The present invention relates to a Josephson signal detector having a sensor with at least one Josephson junction, e.g., a digital SQUID (i.e., Superconducting Quantum Interference Device), capable of sensing with a digital output the magnitude of a signal, and a measurement system using the digital SQUID; and, more particularly, to a reduction of noise, an improvement in measurement accuracy and a prevention of miss operation of such Josephson signal detector (digital SQUID).
The present invention also relates to a Josephson signal detector comprising: a sensor constructed to include at least one Josephson junction; and a comparator constructed to include at least one Josephson junction and capable of discriminating the output voltage or output current of said sensor, wherein the Josephson oscillation generated by the Josephson junction of said sensor at the operating time of said sensor is filtered out in further processing of the output of the sensor. More particularly, this aspect of the present invention relates to a Josephson signal detector, which is stabilized in operation and facilitated to improve measurement accuracy, and to a measurement device which is constructed to include the Josephson signal detector.
The SQUID is an element capable of detecting a micro-magnetic field by making use of the superconducting property. The SQUID is intrinsically a sensor for inputting an analog magnetic flux signal to output an analog voltage signal, but is used as a sensor for outputting a digital signal to improve the function, as is well known as the "digital SQUID" in the art. Like many other sensor techniques, the digital SQUID can facilitate signal processing and data transmission. Therefore, the digitization per se is not intrinsic to the SQUID, but there have been disclosed several specific methods to provide a digital signal from the SQUID.
One method of realizing the digital SQUID is disclosed in Japanese Patent Laid-Open No. 21379/1989. On the other hand, a method of driving a multi-channel digital SQUID magnetic flux device having a plurality of digital SQUIDs is disclosed in Japanese Patent Laid-Open No. 197885/1991, for example.
FIG. 20 is a circuit diagram showing a digital SQUID in the prior art. In the digital SQUID in the prior art, the sensor or a superconducting quantum interference device is powered by an AC current source 59 so that positive or negative pulses are outputted depending upon the magnitude of the magnetic flux inputted. The magnitude of the magnetic flux to be measured can be determined by counting the number of the positive or negative pulses and by feeding back a current proportional to the difference.
Here will be described the operation of the digital SQUID, with reference to FIG. 20. The signal magnetic flux detected by a pickup coil 700 is inputted by an input coil 220 to the SQUID 485. This SQUID is biased by an AC current source 59 and is composed of two Josephson junctions 110 and an inductance 221. The SQUID feeds back the positive or negative current pulses responding to an input magnetic flux corresponding to the difference between the amount of magnetic flux signal coming from the input coil 220 and the amount of feedback magnetic flux, to the feedback circuit 475 through a switching current 810 which is also powered by the AC current source and composed of an AND gate 801 and OR gates 800. The feedback circuit is composed of a superconducting loop having an inductance 223, two Josephson junctions 111, and a write gate for converting the pulses fed through the switching circuit 810 into a magnetic flux quantum. This write gate is composed of another inductance 222 magnetically coupled to the inductance 223. The pulses having passed through the write gate are converted into a magnetic flux quantum and stored in a superconducting loop composed of an inductance 224.
The inductance 224 is coupled to the inductance 221 of the SQUID through a magnetic coupling 900 (each of reference characters 900 represents a magnetic coupling), and the magnetic flux quantum stored in the superconducting loop containing the inductance 224 is fed back to the SQUID. As a result, the feedback circuit can measure the pulses outputted from the SQUID and can feed back the magnetic flux quantity according to the result to the SQUID. In FIG. 20, reference character 320 is the terminal to the output processing circuit and the display circuit, where appropriate. An output signal 499 is passed to these circuits for further processing and display, respectively.
Another method of realizing a digital SQUID is described in detail on pages 623 to 627 of CRYOGENICS, 1986, Vol. 26, by Drung. Drung has described a method of using a comparator powered by an AC current source in combination with the SQUID.
