JP2014173846A - Measuring apparatus and measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measuring apparatus which can be easy multichannelled without reducing magnetic flux resolution and has high response characteristic.SOLUTION: A measuring apparatus 1 comprises: a magnetic flux detection section 10 constituted of a superconducting quantum interference device which outputs a pulse voltage at frequency corresponding to measurement target magnetic flux Φs to be input; a reference magnetic flux output section 12 which outputs pulse voltage at a fixed frequency; and a magnetic flux tank section 11 which generates difference magnetic flux (Φd-Φr) between detection magnetic flux Φd generated in one direction on the basis of a pulse voltage that the magnetic flux detection section 10 outputs and reference magnetic flux Φr generated in a direction cancelling the magnetic flux for detection Φd on the basis of a pulse voltage that the reference magnetic flux output section 12 outputs. The magnetic flux tank section 11 inputs at least a part of the difference magnetic flux (Φd-Φr), which is generated in the section, to the magnetic flux detection section 10 by magnetic coupling in a direction cancelling the measurement target magnetic flux Φs.

Description

  The present invention relates to a measuring apparatus and a measuring method for measuring a minute magnetic field using a superconducting quantum interference device.

  In recent years, micro magnetic field measurement has been used in a wide range of applications such as nondestructive inspection, geological survey, and space physics. In particular, in the field of biomagnetic measurement represented by MRI (Magnetic Resonance Imaging), there is an increasing demand for a highly functional multi-channel magnetic sensing system.

  A magnetic sensor using a superconducting quantum interference device (SQUID) is used in MRI that realizes biomagnetism measurement. Although the SQUID magnetic sensor needs to be placed in a cryogenic environment in order to obtain a superconducting state, it is known as a very sensitive magnetic sensor.

FIG. 8 is a diagram for explaining the function of a superconducting quantum interference device according to the prior art.
As shown in FIG. 8A, the measuring device 8 that performs micro magnetic field measurement according to the prior art includes a magnetic flux detector 80 that is a superconducting quantum interference device (SQUID). The magnetic flux detection unit 80 includes a superconducting ring 800 and two joints 801 formed by Josephson junctions. Superconducting ring 800 is a metal wiring formed in a ring shape, and is made of a material that transitions to a superconducting state at a predetermined temperature or lower. The junction 801 is configured such that a very thin normal conductive film or insulating film is sandwiched between upper and lower superconductors. Here, in a state in which a predetermined DC current is supplied from the current source to the magnetic flux detection unit 80, when the measurement target magnetic flux Φs is linked in the superconducting ring 800, the magnetic flux Φs becomes the magnetic flux quantum Φ0 (= 2) at the output voltage Vout. 0.07 × 10 ⁻¹⁵ Wb) is known to periodically change every n times (n is an integer) (FIG. 8B). By measuring the output voltage Vout, the measuring device 8 can measure the measurement target magnetic flux Φs with a resolution smaller than the magnetic flux quantum Φ0.

  Here, a case where the measurement target magnetic flux Φs changes at the point A in FIG. 8B and a case where the measurement target magnetic flux Φs changes at the point B are considered. When the measurement target magnetic flux Φs changes at the point A, the measurement device 8 can detect the measurement target magnetic flux Φs with high accuracy because the change amount of the output voltage Vout that changes in accordance with the change of the magnetic flux Φs is large. On the other hand, when the measurement target magnetic flux Φs changes at the point B, the change amount of the output voltage Vout that changes in accordance with the change of the magnetic flux Φs is small, and thus the measuring device 8 cannot detect the measurement target magnetic flux Φs with high accuracy. . That is, in this state, the measuring device 8 cannot obtain the linearity of the output voltage Vout with respect to the measurement target magnetic flux Φs.

FIG. 9 is a diagram illustrating a configuration example of a measurement apparatus according to the related art. FIG. 10 is a graph for explaining the function of the magnetic flux lock circuit according to the prior art.
In order to solve the above-described linearity problem, the measuring device 8 according to the prior art using the SQUID includes a magnetic flux lock circuit 81 shown in FIG. The magnetic flux lock circuit 81 is also called an FLL (Flux Locked Loop) circuit. As shown in FIG. 9, the magnetic flux lock circuit 81 mainly includes a voltmeter 810 and a current source 811. Further, as shown in FIG. 9, the magnetic flux detector 80 and the feedback coil 82 are disposed in the low temperature environment region 83.

  The function of the magnetic flux lock circuit 81 will be specifically described. First, it is assumed that the state has been adjusted to the point A (FIG. 10) in the initial state (measurement target magnetic flux Φs = 0). Next, it is assumed that the measurement target magnetic flux Φs is linked to the magnetic flux detection unit 80 (FIG. 10 (1)). Then, the magnetic flux detector 80 outputs an output voltage Vout that increases in accordance with the interlinkage magnetic flux Φs. The voltmeter 810 of the magnetic flux lock circuit 81 inputs the increased Vout (FIG. 10 (2)). Then, the current source 811 of the magnetic flux lock circuit 81 immediately outputs the direct current I corresponding to the input increase of Vout to the feedback coil 82. The feedback coil 82 generates a feedback magnetic flux Φf that cancels the measurement target magnetic flux Φs that links the magnetic flux detector 80 based on the input current I. The current source 811 of the magnetic flux lock circuit 81 outputs the current I to the feedback coil 82 so that the feedback magnetic flux Φf completely cancels the measurement target magnetic flux Φs. Then, the measuring device 8 returns to the state of point A again (FIG. 10 (3)). The measuring device 8 outputs the change amount of the output voltage Vout generated according to the measurement target magnetic flux Φs in the process of FIGS. 10 (1) to 10 (3) through an integration circuit (not shown). That is, the measuring device 8 can measure the measurement target magnetic flux Φs by integrating and outputting the change in the measurement target magnetic flux Φs while always ensuring the linearity of the output voltage Vout with respect to the measurement target magnetic flux Φs. It is said.

