Multi-channel squid fluxmeter

Abstract: In a multi-channel SQUID fluxmeter for detecting magnetic flux signals by use of an array of SQUID flux sensors (1) each comprising a digital SQUID for providing a pulse output, multiplexers (4) and the like are appropriately used to allow pulse signals output from the SQUID flux sensors to be switched on a time series basis in order to obtain a magnetic flux signal for each channel. Thus, only one processing circuit (2) need be provided for a large number of SQUID flux sensors, thus reducing the scale of the fluxmeter and making the device small.

Description: The present invention relates to a multi-channel SQUID (superconducting quantum interferometer device) fluxmeter for detecting magnetic flux signals on the basis of pulse signals from a plurality of SQUID flux sensors. High-sensitivity fluxmetersusing superconducting quantum interferometer devices (SQUIDs) are employed for measuring small magnetic fields generated by an object under examination, such as a living body. By measuring the distribution of magnetic fields in the brain or the heart, for example, it is possible to estimate the currents which generate them. It is pointed out, from the diagnostic point of view, that the estimation of currents provides very meaningful information and is useful for clarifying the neural activity within a living body. Magnetic fields may be measured with a one-channel SQUID fluxmeter which is adapted to sequentially measure time series data from each part. However, this method takes a long time and the person under examination gets tired. In addition, since the simultaneous measurement of magnetic fields cannot be made in different parts, the currents cannot be estimated with great accuracy. A multi-channel SQUID fluxmeter is thus required, which permits simultaneous measurement of magnetic fields from different portions of the organ under examination. In a conventional multi-channel SQUID fluxmeter, a processing circuit for converting a detected signal to a flux signal is provided for each one-channel flux sensor. Thus, if, for example, a 100-channel SQUID fluxmeter is to be realized, 100 processing circuits are needed. In analog SQUIDs, a method has also been proposed for simplifying the SQUID system by driving each of the SQUIDs with a different frequency and multiplexing the output lines of the SQUIDs to be sent over a single output line (Furukawa et al., Japanese Journal of Applied Physics, vol. 28, No. 3, March, 989, pp L456- L458). This method, however, requires as many phase detector circuits for demodulation as there are channels. Therefore, problems with the conventional device are its increased circuit scale and its increased power consumption. Some digital SQUIDs are adapted to provide pulse outputs. One type consists of a two-junction quantum interferometer device and is A.C. biased to output a pulse output (refer to Japanese Unexamined Patent Publication No. 63-290979). Another is a D.C. SQUID which applies a voltage to a superconducting comparator or a 1-bit A/D converter to thereby provide a pulse output (D. Drung, Cryogenics, vol. 26, pp 623 - 627, 1986). Journal of Physics E: Scientific instruments, vol. 17, 1984, pages 504-512 discloses a multi-channel SQUID fluxmeter having the features of the preamble of each accompanying independent claim. This fluxmeter employs analog SQUIDS whose signals are processed by sample-and hold circuits and integrators. A respective sample-and hold circuit and integrator are provided for every SQUID. EP-A-0 111 826 discloses another multi-channel SQUID fluxmeter employing analog SQUIDs. In this case a single set of analog processing circuits is provided for all the SQUIDs. According to a first aspect of the present invention, there is provided a multi-channel SQUID fluxmeter comprising: a plurality of SQUID flux sensors for detecting weak magnetic flux emanating from different portions of an object under examination, each sensor including a SQUID for producing an output signal; a multiplexer to switch the output signals from said plurality of sensors to extract each of said output signals on a time series basis; processing circuit means for processing the extracted output signal to determine the intensity of magnetic flux detected by the selected sensor and comprising feedback means for applying a feedback signal to the selected sensor so that said output signal of said sensor becomes a predetermined value; characterised in that each of said SQUIDS is a digital SQUID flux sensor for outputting a pulse signal made up of positive or negative pulses depending on the direction of magnetic flux being detected; in that said processing circuit means is provided in common for all the sensors, and includes a counter for counting the pulses in the extracted pulse signal; and in that a memory means is arranged to store the value counted by the counter in a respective storage area for the selected sensor, said value representing the intensity of magnetic flux detected by the sensor in terms of the magnitude of said feedback signal; and in that it is provided with means