JPH09257894A - Superconducting quantum interference device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a magnetic field with high sensitivity even out of a magnetic shield by connecting a resistor in parallel with a series circuit of Josephson junctions having sufficiently larger output voltage than a DC superconducting quantum interference device output voltage and the same critical current value as the optimum driving current of the device. SOLUTION: A superconducting quantity interference device 13 comprises a Josephson junction 11, a resistor 12 and a DC superconducting quantity interference device 1. The output voltage of the junction 11 is set to sufficiently larger than the output voltage of the device 1, and the critical current of the junction 11 is set to the same as the bias current necessary to drive the device 1. When a predetermined voltage is applied to the device 13 by a bias power source 2, the device 13 satisfies the driving conditions, the current flowing to the junction 11 does not change, and hence the current flowing to the device 11 does not alter, external magnetic field to output voltage characteristics are held, and no deterioration of the accuracy and sensitivity of the detected magnetic field occurs.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting quantum interference device used outdoors and under an environment where electromagnetic waves fluctuate.

[0002]

2. Description of the Related Art FIG. 4 shows a structure for operating a conventional superconducting quantum interference device, 1 is a DC superconducting quantum interference device, 2 is a DC bias power supply, 3 is a bias cable,
4 is a sensor output cable, 5 is a dewar for cooling, 6 is a drive circuit for magnetic field detection, 7 is an output of a magnetometer, and 8 is an external magnetic field incident on the superconducting quantum interference device 1. Further, in the broken line frame in FIG. 4, 9 indicates a low temperature region during operation and 10 indicates a room temperature region during operation.

A voltage is generated in the sensor output cable 4 by supplying an appropriate DC bias current Ib from the DC bias power source 2 to the DC superconducting quantum interference device 1 through the bias cable 3. In this state, the superconducting quantum interference device 1 outputs the voltage shown in FIG. 51 to the sensor output cable 4 with respect to the external magnetic field 8. Hereinafter, the characteristic of FIG. 5 will be referred to as a Φ-V characteristic. The voltage generated in the sensor output cable 4 is detected as a magnetic field by a magnetic field detection drive circuit 6 such as a flux locked loop, which is a known technique, and is output to the magnetometer output 7.

[0004]

When the DC superconducting quantum interference device 1 is used outside the magnetic shield, electromagnetic waves such as broadcast waves enter the bias cable 3. The mixed electromagnetic waves are applied to the DC superconducting quantum interference device 1 in the form of electric current. As a result, the normal Φ-V characteristic shown in FIG. 5 is compressed in the vertical direction, and the peaks and valleys are displaced in the horizontal direction, resulting in the Φ-V characteristic as shown in FIG. The magnetic field detection drive circuit 6 is Φ− in FIG.
Since the magnetic field is detected assuming the V characteristic, for the Φ-V characteristic of FIG. 6 which is different from the assumption, (1) compression in the vertical direction deteriorates the S / N of the detected magnetic field, and (2) the lateral direction. The deviation in the direction is the deviation from the true value of the detected magnetic field. In order to avoid this, a method of installing the DC bias power source 2 as close as possible to the superconducting quantum interference device 1 and shortening the bias cable 3 can be considered. However, the operating low temperature region 9 and the operating room temperature region 10 are possible. Between them, there is a wall surface of the cooling dewar 5 for shutting off heat, and there is always a certain amount of space between them.
Since the DC bias power source 2 does not operate at a low temperature, it cannot be installed inside the cooling dewar 5, and as a result, the bias cable 3 cannot be made sufficiently short. Therefore, when the conventional DC superconducting quantum interference device 1 is used outside the magnetic shield, there is a problem that its sensitivity is significantly deteriorated and an accurate magnetic field is not shown.

The present invention has been made in order to solve such a problem, and an object thereof is to obtain a superconducting quantum interference device which is accurate and does not deteriorate in sensitivity even in an environment where the electromagnetic wave environment changes. .

[0006]

A superconducting quantum interference device of the first invention comprises a Josephson junction showing a constant critical current value, a DC superconducting quantum interference device for magnetic field detection, and the Josephson junction. And a resistor for voltage control connected in parallel to a series circuit of a DC superconducting quantum interference device.

A second aspect of the present invention is the superconducting quantum interference device of the first aspect, in which the output voltage of the Josephson junction is set higher than the output voltage of the DC superconducting quantum interference device.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1. FIG. 1 shows a first embodiment of a superconducting quantum interference device according to the present invention. 1 in FIG.
Is exactly the same as the above conventional device. Further, 11 is a Josephson junction, 12 is a resistor, and 13 is a superconducting quantum interference device including the entire structure. The output voltage of the Josephson junction 11 is set sufficiently higher than the output voltage of the DC superconducting quantum interference device 1, and the critical current of the Josephson junction 11 is the bias current required to drive the DC superconducting quantum interference device 1. Set the same as Ib.