  Incidentally, a digital SQUID magnetic sensor has been proposed as a measurement method different from the measurement method described in FIGS. 8 to 10 (Non-Patent Document 1). In the micro magnetism measuring apparatus using the digital SQUID magnetic sensor, the magnetic flux Φs to be measured is detected by a superconducting digital circuit placed in a low temperature environment together with the superconducting ring without using the above-described magnetic flux lock circuit. By doing so, the measuring device described in Non-Patent Document 1 enables high response characteristics (high slew rate) and multi-channeling of the magnetic sensor.

U. Fath et al., “Experimental Digital SQUID with Integrated Feedback Circuit”, IEEE TRANSACTION ON APPLIED SUPERCONDUCTIVITY, VOL. 7, NO. 2, JUNE 1997

As described above, the measuring device 8 according to the prior art uses the magnetic flux lock circuit 81. The magnetic sensor using the magnetic flux lock circuit 81 can detect a very small magnetic flux of the order of 10 ⁻²⁰ Wb (10 ⁻⁵ Φ0), while the feedback operation of the magnetic flux lock circuit 81 (FIG. 10 (3)). As a result, it has been found that the response characteristic (slew rate) of the magnetic sensor is very low. This is because the magnetic flux lock circuit 81 requires a certain amount of time for feedback control to the DC current I of the current source 811 based on the detection voltage of the voltmeter 810. That is, there is a problem that the measuring device 8 cannot capture the agile change of the measurement target magnetic flux Φs. In addition, the measuring device 8 has a problem that the dynamic range is narrow because the measurable range of the measurement target magnetic flux Φs is limited to the output limit of the current source 811 constituting the magnetic flux lock circuit 81.

  As shown in FIG. 9, the measuring device 8 has a configuration in which the magnetic flux detection unit 80 and the feedback coil 82 are arranged in the low temperature environment region, while the magnetic flux lock circuit 81 is arranged in the room temperature environment. Due to such a configuration, the measurement apparatus 8 needs to have at least three wires per channel straddling the boundary between the room temperature region and the low temperature region. Then, when constructing a multi-channel measurement apparatus including a plurality of magnetic sensors for realizing a plurality of magnetic flux measurements at the same time, the wiring around the low temperature environment region and the room temperature environment region is remarkably increased. That is, it is difficult to increase the number of channels of the magnetic sensor in the aspect of the measuring device 8.

  On the other hand, as described above, according to the micromagnetic measurement device provided with the superconducting digital circuit described in Non-Patent Document 1, it is possible to solve the low response characteristic and the difficulty of multichanneling of the measurement device 8. However, the superconducting digital circuit is a digital circuit (referred to as a single flux quantum (SFQ) circuit) that uses the magnetic flux quantum Φ0, which is the minimum unit of magnetic flux in the superconductor, as an information carrier. That is, in this superconducting digital circuit, information processing is performed while the magnetic flux quantum Φ0 propagates through the circuit, assuming that the magnetic flux quantum Φ0 is present in the superconducting ring as “1”, and the magnetic flux quantum Φ0 as “0”. It is characterized by being.

Since the measuring apparatus including this superconducting digital circuit is in the above-described manner, it is a principle to realize a measurement resolution less than the magnetic flux quantum Φ0 (2.07 × 10 ⁻¹⁵ Wb) when measuring the magnetic flux Φ. Is impossible. That is, since the measuring apparatus described in Non-Patent Document 1 uses a circuit that expresses bits based on the presence or absence of the magnetic flux quantum Φ0, there is a problem that the most important magnetic flux resolution as a magnetic sensor is limited to Φ0. It was.

  Therefore, an object of the present invention is to provide a measuring apparatus and a measuring method that can solve the above-described problems.

  The present invention has been made to solve the above-described problems. A magnetic flux detection unit including a superconducting quantum interference device that outputs a pulse voltage at a frequency corresponding to an input measurement target magnetic flux, and a constant frequency. A reference magnetic flux output unit that outputs a pulse voltage, a detection magnetic flux generated in one direction based on the pulse voltage output from the magnetic flux detection unit, and the detection based on the pulse voltage output from the reference magnetic flux output unit A magnetic flux tank section that generates a differential magnetic flux with respect to a reference magnetic flux generated in a direction to cancel the magnetic flux for use, and the magnetic flux tank section is at least one of the differential magnetic fluxes generated therein by magnetic coupling. The measuring device is characterized in that the unit is input to the magnetic flux detection unit in a direction to cancel the magnetic flux to be measured.

  Further, the present invention provides the above-described measurement apparatus, wherein the reference magnetic flux output unit outputs a frequency when the magnetic flux detection unit has a relationship in which the measurement target magnetic flux and the frequency of the pulse voltage increase or decrease in proportion. The pulse voltage is output at a frequency equal to.

  Moreover, the present invention is characterized in that, in the above-described measuring apparatus, at least the magnetic flux detection unit, the reference magnetic flux output unit, the magnetic flux tank unit, and the pulse counter unit are formed on the same substrate.

  According to the present invention, in the measuring apparatus described above, the reference magnetic flux output unit is configured by the same superconducting quantum interference device as the superconducting quantum interference device constituting the magnetic flux detection unit.

  Further, the present invention is characterized in that the measuring apparatus further includes a calibration coil for adjusting the frequency of the reference magnetic flux output from the reference magnetic flux output unit by magnetic coupling.

  The present invention may further include a pulse counter unit that outputs the number of pulses of the input pulse voltage as a predetermined number of counts in the measurement apparatus, and the pulse counter unit outputs the pulse voltage output from the magnetic flux detection unit. The count number is increased according to the number of pulses, and the count number is decreased according to the number of pulses of the pulse voltage output from the reference magnetic flux output unit.

  Further, according to the present invention, the magnetic flux detection unit composed of a superconducting quantum interference element outputs a pulse voltage at a frequency corresponding to the input magnetic flux to be measured, and the reference magnetic flux output unit outputs a pulse voltage at a constant frequency. The pulse counter unit inputs the pulse voltage, outputs a predetermined value based on the number of pulses, and the magnetic flux tank unit generates the pulse voltage in one direction based on the pulse voltage output by the magnetic flux detection unit. A differential magnetic flux between the detection magnetic flux and a reference magnetic flux generated in a direction to cancel the detection magnetic flux based on a pulse voltage output from the reference magnetic flux output unit. In the measurement method, at least a part of the differential magnetic flux generated therein is input to the magnetic flux detection unit in a direction to cancel the measurement target magnetic flux.