to preset said value in the counter. According to a second aspect of the present invention, there is provided a multi-channel SQUID fluxmeter comprising: a plurality of SQUID flux sensors for detecting weak magnetic flux emanating from different portions of an object under examination, each sensor including a SQUID (13) for producing an output signal; a multiplexer to switch the output signals from said plurality of sensors to extract each of said output signals on a time series basis; and feedback means for applying a feedback signal to the selected sensor so that said output signal of said sensor becomes a predetermined value; characterised in that each of said SQUIDs is a digital SQUID flux sensor for outputting a pulse signal made up of positive or negative pulses depending on the direction of magnetic flux being detected; in that a counter is provided in common for all the sensors for counting the pulses in the extracted pulse signal; in that a memory means is arranged to store the value counted by the counter in a respective storage area for the selected sensor, said value representing the intensity of magnetic flux detected by the sensor; and in that said feedback means is a superconducting feedback circuit, provided for each of said sensors, which stores magnetic flux quanta corresponding to the pulse signal output from the respective sensor, said feedback signal corresponding to said magnetic flux quanta. An embodiment of the present invention may provide a multi-channel SQUID fluxmeter which enables measurement of pulse signals from an array of a plurality of digital SQUID flux sensors by use of at least one processing circuit, and can thereby measure small magnetic fields from different portions of an object under examination at substantially the same time, said fluxmeter having a reduced circuit scale. Reference is made, by way of example, to the accompanying drawings in which:- Figure 1 illustrates the principal arrangement of the present invention; Figure 2 illustrates the principle of operation of the present invention; Figure 3 illustrates an arrangement of a first embodiment of the present invention; Figure 4 is a diagram for use in explaining the operation of the first embodiment of the present invention; Figure 5 illustrates a specific arrangement of a control circuit in Figure 3; Figure 6 is a timing chart illustrating the principal operation of the control circuit of Figure 5; Figure 7 illustrates an arrangement of a second embodiment of the present invention; Figure 8 illustrates an arrangement of a third embodiment of the present invention; and Figure 9 illustrates a specific arrangement of the principal portion of the third embodiment. Figure 1 illustrates the principal structure of the present invention. In Figure 1, SQUID flux sensors 1 (S1 to Sn), each provided for a respective one of channels 1 to n, detect small magnetic fluxes from different portions of an object under examination such as a living body and are of a digital type which provide pulse outputs. A processing circuit 2 receives switched pulse signals output from SQUID flux sensors to process them in turn, and applies a feedback signal to a corresponding respective SQUID flux sensor 1 so that the pulse signals may have a predetermined magnitude. Switching the pulse signals output from SQUID flux sensors 1 to take out them on a time series basis may be made in a time divisional manner by means of a multiplexer 4 controlled by a control circuit 3, which sequentially takes out the pulse signals output from a plurality of SQUIDs, as illustrated in Figure 1. Alternatively, though not shown, bias currents applied to the respective digital SQUIDs in plurality of SQUID flux sensors 1 may be switched on a time series basis by a multiplexer. This switching refers to sequentially taking out the pulse signals associated with S.1 (channel 1) to S.n (channel n) of SQUID flux sensors 1 for application to processing circuit 2 as illustrated in (1) of Figure 2. The process of switching the pulse signals on a time series basis may also be repeated in plural cycles (t1, t2, ...) and a signal representing the intensity of magnetic flux may be obtained from processing circuit 2 at each repetition. A memory 5 may be provided which stores a flux-intensity representing a signal (feedback signal) obtained by processing circuit 2 for each of channels 1 to n. Where such a memory 5 is provided, it is also possible, as illustrated in (2) to (5) of Figure 2, to store a flux-intensity-representing-signal in each of its storage areas corresponding to a respective individual channel in each of cycles t1, t2, in a sequential switching operation. The processing circuit 2 need not be the sole such circuit, but may be one of a plurality (fewer in number than the channels). In this case, plural SQUID flux sensors 1 may be divided into a plurality of groups, the number of which is equal to that of the processing circuits, so that pulse signals output from SQUID flux sensors in each group are multiplexed for application to respective individual processing circuits. In the present invention, as