign opposite to that of the output current of the comparator when the comparator is switched by the output signal of the SQUID to the read circuit. A digital SQUID characterized by that. 14. The Josephson junction according to claim 1, wherein the two Josephson junctions included in the SQUID have substantially the same critical current value and exist on the left and right with respect to the injection point of the bias current. The digital SQUID, wherein the superconducting wirings forming the superconducting ring have substantially the same inductance value. 15. The digital SQUID according to claim 1, wherein the SQUID also has a function of the comparator. 16. The digital SQUID according to any one of claims 1 to 14, wherein the comparator is provided separately from the SQUID. 17. The digital S according to claim 16, wherein the comparator is driven by an alternating current source.
QUID. 18. The digital S according to claim 16, wherein the comparator is driven by a direct current source.
QUID. 19. The power supply applied to the comparator according to claim 17 or 18, wherein the current value of a direct current source increases and the maximum value of the current increases stepwise in an alternating current source. A digital SQUID that is supplied from a current source and can be regarded as a DC current source between one increase and the next increase in a DC current source. 20. The digital SQ according to any one of claims 16 to 19, wherein two or more comparators are present and constitute an analog-digital converter having a resolution of 2 bits or more as a whole.
UID. 21. The comparator according to claim 1, wherein the comparator is a flip-flop type Josephson circuit driven by a direct current source using a Josephson quantum interferometer. A digital SQUID that is characterized. 22. The Josephson circuit according to claim 21, wherein a series connection body of at least two Josephson quantum interferometers and a series connection body of two load resistors are connected in parallel to each other, and It is configured to include a circuit in which the intermediate contacts of the series connection body are connected by an inductance, and a signal input to the Josephson quantum interferometer causes one of the Josephson quantum interferometers to be in a voltage state and the other to be in a superconducting state. A digital SQUID, which is a flip-flop that operates by: 23. The Josephson circuit according to claim 21, wherein the Josephson circuit includes a circuit in which two Josephson quantum interferometers are connected in series, and the series connection body is connected to a constant voltage source. A digital SQUID, which is a flip-flop operated by putting one of the Josephson quantum interferometers into a voltage state and the other into a superconducting state by a signal input to the quantum interferometer. 24. The Josephson circuit according to claim 21, wherein the Josephson circuit includes a circuit in which two current paths are provided by connecting two series connection bodies of a Josephson quantum interferometer and an inductance in parallel. A digital S, which is a flip-flop that operates by switching a current to either one of the two current paths by a signal input to the Josephson quantum interferometer.
QUID. 25. The comparator according to any one of claims 1 to 14 and 16 to 24, wherein:
A digital SQUID, which operates by directly injecting an output current of a UID into a part of a superconducting wire forming the comparator, a Josephson junction forming the comparator, or the Josephson quantum interferometer. 26. The digital SQUID according to claim 25, wherein a low-pass filter constituted by a resistor, a capacitor, or an inductance is inserted between the SQUID and the comparator. 27. The backflow prevention means according to any one of claims 25 to 26, wherein the backflow prevention means is provided to prevent a current from flowing back from the comparator to the SQUID after the output of the comparator transits to a voltage state. A digital SQUID. 28. The digital SQUID according to claim 27, wherein the backflow prevention means includes at least a Josephson junction provided in a part of a wiring connecting the comparator and the SQUID. 29. The digital S according to any one of claims 1 to 14 and 16 to 24, wherein the SQUID and the comparator are magnetically coupled.