In a comparator, as constructed to use a Josephson junction, whether the Josephson junction is in the superconducting state or in the normal conducting state corresponds to whether an input signal is present or absent. The comparator itself may be used as a sensor, or a sensor constructed of a Josephson junction may be arranged as a front stage, before the comparator. In case a SQUID is arranged at the front stage, for example, there arises an advantage that the SQUID can have its analog voltage signal converted by the comparator into a digital signal to facilitate the processing of signals and the transfer of data. This is described in detail in the aforementioned article on pages 623-627 of CRYOGENICS, 1986, Vol. 26, by Drung.
A method of combining a SQUID 1 and a comparator 2 is shown in FIG. 35. This comparator 2 is constructed to receive a signal from the SQUID 1 by a magnetic coupling 100', thereby to detect the presence of the input signal. The comparator 2, as disclosed in the prior art, is constructed to include two Josephson junctions 32', a superconducting wiring connecting the Josephson junctions 32' for forming a loop containing an inductance 92', and an AC (alternating current) current source 7'. The Josephson junction 32' has a superconducting critical current of 30 .mu.A and an electrostatic capacity of 0.45 pF, and the inductance 92' has a value of 17 pH. In response to the signal from the SQUID, the comparator changes (or switches) from the superconducting state having a zero voltage to the voltage state having a finite voltage. The signal current, which is determined by a voltage established in the comparator and a load resistor, flows out as the output signal from the comparator.
In the digital SQUID disclosed by Drung, there is no reference to a technique of processing the digital signal outputted from the combination circuit of the SQUID and the comparator. Hence, there is not intrinsically present the technical view necessary for making the digital signal processing and the highly accurate magnetic flux signal detection compatible. In the technique of the digital SQUID, the technique necessary for simultaneously effecting the aforementioned digital signal processing and magnetic flux signal detection is disclosed in the foregoing Japanese Patent Laid-Open No. 21379/1989. In this prior art, the SQUID itself is caused to execute the operation as the comparator by using the AC current source for the SQUID. Thus, it is essential to use an AC current source. Moreover, a Josephson logic circuit to be powered by the AC current source is used for processing the digital signal outputted or for controlling by the feedback. In case the digital SQUID is to be constructed by using either the SQUID powered by the AC current source or the Josephson logic circuit, the circuit has to be fed with an AC current having a high frequency, thus raising the following three kinds of problems.
The first problem is the reflection of radio-frequency waves. It is very difficult to make a complete coincidence between the impedance of the wiring from the current source and the impedance of the SQUID or the Josephson logic circuit. Because of this difficulty, the radio-frequency current fed from the outside is reflected at the wiring portion immediately upstream of the SQUID or the Josephson logic circuit so that the input waveforms are collapsed. This causes a miss operation in the SQUID or the digital logic circuit.
The second problem is crosstalk. Since a radio-frequency current as high as several tens to hundreds mA is fed to the wiring, an induction current is established in another peripheral wiring or in the SQUID itself, by the induction. The crosstalk thus established between the wirings makes a noise. Especially, noise due to the crosstalk established in the SQUID causes deterioration of the measurement accuracy, and the noise transmitted to the measurement circuit system such as the feedback circuit causes a miss operation in the circuit, and, still worse, in the system.
The third problem is the fluctuation of the ground potential. The resultant noise likewise causes deterioration of the measurement accuracy.
Like the SQUID, the Josephson logic circuit for the digital signal processing is powered by an AC current source in the prior art, so that the above-specified problems occur to cause deterioration of measurement accuracy and miss operation of the system.
Thus, in the digital SQUID of the prior art, neither a view point is present, nor is there paid sufficient care, to reduction of the noise for-measuring a weak signal such as the magnetic flux, improvement in measurement accuracy and prevention of miss operation of the system, in the case of the digital operation or in the digital signal processing of the output signal.