illustrated in Figures 1 and 2, processing circuit 2, which receives pulse signals output from SQUID flux sensors 1 on a time series basis and takes each of them in, applies a feedback signal to respective corresponding SQUID flux sensors 1 so that the pulse signals have predetermined magnitudes (for example, zero) and measures the intensity of corresponding magnetic fields in terms of the magnitudes of feedback signals. Therefore, to measure field intensity, one or several processing circuits 2 have only to be provided for a large number of SQUID flux sensors 1, thereby enabling simultaneous measurement of small magnetic fields in a living body and reducing the scale of the fluxmeter circuit, thus making the device small. Figure 3 illustrates a first embodiment of the present invention. In Figure 3, n SQUID flux sensors 1, each employing a digital SQUID (superconducting quantum interferometer device) adapted to provide pulse outputs, are provided for channels 1 to n. More specifically, each of SQUID flux sensors 1 includes a pickup coil 11 for detecting a magnetic field in an object under examination such as a living body, a feedback coil 12 responsive to a feedback signal from processing circuit 2 to generate a magnetic field and a digital SQUID 13 formed of a Josephson junction. SQUID flux sensors 1 are arrayed in a matrix over or near the object so as to simultaneously detect magnetic flux radiated from different portions of the object (e.g. a human heart). In SQUID flux sensors 1 configured as above, digital SQUIDs 13 are always supplied with an A.C. bias current from a constant current source 6. When, in this state, magnetic flux radiated from the object is applied to digital SQUID 13 through pickup coil 11, a pulse signal of a polarity (positive or negative) corresponding to the direction (positive or negative) of the magnetic flux is output in a cycle corresponding to that of the A.C. bias current. The magnetic flux can be detected in accordance with the presence or absence of the pulse signals (refer to Japanese Unexamined Patent Publication No. 63-290979). Each of preamplifiers 7 is connected to its respective individual SQUID flux sensor 1 to amplify positive or negative pulse signals output from digital SQUID 13. A multiplexer 4-1 sequentially selects a pulse signal from positive and negative pulse signals obtained from preamplifiers 7 for respective channels and applies the pulse signal to processing circuit 2. Processing circuit 2 counts the pulse signals from multiplexer 4-1 and applies a feedback signal to each of SQUID flux sensors 1 so that pulse signals from it may become, for example, zero. The intensity of magnetic flux is measured in terms of the magnitude of the feedback signal. Specifically, processing circuit 2 is comprised of a counter 21 and a D/A converter 22. Counter 21 is an up-down counter which, for example, counts up when a pulse signal from multiplexer 4-1 is positive and counts down when the pulse signal is negative. D/A converter 22 converts a digital value (for example, a value corresponding to a difference in number between positive pulses and negative pulses obtained within a predetermined length of time) obtained by counter 21 to an analog value. A multiplexer 4-2 operates synchronously with multiplexer 4-1 to apply an analog value from D/A converter 22 to feedback coil 12 of a corresponding one of SQUID flux sensors 1. Feedback coil 12 is adapted to generate magnetic flux of a magnitude corresponding to that of an analog feedback signal output from D/A converter 22. This magnetic flux is applied to digital SQUID 13 in a direction to cancel out externally applied magnetic flux. Feedback control is thereby carried out so that the number of pulses input to counter 21 becomes zero. Memory 5 stores a signal representing the intensity of magnetic flux measured by processing circuit 2, i.e., a count value of counter 21, for each of channels 1 to n. Control circuit 3 is adapted to apply switching signals to multiplexers 4-1 and 4-2, control addressing of memory 5 in storing counts of counter 21 and presets a predetermined value (a value stored in memory 5 in the previous cycle) in counter 21. Next, the operation of the present embodiment will be described with reference to Figures 2 and 4. First, the basic operation of the present embodiment, or the process of switching pulse signals by means of multiplexer 4-1 and the process of storing counted values of counter 21 in memory 5 will be described below with reference to Fig. 2. As depicted in (1) of Figure 2, pulse signals from S.1 (channel 1) to S.n (channel n) of SQUID flux sensors 1 are sequentially switched by multiplexer 4-1 to be entered into processing circuit 21 and the entered pulse signals are counted by counter 21. This switching process is repeated for a plurality of cycles (t1, t2, ...). As illustrated in (2) to (5) of Figure 2, on the other hand, the count in counter 21 for each of the channels is stored in its corresponding respective storage area of memory 5 as magnetic flux data (a signal representing the intensity of magnetic flux) in each