  According to the present invention, there is provided a measuring device that is easy to be multi-channeled to realize simultaneous detection of a plurality of magnetic fluxes without reducing the measurement resolution of the magnetic flux and has a high response characteristic that captures an agile change in magnetic flux. Can be provided.

It is a figure which shows the function structure of the measuring apparatus by one Embodiment of this invention. It is a figure explaining the function of the magnetic flux detection part by one Embodiment of this invention. It is a figure explaining the function of the magnetic flux tank part by one Embodiment of this invention. It is a graph explaining operation | movement of the measuring apparatus by one Embodiment of this invention. It is a figure which shows the structure of the reference | standard magnetic flux output part by one Embodiment of this invention. It is a 1st figure explaining the structure of the measuring device by one Embodiment of this invention. It is a 2nd figure explaining the structure of the measuring device by one Embodiment of this invention. It is a figure explaining the function of the superconducting quantum interference element by a prior art. It is a figure which shows the structural example of the measuring apparatus by a prior art. It is a graph explaining the function of the magnetic flux lock circuit by a prior art.

Hereinafter, a measuring apparatus according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing a functional configuration of a measuring apparatus according to an embodiment of the present invention. In this figure, reference numeral 1 denotes a measuring device. As shown in FIG. 1, the measuring apparatus 1 includes a magnetic flux detection unit 10, a magnetic flux tank unit 11, a reference magnetic flux output unit 12, and a pulse counter unit 13.

  The magnetic flux detection unit 10 is a so-called superconducting quantum interference device (SQUID), and includes a superconducting ring 100 and a joint 101. Superconducting ring 100 is a metal wiring formed in a ring shape, and is made of a material that transitions to a superconducting state at a predetermined temperature or lower. For example, Nb (niobium) is used for the superconducting ring 100. In this case, the superconducting ring 100 becomes superconductive at about 9K. The bonding portion 101 is a bonding interface configured such that a very thin normal conductive film or insulating film is sandwiched between upper and lower superconductors, and is called a Josephson Junction (JJ).

  The magnetic flux detection unit 10 according to the present embodiment is a functional unit that outputs a pulse voltage at a frequency (detection frequency fd) corresponding to the measurement target magnetic flux Φs interlinking the superconducting ring 100. Details of the function of the magnetic flux detector 10 will be described later. In the following description, the pulse voltage output by the magnetic flux detection unit 10 at the detection frequency fd is referred to as a “detection pulse voltage”, and a bundle of magnetic flux quantum Φ0 generated in the superconducting ring based on the detection pulse voltage is expressed as “ The magnetic flux for detection Φd ”.

  The reference magnetic flux output unit 12 is a functional unit that outputs a pulse voltage at a constant frequency (reference frequency fr). The reference magnetic flux output unit 12 according to the present embodiment outputs the same frequency as the detection frequency fd output from the magnetic flux detection unit 10 when the measurement target magnetic flux Φs interlinking the magnetic flux detection unit 10 is zero. To be adjusted. That is, when the measurement target magnetic flux Φs = 0, fd = fr. Specific functions and configurations of the reference magnetic flux output unit 12 will be described later. In the following description, the bundle of magnetic flux Φ0 generated in the superconducting ring based on the pulse voltage output from the reference magnetic flux output unit 12 at the reference frequency fr is referred to as “reference magnetic flux Φr”.

  The magnetic flux tank unit 11 is a functional unit that inputs the pulse voltage output from the magnetic flux detection unit 10 and the pulse voltage output from the reference magnetic flux output unit 12. As shown in FIG. 1, the magnetic flux tank unit 11 is connected to the magnetic flux detection unit 10 and the reference magnetic flux output unit 12 by wiring, and inputs the detection pulse voltage and the reference pulse voltage.

  Here, the wiring constituting the magnetic flux tank unit 11 is also a superconductor, and a superconducting ring is formed through a Josephson junction (indicated by x on the wiring in FIG. 1) and the ground. Therefore, when the detection pulse voltage is input from the magnetic flux detection unit 10, the magnetic flux tank unit 11 generates the detection magnetic flux Φd in one direction inside the superconducting ring. Similarly, when the reference pulse voltage is input from the reference magnetic flux output unit 12, the magnetic flux tank unit 11 generates the reference magnetic flux Φr in the superconducting ring. At this time, the magnetic flux tank unit 11 generates the reference magnetic flux Φr in a direction opposite to the direction of the detection magnetic flux Φd (direction in which the detection magnetic flux Φd is canceled). As a result, the magnetic flux tank unit 11 generates only the difference magnetic flux (difference magnetic flux (Φd−Φr)) between the detection magnetic flux Φd and the reference magnetic flux Φr.

  As shown in FIG. 1, the magnetic flux tank unit 11 has a magnetic coupling unit 110. The magnetic coupling unit 110 is magnetically coupled to the magnetic flux detection unit 10 and acts to cancel the measurement target magnetic flux Φs input to the magnetic flux detection unit 10 in accordance with the differential magnetic flux (Φd−Φr) generated therein. Details of the function of the magnetic flux tank unit 11 will be described later.

  The pulse counter unit 13 is a functional unit that inputs a predetermined pulse voltage and outputs a predetermined value based on the number of pulses. The pulse counter unit 13 according to the present embodiment is configured by a digital circuit that performs arithmetic processing for counting the number of pulses of the sequentially input pulse voltage. The pulse counter unit 13 according to the present embodiment is mainly formed of a superconductor, and is configured by a digital circuit (hereinafter referred to as an SFQ circuit) using the flux quantum Φ0 as an information carrier. The pulse counter unit 13 according to the present embodiment is an aspect of a “measurement unit” that has a function of inputting the detection pulse voltage and specifying the measurement target magnetic flux Φs based on the detection pulse voltage. Details of the function of the pulse counter unit 13 will be described later.