QUID. 30. A low-pass filter formed by a resistor, a capacitor, or an inductance is inserted between an inductance portion of the SQUID magnetically coupled to the comparator and a Josephson junction included in the SQUID. A digital SQUID that is characterized. 31. The S according to claim 29 or 30,
The digital SQUID is characterized in that there is no component constituted by a superconductor such as a ground plane or a wiring for driving, between the QUID and the comparator, except for a wiring for connecting the both. 32. The feedback means for returning a control signal to the SQUID according to claim 3, wherein at least the superconducting loop and a Josephson junction provided in a part of the superconducting loop are provided. A digital SQUID characterized in that it is configured to include. 33. The return means according to claim 32,
A digital SQUID characterized in that it is configured to return a control signal to the SQUID by a magnetic field generated by a permanent current flowing in the superconducting loop which constitutes the SQUID. 34. The digital SQUID according to claim 3, wherein the feedback means for feeding back the control signal to the SQUID includes at least a superconducting coil. 35. The return means according to claim 34,
By the current flowing in the superconducting coil that constitutes this,
A digital SQUID, characterized in that it is configured to provide feedback of a control signal to the SQUID. 36. The read circuit according to claim 1, wherein the readout circuit is a flip-flop type Josephson circuit driven by a direct current source using a Josephson quantum interferometer. A digital SQUID that is characterized. 37. The Josephson circuit according to claim 36, wherein a series connection body of at least two Josephson quantum interferometers and a series connection body of two load resistors are connected in parallel to each other, and It is configured to include a circuit in which an intermediate contact of a connection body is connected by an inductance, and by inputting a signal to the Josephson quantum interferometer, one of the Josephson quantum interferometers is brought into a voltage state and the other is brought into a superconducting state. A digital SQUID characterized in that it is a flip-flop that operates according to the above. 38. The Josephson circuit according to claim 36, wherein the Josephson circuit includes a circuit in which two Josephson quantum interferometers are connected in series, and the series connection body is connected to a constant voltage source. A digital SQUID, which is a flip-flop operated by putting one of the Josephson quantum interferometers into a voltage state and the other into a superconducting state by a signal input to the quantum interferometer. 39. The Josephson circuit according to claim 36, wherein the Josephson circuit includes a circuit in which two current paths are provided by connecting two series connection bodies of a Josephson quantum interferometer and an inductance in parallel. A digital S, which is a flip-flop that operates by switching a current to either one of the two current paths by a signal input to the Josephson quantum interferometer.
QUID. 40. A digital SQUID measuring system according to any one of claims 1 to 39, wherein a plurality of said digital SQUIDs are included. 41. A feedback means for feeding back a control signal to the SQUID in response to an output signal of the readout circuit or an output signal of the comparator, according to claim 40. Digital SQUID measuring system. 42. The digital SQ according to claim 41.
A digital SQUID measuring system, wherein at least a part of the feedback unit of the UID excluding the superconducting loop or the superconducting coil is shared among a plurality of the digital SQUIDs. 43. A memory circuit according to claim 41 or 42, further comprising a memory circuit for storing an amount corresponding to a feedback amount returned to the superconducting coil or the superconducting loop by the feedback means of the digital SQUID. Digital SQUID measuring system. 44. The flip-flop type Josephson circuit driven by a direct current source, comprising at least a Josephson quantum interferometer according to claim 41, wherein said feedback means comprises at least a Josephson quantum interferometer. A digital SQUID measurement system characterized by being configured. 45. The Josephson circuit according to claim 44, wherein a series connection body of at least two Josephson quantum interferometers and a series connection body of two load resistors are connected in parallel to each other, and It is configured to include a circuit in which an intermediate contact of a connection body is connected by an inductance, and by inputting a signal to the Josephson quantum interferometer, one of the Josephson quantum interferometers is brought into a voltage state and the other is brought into a superconducting state. A digital SQUID measurement system characterized by being a flip-flop that operates according to the above. 46. The Josephson circuit according to claim 44, wherein the Josephson circuit includes a circuit in which two Josephson quantum interferometers are connected in series, and the series connection body is connected to a constant voltage source. A digital SQUID measuring system, which is a flip-flop that operates by putting one of the Josephson quantum interferometers into a voltage state and the other into a superconducting state by a signal input to the quantum interferometer. 47. The Josephson circuit according to claim 44, wherein the Josephson circuit includes a circuit in which two current paths are provided by connecting two series connection bodies of a Josephson quantum interferometer and an inductance in parallel. A digital S, which is a flip-flop that operates by switching a current to either one of the two current paths by a signal input to the Josephson quantum interferometer.