of cycles t1, t2, ... That is, the storage area in memory 5 which corresponds to channel 1 stores flux signals S1(t1), S2(t2)... from S1 of SQUID flux sensors 1 in corresponding respective cycles t1, t2,.., as shown in (2) of Figure 2. The storage area in memory 5 which corresponds to channel 2 stores flux signals S2(t1), S2(t2),... from S2 of SQUID flux sensors 1 in corresponding respective cycles t1, t2,... as shown in (3) of Figure 2. The storage area in memory 5 which corresponds to channel 3 stores flux signals S3(t1), S3(t2),... from S3 of SQUID flux sensors 1 in corresponding respective cycles t1, t2,...as shown in (4) of Figure 2. Likewise, the storage area in memory 5 which corresponds to channel n stores flux signals Sn(t1), Sn(t2),... from Sn of SQUID flux sensors 1 in corresponding respective cycles t1, t2,..as shown in (5) of Figure 2. Subsequently, the more specific operation associated with a channel i (i= 1 - n) will be described below with reference to Figure 4. In this Figure, the abscissa represents time, while the ordinate represents the magnitude of feedback signal and the intensity of input magnetic flux. First, at time (1), a feedback loop for channel 1 extending from SQUID flux sensor 1 through preamplifier 7, multiplexer 4-1, counter 21, D/A converter 22 and multiplexer 4-2 to feedback coil 12 is closed to initiate feedback control. This feedback control is performed only for a predetermined time period indicated at (2), during which time counter 21 counts up with positive pulses from SQUID flux sensor 1 and counts down with negative pulses. After the predetermined time period indicated by (2) has elapsed, at time (3) ((A)), the counter value of counter 21 is stored as magnetic flux data in the storage area of memory 5 corresponding to channel i. Subsequently, during the time period indicated by (4), the counting process and data storing process are also performed for other channels as in the case of channel i. After the measurement of magnetic flux for all of channels 1 to n has been terminated in that way, the flux is measured again for channel i during the next cycle. In this case, at time indicated at (5) ((B)), the flux signal data (the count of counter 21) which has been stored in memory 5 in the previous cycle is read from memory 5 and then preset in counter 21. At the same time, the feedback loop for channel i is closed so that counter 21 initiates its counting operation, as was performed at time (1). After the counting operation is performed, a predetermined time period indicated by (6), at time (7) ((C)), the count value of counter 21 is stored as magnetic flux data in the corresponding storage area to channel i in memory 5. The above operation of presetting counter 21 and storing data is repeated for each of channels 1 to n to measure the intensity of magnetic flux for each of channels 1 to n every cycle. Thus, the magnetic flux signal data corresponding to the intensity of input magnetic flux can be obtained for each channel and every cycle. A specific configuration of control circuit 3 for realizing the above operation and a timing chart associated with its principal operation are illustrated in Figs. 5 and 6, respectively. Basically, the control by control circuit 3 can be entirely executed by a microcomputer 31 as shown in Fig. 5. In this configuration, microcomputer 31 is subjected to forced hardware interruption by an interval counter 32 every time a channel is selected. When microcomputer 31 is interrupted, a program begins to run so that, in the beginning, channel select data is output onto a port A and multiplexer 4-1 selects a channel in response to the data. A multiplexer adapted for channel selection usually includes a decoder. To turn eight switches on or off one at a time, therefore, only log₂ 8 = 3 signal lines have to be provided. Next, a preset command is output onto a port D and preset data (namely, the count of one cycle before) is output onto a port C so that counter 21 is preset with the data. Thereafter, the count of counter 21 at a desired point of time is entered into microcomputer 31 via port B to be stored in its memory (corresponding to memory 5). When microcomputer 31 is subsequently interrupted, the next channel is selected for the same processing. According to the present embodiment, as described above, only one processing circuit 2 can sequentially receive signals from SQUID flux sensors 1 of a large number of channels to measure the intensity of magnetic flux for each channel. Therefore, small magnetic fields from the object can be measured in the channels at substantially the same time and one processing circuit 2 suffices for a large number of SQUID flux sensors 1, thereby reducing the circuit scale and enabling the device to be made small. Moreover, in the present embodiment, counter 21 is preset with the count obtained in the previous cycle for each channel and initiates its counting operation using the preset value as an initial value. That is, as illustrated in Figure 4, a count initial value (a count at the point of time indicated by (B) in Figure 4) of counter 21 