FIG. 2 is a diagram illustrating the function of the magnetic flux detector according to an embodiment of the present invention.
FIG. 2A shows the average value of the pulse voltage output from the magnetic flux detector 10. Here, when the measurement target magnetic flux Φs is linked in the superconducting ring 100 with a predetermined DC current flowing from the current source to the magnetic flux detection unit 10, the output voltage Vout changes to the magnetic flux quantum Φ0 ( = 2.07 × 10 ⁻¹⁵ Wb) is known to change periodically (FIG. 2A). By measuring the output voltage Vout, the measuring device 8 can measure the measurement target magnetic flux Φs with a resolution smaller than the magnetic flux quantum Φ0.

  However, Vout output from the magnetic flux detector 10 is actually an average of pulse voltages output at a very high frequency (in the order of several tens of GHz). Each pulse of the pulse voltage is an induced voltage generated by the amount of change of the magnetic flux quantum Φ0 in the magnetic flux Φ interlinking the superconducting ring 100. Here, it is known that the magnetic flux Φ interlinking in the superconducting ring formed of a superconductor is subjected to a constraint condition that can change only in units of the magnetic flux quantum Φ0. On the other hand, the Josephson junction (joint 101) has a property equivalent to that of a superconductor when no current flows, but the current passing through the junction 101 exceeds a predetermined critical current Ic. The state changes from the superconducting state to the normal conducting state, and the superconducting ring 100 has an effect of cutting the superconducting region. Therefore, when the current passing through the junction 101 exceeds the critical current Ic, the superconducting ring (in units of the flux quantum Φ0) from the junction 101 in which the magnetic flux Φ interlinking the superconducting ring 100 is in a normal conduction state. The pulse voltage is excited in the magnetic flux detection unit 10 (junction 101) as a result of the movement of the magnetic flux Φ (change of the magnetic flux quantum Φ0).

  When the pulse voltage is excited, the current flowing through the junction 101 instantaneously falls below the critical current Ic and the junction 101 returns to the superconducting state. However, since the current is constantly applied, the critical current Ic is again applied. And the pulse voltage is excited. By repeating this phenomenon, the magnetic flux detector 10 outputs a pulse voltage at a predetermined frequency. Note that, as the magnetic flux Φ interlinking the superconducting ring 100 increases, the current around the superconducting ring 100 increases, so the time until the critical current Ic is exceeded is also shortened. Therefore, the frequency f of the pulse voltage increases as the magnetic flux Φ interlinking the superconducting ring 100 increases.

  FIGS. 2B and 2C show the state of pulse voltages actually generated at point A (average voltage Vout1) and point B (average voltage Vout2) in FIG. 2A, respectively. That is, the average voltage Vout1 at the point A (FIG. 2A) is actually observed by averaging the pulse voltage output at the frequency f1 as shown in FIG. 2B. Similarly, the average voltage Vout2 (> Vout1) at the point B is actually observed by averaging the pulse voltage output at the frequency f2 higher than the frequency f1, as shown in FIG. 2C. .

  As described above, the voltage waveform for one pulse corresponds to the change of the flux quantum Φ0. Therefore, Equation (1) is established between the average voltage Vout and the frequency f.

  Since an equation like equation (1) holds, the average value (vertical axis) of Vout in the graph of FIG.

FIG. 3 is a diagram illustrating the function of the magnetic flux tank unit according to the embodiment of the present invention.
As described above, the magnetic flux tank portion 11 is formed of the superconducting ring 111 (FIG. 3) having a Josephson junction (joint portion 112). Here, the magnetic flux tank unit 11 receives a pulse voltage for detection from the magnetic flux detection unit 10. As described above, the pulse voltage corresponds to the amount of change of the flux quantum Φ0 for one pulse. The magnetic flux tank unit 11 generates a circulating current around the superconducting ring 111 of the magnetic flux tank unit 11 when the pulse voltage for one pulse is excited by itself, and a magnetic flux Φ0 interlinking the superconducting ring 111 is generated. Occur. Here, in the superconductor, the voltage V and the magnetic flux Φ are converted without energy loss. Therefore, as shown in FIG. 3, the phenomenon described above causes a part of the magnetic flux Φ interlinking the superconducting ring 100 of the magnetic flux detection unit 10 (magnetic flux quantum Φ0) to be generated in the magnetic flux tank unit 11 via the pulse voltage. It can be seen as a phenomenon that moves to. In this way, the magnetic flux tank unit 11 generates the magnetic flux for detection Φd interlinking the superconducting ring 111 by the magnetic flux quantum Φ0 for each pulse of the detection pulse voltage continuously input at the detection frequency fd. Increase (Figure 3). Here, it is assumed that the direction of the magnetic flux for detection Φd is the front side in FIG.

  Similarly, the magnetic flux tank unit 11 continuously receives the reference pulse voltage from the reference magnetic flux output unit 12 at the reference frequency fr, and generates the reference magnetic flux Φr interlinking the superconducting ring 111 (FIG. 3). . At this time, the magnetic flux tank unit 11 generates the reference magnetic flux Φr in the direction opposite to the direction of the detection magnetic flux Φd (the direction in which the detection magnetic flux Φd is canceled (the back direction in FIG. 3)). Specifically, in the magnetic flux tank unit 11, the circulating current generated in the magnetic flux tank unit 11 by the reference pulse voltage from the reference magnetic flux output unit 12 is in the opposite direction to the circulating current generated by the detection pulse voltage from the magnetic flux detection unit 10. It has a circuit configuration that goes around. As a result, the magnetic flux tank unit 11 generates only the differential magnetic flux (Φd−Φr) between the detection magnetic flux Φd and the reference magnetic flux Φr. The circulating current that generates the differential magnetic flux (Φd−Φr) in the superconducting ring 111 is limited to a range that does not exceed the critical current Ic.