FIG. 2 is a schematic diagram showing the relationship between the voltage Vj applied to both ends of the Josephson junction 11 and the current Ij flowing through the Josephson junction 11 at that time. According to FIG. 2, the Josephson junction 11 has a voltage Vj at a current Ij = Ic.
Changes from 0 to ΔV, almost no current change occurs. However, ΔV is a constant characterizing the junction.

A voltage of about ΔV / 2 is applied across the resistor 12. The output voltage of the Josephson junction 11 is set to be sufficiently higher than the output voltage of the DC superconducting quantum interference device 1. Therefore, at both ends of the Josephson junction 11,
A voltage of approximately ΔV / 2 is applied. At this time, the critical current Ic flows through the Josephson junction 11, and accordingly, the bias current appropriate for driving flows through the DC superconducting quantum interference device 1, so that the Φ-V characteristic shown in FIG. 5 is obtained. FIG. 3 shows a configuration for driving the superconducting quantum interference device 13. In FIG. 3, 2 to 10 are the same as those of the conventional device. A voltage of ΔV / 2 is applied to the DC superconducting quantum interference device 13 from the DC bias power supply 2. Since the superconducting quantum interference device 13 satisfies the above driving conditions, it exhibits the Φ-V characteristic of FIG. Therefore, the magnetic field detection drive circuit 6 outputs the magnetometer output 7 as in the prior art.

Next, consider the case where a disturbance due to electromagnetic waves is introduced. It is assumed that electromagnetic waves are applied to the bias cable 3 during magnetic field detection. Electromagnetic waves become voltage, and the bias cable 3
Is applied to the superconducting quantum interference device 13 via. At this time, if the amplitude of the current is within the range of ± ΔV / 2,
Due to the property of the Josephson junction 11 shown in (4), the current flowing in the Josephson junction 11 does not change, and therefore the current flowing in the DC superconducting quantum interference device 1 does not change. Therefore, FIG.
Since the .PHI.-V characteristic shown in (3) can be maintained, the detection magnetic field does not deteriorate in accuracy and sensitivity.

As described above, the superconducting quantum interference device of the present invention has a DC superconducting quantum interference device and an output voltage sufficiently larger than the output voltages of the DC superconducting quantum interference device and the DC superconducting quantum interference device. By connecting a Josephson junction having the same critical current value as the optimum drive current of in series and connecting a resistor in parallel to the series circuit and controlling the voltage across the resistor, By keeping the flowing current constant and allowing an appropriate bias current of the DC superconducting quantum interference device to flow, it is possible to prevent the voltage due to the electromagnetic waves mixed in the bias cable from shifting the bias current of the DC superconducting quantum interference device, The magnetic field can be detected with high sensitivity even outside.

[0013]

According to the present invention, it is possible to detect a magnetic field with high accuracy without sensitivity deterioration even in a bad electromagnetic environment.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a first embodiment of the present invention.

FIG. 2 is a diagram showing a current-voltage characteristic of a Josephson junction.

FIG. 3 is a diagram showing a configuration for driving Embodiment 1 of the present invention.

FIG. 4 is a diagram showing a configuration for driving a conventional superconducting quantum interference device.

FIG. 5 is a diagram showing a Φ-V characteristic in a normal state.

FIG. 6 is a diagram showing a Φ-V characteristic affected by an electromagnetic wave.

[Explanation of symbols]

1 DC superconducting quantum interference device, 2 DC bias power supply,
3 Bias cable, 4 Sensor output cable, 5
Dewar for cooling, 6 magnetic field detection drive circuit, 7 detection magnetic field,
8 external magnetic field, 9 low temperature region during operation, 10 room temperature region during operation, 11 Josephson junction, 12 resistor, 13 superconducting quantum interference device.

Claims (2)

[Claims] 1. A superconducting quantum interference device for applying a DC bias current, comprising: a Josephson junction showing a constant critical current value; a DC superconducting quantum interference device for magnetic field detection; and the Josephson junction. A superconducting quantum interference device, comprising: a voltage control resistor connected in parallel to a series circuit of a DC superconducting quantum interference device. 2. The superconducting quantum interference device according to claim 1, wherein the output voltage of the Josephson junction is set higher than the output voltage of the direct current superconducting quantum interference device.