becomes equal to the count (the count at the point of time indicated by (A) in Figure 4) obtained in the previous cycle for the same channel. Thus, a difference between the actual intensity of magnetic flux at the point of time when counter 21 starts its counting operation and the intensity of magnetic flux corresponding to a count initial value can be made very small. Therefore, as seen from the transit from (B) to (C) in Figure 4, the count after counter 21 starts its count operation, or the magnitude of a feedback signal can be made to rapidly follow a value corresponding to the actual intensity of magnetic flux. Figure 7 illustrates a second embodiment of the present invention. This embodiment is arranged such that a multiplexer 4-3 is connected between current source 6 and SQUID flux sensors 1 to sequentially supply an A.C. bias current to SQUIDs 13 in one of the SQUID flux sensors 1 and sequentially generated pulse signals are entered into counter 21. The switching timing of multiplexer 4-3 may be the same as that of multiplexer 4-1 of Figure 3 in the first embodiment described above. In other respects, the second embodiment is the same as the first embodiment so the description thereof is unnecessary. In the present embodiment as well, the intensity of magnetic flux is measured for each channel on a time series basis, and the count obtained in the previous cycle is preset into counter 21 for each channel , thus providing the same advantages as the first embodiment. Next, Figure 8 illustrates a third embodiment of the present invention, and Figure 9 illustrates a specific arrangement of the principal portion of the third embodiment. As illustrated in Figure 8, the third embodiment is the same as the second embodiment shown in Figure 7 in that any one of SQUID flux sensors 1 may be selected by multiplexer 4-3, but is mainly distinct from the second embodiment in that a superconducting feedback circuit 8 comprised of a write gate 81 and a superconducting inductance 82 is provided for each channel instead of the feedback circuits in Figure 7. As illustrated specifically in Figure 9, such a superconducting feedback circuit 8 stores magnetic flux quanta corresponding to the number of pulse signals output from digital SQUID 13 of SQUID flux sensor 1 in superconducting inductance 82 by use of write gate 81 and thereby applies magnetic flux to digital SQUID 13 so that the number of pulse signals becomes a predetermined number (for example, zero). The circuit provides the same function as the feedback circuit comprised of counter 21 and D/A converter 22 shown in Figures 3 and 7. This type of superconducting feedback circuit is described in detail, for example, in "A Single-Chip SQUID Magnetometer", N.Fujimaki et al., IEEE Trans. Electron Device, vol. 35, no. 12 pp 2412-2418, 1988 and Japanese Patent Publication No. 63-290979. With superconducting feedback circuit 8, a digital quantity of feedback cannot be obtained and thus it cannot be stored in memory 5 as it is with an intensity of magnetic flux. For this reason, the present embodiment is arranged to amplify pulse signals from SQUID flux sensors 1 which are subjected to feedback by superconducting feedback circuits 8 by preamplifiers 7, and then count the number of pulses only on a channel selected by multiplexer 4-1 synchronized with multiplexer 4-3 with counter 21 so as to store its count in memory 5. The third embodiment requires as many superconducting feedback circuits 8 as there are channels. As compared with counter 21 and D/A converter 22, however, write gate 81 and superconducting inductance 82 can be configured on a reduced scale by using a Josephson circuit (they can be configured on a chip of, for example, a few mm x 10 mm together with SQUIDs) and consume less power. Therefore, the third embodiment can be of smaller scale and lower power consumption than the first and second embodiments. In addition, the function of presetting counter 21 as used in the first and second embodiments is not required because superconducting inductance 82 retains the feedback quantity as it is unless a new pulse is written into it. In the above embodiments, pulse signals output from SQUID flux sensors 1 associated with channels 1 to n are sequentially switched to be entered into single counter 21 for sequential measurement of the intensity of magnetic flux in each channel. Alternatively, modifications may be made such that a plurality of counters 21 are provided and channels are divided into as many groups as there are counters so as to make simultaneous measurement of magnetic flux in the groups. According to the present invention, as described above, signals output from a plurality of SQUID flux sensors are entered into at least one or a plurality of processing circuits on a time series basis to measure magnetic flux. Accordingly, only one or a few processing circuits are needed for a large number of SQUID flux sensors and small magnetic fields in an object under examination such as a living body can be measured at substantially the same time and a circuit scale can be reduced to make a device small.