Moreover, in prior proposals having its SQUID arranged at the front stage, before the comparator, this comparator directly detects the output voltage, which is generated by the Josephson junction contained in the SQUID, by converting it into an output current through a resistor. FIG. 36 is a diagram schematically illustrating the change in the characteristics in case a magnetic flux signal is inputted to the SQUID. The current-voltage characteristics of the SQUID continuously change from n.PHI..sub.o to (n+0.5).PHI..sub.o, as shown. If, therefore, the bias current of the SQUID is selected, as shown in FIG. 36, the operating voltage range of the SQUID is determined, as shown. In FIG. 36, 500' and 501' respectively represent the operating voltage range and the bias current.
At this time, an AC current due to the AC Josephson effect is superposed on an output current of the SQUID. FIG. 37 illustrates one example of the relation between the magnitude of an input magnetic flux and the current converted from the output voltage of the SQUID by a resistor, in case the input magnetic flux of the SQUID increases in proportion to time. In FIG. 37, a solid curve plots the output current of an actual SQUID on which is superposed an AC current due to the AC Josephson effect. A broken curve plots the current which is converted by a resistor from the output voltage of the SQUID after having been passed through a low-pass filter and which is the output of the SQUID usually observed on an oscilloscope. The frequency of the AC current due to the AC Josephson effect takes a value which is determined by subtracting the output voltage of the SQUID by a magnetic flux quantum .PHI..sub.o (2/10.sup.15 Wb), and normally ranges from about 100 MHz to about 50 GHz. This AC current has its peak value modulated due to the electrostatic capacitance and the inductance contained in the SQUID by another lower frequency than that of the AC Josephson effect, as indicated by the solid curve of FIG. 37, so that it will not always change monotonously with respect to the output voltage of the SQUID.
The output of the SQUID to be observed by the oscilloscope or the like does not contain the AC current due to such AC Josephson effects. This is partly because the AC current attenuates due to the stray capacitance and the inductance existing in the wiring from the SQUID placed in a cryogenic state, to the oscilloscope placed at room temperature, and partly because the measurement band of the oscilloscope falls short of such high frequency. If, however, a comparator using the Josephson junction is arranged in the vicinity of the SQUID, the AC current due to the AC Josephson effect reaches the comparator without any attenuation. Moreover, the Josephson junction contained in the comparator can be switched at a high speed. If, therefore, the comparator is manufactured without any special consideration, as in prior proposals, it switches in response to the AC current due to the AC Josephson effect.
Thus, in prior proposals in which the SQUID is arranged at the front stage of the comparator, the comparator is constructed without any special consideration of the influences of the AC current due to the AC Josephson effect upon the measurement accuracy and the stability of the operations of the device, and the signal containing the AC current due to the AC Josephson effect is read out by the comparator. In prior proposals, therefore, there is a problem that the value of the output voltage of the SQUID read out by the comparator usually fails to correspond to the output voltage of the SQUID observed by the oscilloscope.
More specifically, the following two points belong to this problem. Firstly, the peak value or waveform of the AC current is not timewise steady but changes. This raises a problem that the output of the SQUID read out by the comparator seriously disperses to deteriorate measurement accuracy.
Moreover, the amplitude of the AC current is not proportional to a DC (direct current) component of the output voltage of the SQUID but may have its magnitude increased, as the case may be, to ten times as high as the DC component of the output voltage of the SQUID. This raises another problem that the linearity between the output of the comparator and the input magnetic flux to the SQUID is deteriorated, to make the measurement itself difficult and to make the circuit operation unstable.
Here has been described the problem in the case in which a SQUID is arranged at the front stage, before the comparator. This problem is common among all the comparators that receive a signal from a sensor composed to a Josephson junction. Thus, in prior proposals neither a view point is present upon, nor is taken a sufficient care of, the drop of measurement accuracy of the comparator or the prevention of the unstable operation of the device, which is caused by the AC current due to the AC Josephson effect.