  As described above, the frequency of the reference pulse voltage output by the reference magnetic flux output unit 12 (reference frequency fr) is the same as the detection frequency fd when the measurement target magnetic flux Φs linked in the magnetic flux detection unit 10 is zero. The frequency is set. Therefore, when the measurement target magnetic flux Φs is zero, since fd = fr, the detection magnetic flux Φd and the reference magnetic flux Φr that can be generated inside the magnetic flux tank unit 11 are equal, and both cancel each other completely. Therefore, in this case, no magnetic flux is generated in the magnetic flux tank unit 11. On the other hand, when the measurement target magnetic flux Φs has a finite value, the detection frequency fd changes by the measurement target magnetic flux Φs. Here, it is assumed that the direction of the measurement target magnetic flux Φs is the back direction in FIG. 3 and the frequency fd is increased based on the measurement target magnetic flux Φs. Then, since fd> fr, the balance between the detection magnetic flux Φd generated in the magnetic flux tank 11 and the reference magnetic flux Φr is lost, and Φd> Φr. Then, a differential magnetic flux (Φd−Φr) corresponding to the magnetic flux amount of the measurement target magnetic flux Φs is generated in the magnetic flux tank unit 11.

  Furthermore, the magnetic flux tank unit 11 according to the present embodiment is arranged so as to be magnetically coupled to the magnetic flux detection unit 10 (FIG. 3), and the differential magnetic flux (Φd−Φr) generated inside by magnetic coupling. At least a part is input to the magnetic flux detection unit 10 in a direction to cancel the measurement target magnetic flux Φs. Specifically, in the magnetic flux tank unit 11, at least a part of the differential magnetic flux (Φd−Φr) is linked to the superconducting ring 100 of the magnetic flux detection unit 10 in the magnetic coupling unit 110 constituting a part thereof. They are arranged close to each other so as to generate a feedback magnetic flux Φf. Further, the magnetic flux tank unit 11 is arranged so that the feedback magnetic flux Φf is generated in a direction that cancels the measurement target magnetic flux Φs (the front side in FIG. 3). The feedback magnetic flux Φf increases and decreases in proportion to the magnitude of the differential magnetic flux (Φd−Φr).

  As a result of the feedback magnetic flux Φf interlinking the superconducting ring 100 of the magnetic flux detection unit 10, the influence of the measurement target magnetic flux Φs on the magnetic flux detection unit 10 is reduced, and as a result, the detection increased due to the influence of the measurement target magnetic flux Φs. The frequency fd decreases. As long as the condition of fd> fr is satisfied, the differential magnetic flux (Φd−Φr) continues to increase, and the feedback magnetic flux Φf is increased. Therefore, the feedback magnetic flux Φf increases until the stage where fd = fr, that is, the stage where the measurement target magnetic flux Φs is completely canceled (Φf = Φs). When the feedback magnetic flux Φf increases until it completely cancels the measurement target magnetic flux Φs, fd = fr and the differential magnetic flux (Φd−Φr) generated by the magnetic flux tank unit 11 does not increase any more. At equilibrium. In this way, the magnetic flux tank unit 11 functions as a negative feedback circuit that inputs the fluctuation component (increase / decrease in the detection frequency fd) generated in the magnetic flux detection unit 10 and acts in a direction to suppress the fluctuation.

  Even if the detection frequency fd varies temporarily according to the change in the measurement target magnetic flux Φs interlinking the superconducting ring 100 of the magnetic flux detection unit 10, the detection frequency fd is changed to the reference frequency fr by the function of the magnetic flux tank unit 11. The feedback action works until they are equal.

  Here, a case where the measurement target magnetic flux Φs changes at the point A in FIG. 2A and a case where the measurement target magnetic flux Φs changes at the point B are considered. When the measurement target magnetic flux Φs changes at the point A, the output average voltage Vout that changes in accordance with the change of the magnetic flux Φs, that is, the amount of change in the detection frequency fd (see Equation (1)) is large. The measurement target magnetic flux Φs can be detected with high accuracy. On the other hand, when the measurement target magnetic flux Φs changes at the point B, the amount of change in the detection frequency fd that changes in accordance with the change in the magnetic flux Φs is small, and thus the measurement apparatus 1 cannot accurately detect the measurement target magnetic flux Φs. . That is, the measuring device 1 needs to ensure the linearity of the detection frequency fd with respect to the measurement target magnetic flux Φs.

  Here, attention is focused on the reference frequency fr of the reference pulse voltage output from the reference magnetic flux output unit 12. The reference magnetic flux output unit 12 generates a reference pulse voltage at a frequency equal to the output frequency when the magnetic flux detection unit 10 has a relationship (linearity) in which the measurement target magnetic flux Φs and the detection frequency fd increase or decrease in proportion. Output. That is, the reference frequency fr is set to a frequency equal to the detection frequency fd output by the magnetic flux detection unit 10 at point A in FIG. By doing in this way, the average voltage Vout output from the magnetic flux detector 10 has a feedback action that is in an equilibrium state at the point A in FIG. Therefore, even if the measurement target magnetic flux Φs interlinking the magnetic flux detection unit 10 changes and the detection frequency fd output by the magnetic flux detection unit 10 varies temporarily according to this change, the magnetic flux tank unit 11 immediately measures the measurement target. A feedback magnetic flux Φf that cancels the magnetic flux Φs is generated and returned to the state of point A.

  On the other hand, the pulse counter unit 13 (FIGS. 1 and 3) according to the present embodiment is composed of a digital circuit that inputs at least a detection pulse voltage and counts the number of pulses. Specifically, every time one pulse of the detection pulse voltage is input, the pulse counter unit 13 performs a calculation process to detect the pulse and increase the count number by one. That is, the pulse frequency of the detection frequency fd that fluctuates according to the increase / decrease of the measurement target magnetic flux Φs is counted by the pulse counter unit 13. The measuring apparatus 1 can measure the measurement target magnetic flux Φs by appropriately associating the count number calculated by the pulse counter unit 13 with the measurement target magnetic flux Φs.

  For example, the pulse counter unit 13 according to the present embodiment is an up / down counter circuit that receives the detection pulse voltage output from the magnetic flux detection unit 10 and the reference pulse voltage output from the reference magnetic flux output unit 12 as inputs. May be. The up / down counter circuit (pulse counter unit 13) has a circuit configuration that increases the count number according to the number of pulses of the detection pulse voltage and decreases the count number according to the number of pulses of the reference pulse voltage. In this way, the pulse counter unit 13 functions to output only the difference between the number of pulses of the detection pulse voltage and the number of pulses of the reference pulse voltage, that is, the number of pulses increased or decreased due to the change in the measurement target magnetic flux Φs. Therefore, the output count number can be directly linked to the measurement target magnetic flux Φs.