Claims: A multi-channel SQUID fluxmeter comprising: a plurality of SQUID flux sensors (1) for detecting weak magnetic flux emanating from different portions of an object under examination, each sensor including a SQUID (13) for producing an output signal (S1, S2, S3,...); a multiplexer (4; 4-1; 4-3) to switch the output signals from said plurality of sensors to extract each of said output signals on a time series basis; processing circuit means (2) for processing the extracted output signal to determine the intensity of magnetic flux detected by the selected sensor and comprising feedback means for applying a feedback signal to the selected sensor so that said output signal of said sensor becomes a predetermined value; characterised in that each of said SQUIDs (13) is a digital SQUID flux sensor for outputting a pulse signal made up of positive or negative pulses depending on the direction of magnetic flux being detected; in that said processing circuit means (2) is provided in common for all the sensors, and includes a counter (21) for counting the pulses in the extracted pulse signal; and in that a memory means (5) is arranged to store the value counted by the counter (21) in a respective storage area for the selected sensor, said value representing the intensity of magnetic flux detected by the sensor in terms of the magnitude of said feedback signal; and in that it is provided with means (3) to preset said value in the counter (21). A multi-channel SQUID fluxmeter comprising: a plurality of SQUID flux sensors (1) for detecting weak magnetic flux emanating from different portions of an object under examination, each sensor including a SQUID (13) for producing an output signal (S1, S2, S3,...); a multiplexer (4-1, 4-3) to switch the output signals from said plurality of sensors to extract each of said output signals on a time series basis; and feedback means (8) for applying a feedback signal to the selected sensor so that said output signal of said sensor becomes a predetermined value; characterised in that each of said SQUIDs (13) is a digital SQUID flux sensor for outputting a pulse signal made up of positive or negative pulses depending on the direction of magnetic flux being detected; in that a counter (21) is provided in common for all the sensors for counting the pulses in the extracted pulse signal; in that a memory means (5) is arranged to store the value counted by the counter (21) in a respective storage area for the selected sensor, said value representing the intensity of magnetic flux detected by the sensor; and in that said feedback means is a superconducting feedback circuit (8), provided for each of said sensors (1), which stores magnetic flux quanta corresponding to the pulse signal output from the respective sensor, said feedback signal corresponding to said magnetic flux quanta. A fluxmeter according to claim 1 or 2, wherein the multiplexer comprises a first multiplexer (4-1) which is connected to receive the pulse signals outputted by all the sensors (1), and which enables one of the pulse signals by connecting it to said counter (21), said connection being changed at said constant time interval. A fluxmeter according to claim 1 or 2, wherein the multiplexer comprises a third multiplexer (4-3) which is connected to the digital SQUID (13) of each of the sensors (1), and which enables one of the pulse signals by connecting a bias current source (6) only to the digital SQUID of the corresponding sensor, said connection being changed at said constant time interval. A fluxmeter according to claim 1, wherein said feedback means comprises a D/A converter (22) for converting the value counted by said counter (21) into an analog value, and a second multiplexer (4-2) operable in synchronism with said other multiplexer (4-1, 4-3) to supply said analog value to the selected sensor as said feedback signal. A fluxmeter according to claims 4 and 2 in combination, wherein said multiplexer further comprises a first multiplexer (4-1) operable in synchronism with said third multiplexer (4-3) to supply said enabled pulse signal to said counter (21). A fluxmeter according to any preceding claim, wherein said counter (21) is an up/down counter which is incremented by receipt of a positive pulse signal and decremented by receipt of a negative pulse signal. A fluxmeter according to claim 1 or 5, further comprising a control circuit (3) for controlling said multiplexers (4; 4-2 and 4-1 or 4-3), said counter (21) and said memory means (5), and operable to cause said multiplexer (4; 4-1; 4-3) to repeatedly enable the pulse signal of each sensor in turn so as to perform a cycle (t₁, t₂,...), and further operable, before enabling the same pulse signal again, to preset said counter (21) with the count value of said counter (21) which has been stored in said memory means (5) in the preceding cycle. A fluxmeter according to any preceding claim, wherein said feedback means (22, 4-2; 8) supplies said feedback signal to the selected sensor so that the enabled pulse signal becomes zero. A fluxmeter according to claims 1 and 9 in combination, wherein each sensor (1) is provided with a feedback coil (12) for receiving said feedback signal and generating a magnetic flux in response thereto so as to cancel the magnetic flux detected by the digital SQUID (13). A fluxmeter according to claim 2, wherein each superconducting feedback circuit (8) comprises a superconducting inductance (82) provided with said digital SQUID (13) and a write-gate (81) for feeding said magnetic flux quanta to said superconducting inductance (82).

Inventors: GOTO TAKAO, HAYASHI TAKEHIKO, IGARASHI YUTAKA, GOTO, TAKAO, HAYASHI, TAKEHIKO, IGARASHI, YUTAKA
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G01R33
IPC codes: G01R33