QUID measurement system. 48. A control signal according to any one of claims 40 to 47, wherein a control signal is sent to said SQUID via a circuit for converting an output signal of each digital SQUID read circuit into serial data and said feedback means. A digital SQUID measuring system, characterized in that when returning, the serial data is converted into parallel data and returned. 49. A flip-flop type circuit driven by a direct current source, wherein a Josephson quantum interferometer is used as a means for converting the output signal of the readout circuit into the serial data. A digital SQUID measurement system using a multiplexer configured by using the. 50. The multiplexer according to claim 49, wherein at least two series connection bodies of Josephson quantum interferometers and two load resistance series connection bodies are connected in parallel, and the two series connection bodies are connected. By including a circuit in which the intermediate contacts of are connected by an inductance, by inputting a signal to the Josephson quantum interferometer, one of the Josephson quantum interferometers is brought into a voltage state and the other is brought into a superconducting state. A digital SQUID measurement system characterized by being a flip-flop that operates. 51. The multiplexer of claim 49, wherein the multiplexer connects two Josephson quantum interferometers in series,
It is configured to include a circuit in which this series connection body is connected to a constant voltage source, and a signal input to the Josephson quantum interferometer causes one of the Josephson quantum interferometers to be in a voltage state and the other to be in a superconducting state. Digital S characterized by being a flip-flop that operates by
QUID measurement system. 52. The multiplexer according to claim 49, wherein the multiplexer includes a circuit in which two current paths are provided by connecting two series connection bodies of a Josephson quantum interferometer and an inductance in parallel. A digital SQU, which is a flip-flop that operates by switching a current to either one of the above two current paths by a signal input to a Son quantum interferometer.
ID measurement system. 53. In any one of claims 40 to 52, the SQUID and the ground plane of the comparator are arranged as separate superconductors which are not superconductingly connected in a one-to-one correspondence. A digital SQUID measurement system characterized in that 54. The SQUID, the comparator, and the multiplexer according to claim 40, wherein the SQUID, the comparator, and the multiplexer are installed in a cold bath, and the data output from each digital SQUID is serialized by the multiplexer. A digital SQUID measurement system characterized by converting to data and outputting to a circuit on the room temperature side. 55. An SQUID including two Josephson junctions and a superconducting ring arranged by connecting the two Josephson junctions, a magnetic flux input means to the SQUID, and a Josephson junction. A read circuit for reading the magnitude of the magnetic flux or the output of the comparator, which includes a Josephson junction, and a comparator for identifying the magnitude of the output voltage or output current of the SQUID A feedback means for feeding back a control signal to the SQUID in response to the output signal of the SQUID or the output of the comparator.
The magnetic flux input means, the comparator, the readout circuit, and the feedback means are integrated on the same chip, and the SQUID and the readout circuit are driven by a direct current source. SQ
UID. 56. An SQUID including two Josephson junctions and a superconducting ring arranged by connecting the two Josephson junctions, and an output voltage or output of the SQUID including Josephson junctions. A comparator for identifying the magnitude of the current, and a read circuit for reading the magnitude of the magnetic flux or the output of the comparator, which is configured to include a Josephson junction,
The read circuit is configured to include at least a plurality of current paths, and is configured using a flip-flop type logic circuit that operates by switching the current path through which the current flows, and a current flowing to the ground. Is configured to be a constant amount, a digital SQUID. 57. A SQUID including two Josephson junctions and a superconducting ring arranged by connecting the two Josephson junctions, and an output voltage or output of the SQUID including the Josephson junctions. A comparator for identifying the magnitude of the current, and a read circuit for reading the magnitude of the magnetic flux or the output of the comparator, which is configured to include a Josephson junction,
In the digital SQUID configured to include at least, a circuit for inputting a digital signal and outputting a digital signal, and a circuit for inputting an analog signal and outputting an analog signal are driven by a direct current source, digital SQUID . 58. An SQUID configured to include two Josephson junctions and a superconducting ring arranged to connect the two Josephson junctions, and an output voltage or output of the SQUID configured to include the Josephson junctions. A comparator for identifying the magnitude of the current, and a read circuit for reading the magnitude of the magnetic flux or the output of the comparator, which is configured to include a Josephson junction,
A measurement system using a digital SQUID, which is configured to include at least the SQUID and the read circuit are driven by a direct current source.