  As described above, since the pulse counter unit 13 is formed by an SFQ circuit, high-speed data processing can be realized. The detection pulse voltage and the reference pulse voltage described above are high-frequency signals each extending over several tens of GHz, and are pulse signals on the order of several psec per pulse. However, the SFQ circuit formed of a superconductor can stably operate against such a high-frequency signal.

  In the above description, the detailed description of the up / down counter circuit based on the SFQ circuit is described in “T. Onomi and two others,“ Implementation of High-Speed Single-Flux-Quantum Up / Down Counter for the National Computation Lumps ”. , IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 19, NO. 3, JUNE 2009 ".

  As described above, the measurement apparatus 1 according to the present embodiment uses the pulse counter unit 13 as one aspect of the measurement unit that specifies the measurement target magnetic flux Φs from the detection pulse voltage input from the magnetic flux detection unit 10. The measuring apparatus 1 according to the embodiment is not limited to this mode. For example, the measurement apparatus 1 according to another embodiment may be configured such that the measurement target magnetic flux Φs is specified by linking the average voltage Vout of the detection pulse voltage and the measurement target magnetic flux Φs as the measurement unit. .

FIG. 4 is a graph illustrating the operation of the measurement apparatus according to one embodiment of the present invention.
The graph shown in FIG. 4 is a circuit simulation result showing how the frequency of each pulse voltage fluctuates when the measurement target magnetic flux Φs increases in the measuring apparatus 1. When a predetermined measurement target magnetic flux Φs is input to the magnetic flux detection unit 10, each pulse voltage output from the magnetic flux detection unit 10 and the reference magnetic flux output unit 12 exhibits characteristics as shown in FIG. 4. For example, in the period T1 in which the measurement target magnetic flux Φs is zero, the detection pulse voltage output from the magnetic flux detection unit 10 is output at the same frequency as the reference pulse voltage output from the reference magnetic flux output unit 12 ( fd = fr). At this time, since the pulse counter unit 13 inputs the detection pulse voltage and the reference pulse voltage at the same frequency as an up / down counter circuit, the pulse count number maintains a constant value (eg, “0”) without increasing or decreasing doing. Note that in the period T1, the reference frequency fr is set so that the measurement target magnetic flux Φs and the detection frequency fd change to have a linearity (point A in FIG. 2A). And

  Next, it is assumed that the measurement target magnetic flux Φs (= 0.5Φ0) is input to the magnetic flux detection unit 10 at the first time point of the period T2 illustrated in FIG. Then, at the next moment, the detection frequency fd of the detection pulse voltage increases linearly according to the increased magnetic flux to be measured Φs. However, due to the negative feedback action of the magnetic flux tank unit 11 described above, the feedback magnetic flux Φf is generated in the superconducting ring 100 of the magnetic flux detection unit 10 in the direction to cancel the input measurement target magnetic flux Φs, and the detection frequency fd gradually decreases. To do. Then, in the period T3, it is stabilized again at fd = fr (point A in FIG. 2A) (FIG. 4).

  On the other hand, the pulse counter unit 13 continues to input the detection pulse voltage and the reference pulse voltage during the periods T2 to T3. Here, since the number of pulses of the detection pulse voltage is input more than the number of pulses of the reference pulse voltage in the period T2, the pulse counter unit 13 increases the count by the number of pulses increased in the period T2. To do. The measuring apparatus 1 makes it possible to measure the measurement target magnetic flux Φs based on the pulse count number by storing the pulse count number and the measurement target magnetic flux Φs in association with each other. Further, even if the measurement target magnetic flux Φs further changes after the period T3, the magnetic flux detection unit 10 is stable at a point having linearity (point A in FIG. 2A) at this time point. The change of the measurement object magnetic flux Φs can be read with high accuracy.

FIG. 5 is a diagram illustrating a configuration of a reference magnetic flux output unit according to an embodiment of the present invention.
As shown in FIG. 5, the reference magnetic flux output unit 12 according to the present embodiment may be configured by a superconducting ring 120 having a joint 121. Here, it is assumed that the superconducting ring 120 and the joining part 121 have the same configuration as the superconducting ring 100 and the joining part 101 constituting the magnetic flux detection part 10. A DC current equivalent to the DC current applied to the superconducting ring 100 is applied to the superconducting ring 120.

  The reference magnetic flux output unit 12 includes a calibration coil 122. The calibration coil 122 is disposed so as to be magnetically coupled to the superconducting ring 120, and an arbitrary DC current is input from another current source (not shown). By appropriately selecting the DC current to the calibration coil 122, the reference pulse voltage output from the reference magnetic flux output unit 12 is set to a desired reference frequency fr by magnetic coupling between the calibration coil 122 and the superconducting ring 120. Can be set. Here, the operator of the measuring apparatus 1 sets the reference frequency fr to a frequency at which linearity is ensured in the magnetic flux detection unit 10 in the previous stage of magnetic flux measurement.

6 and 7 are a first diagram and a second diagram illustrating the structure of the measuring apparatus 1 according to an embodiment of the present invention.
Next, the specific structure of the measuring apparatus 1 according to the present embodiment will be described with reference to FIGS. The present embodiment is characterized in that all the functional units constituting the measuring apparatus 1 are formed on one substrate. More specifically, the magnetic flux detection unit 10, the reference magnetic flux output unit 12, the magnetic flux tank unit 11, and the pulse counter unit 13 are formed on the same substrate with a layer (made of a material that becomes a superconducting state at a predetermined temperature or lower) Hereinafter, it is formed by forming a multi-layer film of “niobium wiring layer” as an example) and an insulating layer. Each niobium wiring layer and insulating layer are laminated with a film thickness of about 300 nm to 400 nm, for example.

  FIG. 6 shows a structure when the substrate is viewed from above. In this figure, the upper side of the substrate is shown on the front side of the paper and the lower side of the substrate is shown on the back side of the paper. FIG. 6A mainly shows the structure of the superconducting ring 100 of the magnetic flux detection unit 10 and the superconducting ring 111 of the magnetic flux tank unit 11.

  Three layers of niobium wiring layers are formed on the substrate with an insulating layer interposed therebetween. Each niobium wiring layer is a ground plane layer (G1), a BAS layer (BAS1 to BAS5), and a COU layer (COU1 to COU3) in order from the lower layer (the back side in the drawing). As shown in FIG. 6, the ground plane layer (G1) may be formed so as to cover the entire surface of the substrate. The wiring patterns of the BAS layer and the COU layer are routed with a wiring width of about 3 μm to 5 μm.

In addition, each niobium wiring layer is provided with a contact in various places, and is electrically connected to each layer through the contact. Here, each contact shown in FIG. 6 will be described. The ground contacts (GC1 to GC4) are contacts that electrically short-circuit the BAS layer and the ground plane layer. The BAS-COU contacts (BCC1 to BCC2) are contacts that electrically short-circuit the BAS layer and the COU layer. The Josephson junctions (JJ1 to JJ4) are contacts in which the BAS layer and the COU layer are connected by a predetermined resistance film. Each contact as described above are all formed in 1.5μm ² ~3μm ² about. In the following description, each niobium wiring layer and each contact are represented only by reference numerals.

  As shown in FIG. 6, the magnetic flux detection unit 10 is mainly formed by niobium wiring layers BAS1 to BAS3, COU1, COU2, and G1 and contacts JJ1, JJ2, BCC1, BCC2, GC1, and GC2. The DC current from the current source is applied via BAS3. FIG. 7A shows a cross-sectional structure of the magnetic flux detection unit 10 formed in this way. However, it should be noted that the cross-sectional view of the magnetic flux detection unit 10 shown in FIG. 7A is not shown as an accurate cross-sectional view of FIG. 6 for convenience of illustration.

  As shown in FIG. 7A, the superconducting ring 100 of the magnetic flux detection unit 10 includes a niobium wiring layer BAS1− from the niobium wiring layer BAS3-BCC1-COU1 which is a superconductor through the Josephson junction JJ1. Grounded via GC1-G1. Further, the superconducting ring 100 is connected to the niobium wiring layer COU2-BCC2-BAS3 from the niobium wiring layer G1-GC2-BAS2 via the Josephson junction JJ2, thereby forming a superconducting ring. When the magnetic flux Φs to be measured interlinks in the superconducting ring 100 formed in this way, JJ1 and JJ2 correspond to the joint portion 101 (FIG. 1). For convenience of explanation, although not shown, an insulating layer is provided between the niobium wiring layers as described above.

  As shown in FIG. 6, the magnetic flux tank 11 is mainly formed by niobium wiring layers BAS4, BAS5, COU3 and contacts JJ3, JJ4, GC3, GC4. FIG. 7B shows a cross-sectional structure of the magnetic flux tank portion 11 formed in this way. However, it should be noted that the cross-sectional view of the magnetic flux tank portion 11 shown in FIG. 7B is not shown as an accurate cross-sectional view of FIG. 6 for convenience of illustration.

  As shown in FIG. 7 (b), the superconducting ring 111 of the magnetic flux tank unit 11 connects the niobium wiring layer BAS4-GC3-G1 from the niobium wiring layer COU3, which is a superconductor, via the Josephson junction JJ3. Is connected to ground. Further, the superconducting ring 111 is connected to the niobium wiring layer COU3 from the niobium wiring layer G1-GC4-BAS5 via the Josephson junction JJ4 to form a superconducting ring. JJ3 and JJ4 correspond to the joint 112 (FIG. 3).

  As shown in FIG. 6, the magnetic flux tank unit 11 inputs the detection pulse voltage output from the magnetic flux detection unit 10 from the right side of FIG. 6, so that the superconducting ring 111 formed as shown in FIG. The magnetic flux for detection Φd is generated inside. On the other hand, the magnetic flux tank unit 11 inputs a reference pulse voltage from a reference magnetic flux output unit 12 (not shown in FIGS. 6 and 7) from the left side of FIG. Give rise to However, the directions of the detection magnetic flux Φd and the reference magnetic flux Φr are opposite to each other (directions in which they cancel each other). As a result, a differential magnetic flux (Φd−Φr) is generated in the superconducting ring 111 of the magnetic flux tank unit 11.

  Further, as shown in FIG. 6, the BAS 3 constituting the superconducting ring 100 of the magnetic flux detection unit 10 and the COU 3 constituting the superconducting ring 111 of the magnetic flux tank unit 11 are arranged so as to overlap in a plan view. The magnetic coupling unit 110 of the magnetic flux tank unit 11 is an area where the BAS 3 and the COU 3 overlap, and the magnetic flux detection unit 10 and the magnetic flux tank unit 11 are magnetically coupled mainly at this part. That is, at least a part of the differential magnetic flux (Φd−Φr) interlinking the superconducting ring 111 shown in FIG. 7B is simultaneously chained using the superconducting ring 100 of FIG. 7A as the feedback magnetic flux Φf. Interact. Here, the feedback magnetic flux Φf is generated in a direction opposite to the measurement target magnetic flux Φs (direction in which the measurement target magnetic flux Φs is canceled).

  Although not shown in FIG. 6, the detection pulse voltage output from the superconducting ring 100 of the magnetic flux detection unit 10 is a superconductivity formed equivalent to a predetermined SFQ circuit (for example, the superconducting ring 100 or the like). And is input to the magnetic flux tank unit 11 and the pulse counter unit 13. Further, the superconducting ring 120 (FIG. 5) constituting the reference magnetic flux output unit 12 according to the present embodiment may be formed with the same structure as the superconducting ring 100 of the magnetic flux detection unit 10.

  As described above, each functional unit, magnetic flux detection unit 10, magnetic flux tank unit 11, reference magnetic flux output unit 12 and pulse counter unit 13 constituting the measuring apparatus 1 according to the present embodiment are all formed on the same substrate. It will be arranged in the same low temperature region. By doing in this way, the measuring apparatus 1 can abolish complicated routing wiring. Further, by eliminating the routing wiring and forming it on the same substrate, it contributes to speeding up the circuit operation.

  The measuring apparatus 1 according to the present embodiment is characterized in that the function of a conventional magnetic flux lock circuit (FLL circuit) is realized by a reference magnetic flux output unit 12 and a magnetic flux tank unit 11. Here, since both the reference magnetic flux output unit 12 and the magnetic flux tank unit 11 according to the present embodiment are formed of a superconductor like the magnetic flux detection unit 10, the feedback action is also performed at a high speed. That is, the measuring apparatus 1 eliminates the conventional magnetic flux lock circuit and achieves high response characteristics with respect to changes in the magnetic flux Φs to be measured by realizing equivalent functions with various circuits formed of superconductors. Can do.

  In addition, the magnetic flux lock circuit used in the conventional magnetic measurement apparatus requires routing to the room temperature region, but both the magnetic flux tank unit 11 and the reference magnetic flux output unit 12 according to the present embodiment detect the magnetic flux. Since it is arranged in the same low temperature region as the portion 10, complicated routing wiring can be suppressed to a minimum. Therefore, the measuring apparatus 1 can easily realize multi-channeling.

  In addition, the magnetic flux lock circuit used in the conventional magnetic measurement device realizes the feedback action by inputting the output current I to a predetermined feedback coil. In this case, the conventional magnetic measurement device can measure. The range (dynamic range) of the amount of magnetic flux is limited by the limit of the output current I that can be output in the magnetic flux lock circuit. However, according to the measuring apparatus 1 according to the present embodiment, as the measurement target magnetic flux Φs increases, the feedback magnetic flux Φf also increases following this, so that in principle there is no measurement limit. Therefore, the measuring device 1 can have a larger dynamic range than the conventional magnetic measuring device.

  In addition, since the micro magnetism measuring apparatus using the SFQ circuit according to the prior art uses a circuit that expresses bits by the presence or absence of the magnetic flux quantum Φ0, the most important magnetic flux resolution as a magnetic sensor is limited to Φ0. . However, since the measuring apparatus 1 according to the present embodiment measures the magnetic flux Φs to be measured by reading the frequency (corresponding to the output voltage Vout) of the pulse voltage that periodically varies in units of the magnetic flux quantum Φ0, A high measurement resolution equivalent to that of a magnetic measurement device using a magnetic flux lock circuit can be realized.

  In the measuring apparatus 1 according to the present embodiment, as described above, the magnetic flux detection unit 10, the reference magnetic flux output unit 12, the magnetic flux tank unit 11, and the pulse counter unit 13 can be formed on the same substrate. By doing in this way, the measuring apparatus 1 can further improve the operation speed and response characteristics, and can easily increase the number of channels by eliminating the routing wiring.

  As described above, according to the measuring apparatus of the present invention, it is possible to provide a measuring apparatus that can easily be multi-channeled and has high response characteristics without reducing the measurement resolution.

  In addition, although the measuring apparatus 1 by this embodiment is implement | achieved by the aspect mentioned above, the measuring apparatus by other embodiment of this invention is not limited to this aspect. For example, in the measurement apparatus according to another embodiment, the structures of the magnetic flux detection unit 10, the magnetic flux tank unit 11, and the reference magnetic flux output unit 12 are not limited to the modes illustrated in FIGS. Changes can be made to the extent that the objective is achieved.

DESCRIPTION OF SYMBOLS 1 ... Measuring apparatus 10 ... Magnetic flux detection part 100, 111, 120 ... Superconducting ring 101, 112, 121 ... Joint part 11 ... Magnetic flux tank part 110 ... Magnetic coupling part 12. ..Reference magnetic flux output section 122 ... calibration coil 13 ... pulse counter section

Claims (7)

A magnetic flux detector composed of a superconducting quantum interference device that outputs a pulse voltage at a frequency corresponding to the input magnetic flux to be measured;
A reference magnetic flux output unit that outputs a pulse voltage at a constant frequency;
A detection magnetic flux generated in one direction based on the pulse voltage output from the magnetic flux detection unit, and a reference magnetic flux generated in a direction to cancel the detection magnetic flux based on the pulse voltage output from the reference magnetic flux output unit And a magnetic flux tank section for generating a differential magnetic flux of
With
The magnetic flux tank unit inputs at least a part of the differential magnetic flux generated therein by magnetic coupling to the magnetic flux detection unit in a direction to cancel the magnetic flux to be measured. . The reference magnetic flux output unit is
The pulse voltage is output at a frequency equal to a frequency that is output when the magnetic flux to be measured and the frequency of the pulse voltage have a proportional increase or decrease in the magnetic flux detection unit. The measuring device described in 1. The measuring apparatus according to claim 1, wherein at least the magnetic flux detection unit, the reference magnetic flux output unit, and the magnetic flux tank unit are formed on the same substrate. The reference magnetic flux output unit is
The measurement apparatus according to any one of claims 1 to 3, wherein the measurement apparatus includes the same superconducting quantum interference element as that constituting the magnetic flux detection unit. The measuring apparatus according to any one of claims 1 to 4, further comprising a calibration coil that adjusts a frequency of a reference magnetic flux output from the reference magnetic flux output unit by magnetic coupling. A pulse counter unit for outputting the number of pulses of the input pulse voltage as a predetermined count number;
The pulse counter unit
The count number is increased according to the number of pulses of the pulse voltage output from the magnetic flux detection unit, and the count number is decreased according to the number of pulses of the pulse voltage output from the reference magnetic flux output unit. The measuring device according to any one of claims 1 to 5. A magnetic flux detection unit composed of a superconducting quantum interference device outputs a pulse voltage at a frequency corresponding to the input magnetic flux to be measured,
The reference magnetic flux output unit outputs a pulse voltage at a constant frequency,
A pulse counter unit inputs the pulse voltage, outputs a predetermined value based on the number of pulses,
The magnetic flux tank unit cancels the detection magnetic flux based on the detection magnetic flux generated in one direction based on the pulse voltage output from the magnetic flux detection unit and the pulse voltage output from the reference magnetic flux output unit. Generate a differential magnetic flux between the generated reference magnetic flux and
The magnetic flux tank unit further inputs at least a part of the differential magnetic flux generated therein by magnetic coupling in a direction to cancel the measurement target magnetic flux to the magnetic flux detection unit. Measuring method.

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