JP3103866B2 - Pulse output type squid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse output type squid which uses a superconducting quantum interferometer (SQUID), which is useful as an ultra-sensitive magnetic sensor, in a digital system.

[0002]

2. Description of the Related Art A junction in which a thin insulator barrier is sandwiched between superconductors is called a tunnel-type Josephson junction. As an application example, a superconducting quantum interferometer (ie, squid) is widely known. This squid is applied to magnetic measurement, exploration of minerals and geothermal sources, meteorological observation, or measurement of magnetic properties of materials and precise electromagnetic measurement, which occur with activities of the heart, muscles, eyes, and the like.

A squid having two Josephson junctions related to the present invention has a Josephson junction formed of a tunnel junction. Typical the I-V characteristic of the tunnel junction is a characteristic with hysteresis as shown in FIG. 1, the transition to promptly voltage state when the voltage is to a certain current value I _c reaches I _c does not occur I do. The voltage _Vg is determined by the material forming the junction. When the current is further increased, the state where the proportional relationship between the current and the voltage is maintained as in the case of the one having a finite electric resistance.
On the other hand, while the current there when going subtracting current from the voltage V _g is returned to 0 through the path shown in the figure decreases. The same happens with opposite currents and voltages.

[0004] Two junctions having such hysteresis are arranged on one link. The current terminals are arranged in such a position in the squid loop such that each junction has one junction in each branch. The presence or absence of voltage generation between the voltage terminals is determined by whether or not each Josephson junction is in a voltage state according to the current value flowing from the current terminals.

More importantly, the threshold current value of the voltage generation changes periodically with the magnitude of the magnetic flux quantum as one cycle as the external magnetic flux applied to the loop changes. The inside surrounded by the threshold current value curve expressed as a function of the external magnetic flux is a zero voltage state, and the outside is a voltage state. That is, the curve changes periodically with respect to the external magnetic flux by the threshold current value itself, and the cycle is a magnetic flux quantum.

FIG. 4 shows a block diagram of a conventional drive detecting device. In this method, the bias current source 24
Then, an AC bias as shown in FIG. The operating point of the squid is point A shown in FIG. The magnetic flux corresponding to φ ₁ in FIG. 5B is supplied from the variable voltage source 26 to the adder 27 and the voltage / current converter 2.
8 from a feedback coil 29.

The output pulse from the squid is supplied to an amplifier 30.
The output is amplified to about the TTL level, the output is shaped by a single-shot pulse generator 31, and converted into an analog signal by an inverting integrator 32 at the next stage. FIG. 5C shows the output of the single shot pulse generator 31. The output of the adder 33 is supplied from the variable voltage source 34 to the inverting integrator 3 corresponding to the case where the voltage transition probability of the squid is １ ／.
Since the output of No. 2 is canceled, the output is inverted, but becomes an output corresponding to the applied external magnetic field.

A negative feedback current is applied to this output through a switch 35, an adder 27, and a voltage / current converter 28 to a feedback coil 29 to cancel an applied external magnetic field. The change in the external magnetic field can be known by monitoring the output of the adder 33. The frequency counter 36 has a role of monitoring the frequency of the bias current and the frequency of the output pulse of the output pulse generator 31 of the single-shot pulse generator 31.

[0009]

In such a drive detection device of the prior art, the voltage transition probability of the squid changes even when the amplitude of the bias current or the critical current value generated when the temperature of the squid fluctuates. However, since this is detected as a change in the external magnetic field, there arises a problem that noise superimposed on the bias current or a change in temperature lowers the sensitivity of the squid.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to compensate for fluctuations in output pulses due to fluctuations in bias current applied to the squid and fluctuations in output pulses due to changes in the critical current of the squid due to temperature changes. The purpose is to improve the sensitivity.

[0011]

According to the present invention, in order to solve the above-mentioned problems, an AC bias current is applied to a pulse output type squid, and an offset of a magnetic field is further applied to the squid to obtain a squid threshold curve. Having two or more operating points, detecting the amount of output pulse from the operating point,
By performing arithmetic processing, a signal corresponding to noise superimposed on the bias current is discriminated from a signal corresponding to the external magnetic field applied to the squid, and each signal is negatively fed back to perform control by the null method. Adopt means.

More specifically, the present invention provides a pulse output type squid having two Josephson junctions and a bias current injection terminal in a superconducting closed loop to provide an offset of a magnetic field and to plot a squid threshold curve. A magnetic field modulated wave source having two or more operating points, a device for calculating a change in the probability of occurrence of output pulses from the two or more operating points and detecting noise of bias current and / or temperature change of the squid, and an applied magnetic field The present invention provides a pulse output type squid having a device for calculating a change in the pulse occurrence probability at two or more operating points with respect to a change in the pulse output type.

[0013]

The critical current value of the two-junction squid has a dependence on the applied magnetic field (φ) as shown at 38 in FIG. The squid is in a superconducting state inside the threshold curve and in a voltage state outside. Squid 3 in Figure 2
A bias current as shown in FIG. 7 is applied (a sine wave, a triangular wave or the like may be used). The magnitude of the amplitude is equal to or less than the critical current value of the squid (I _c1 in the figure). In this state, 39 in FIG.
By giving a magnetic field modulation waveform as shown in FIG. 2, A on the threshold curve of the magnetic field dependence of the critical current shown at 38 in FIG.
The points B and B can be set as operating points of the squid.

The points A and B serve as operating points while alternating in a half cycle of the magnetic field modulation waveform. That is, when the cycle of the magnetic field modulation waveform is T, the operating points A and B alternately become T / 2 by T / 2. At point A and point B, there is a probability that the squid is in a voltage state beyond the threshold curve due to the amplitude of the bias current during a half cycle of the magnetic field modulation waveform. By properly adjusting the amplitude of the magnetic field modulation waveform and the bias current, the probability of occurrence of this voltage can be reduced to 50%.
The generation probability of the pulse in the half cycle of the magnetic field modulation waveform at the point A and the point B is such that a pulse of 1/4 of the frequency of the bias current is generated, and 1 cycle of the frequency of the bias current in one cycle of the magnetic field modulation waveform. Two pulses will be generated. This can be realized because the threshold curve fluctuates due to thermal noise.

The output pulses at the operating points A and B are shown at 40 and 41 in FIG. 2, respectively. When an external magnetic field is applied to the squid in this state, the occurrence probability of the pulses at the operating points A and B changes, and by detecting this, the change in the magnetic field can be known. With respect to the applied magnetic field, one of the changes in the occurrence probability of the two types of pulses of the operating points A and B output from the squid decreases, and the other increases. However, when noise from a commercial power supply or the like overlaps with the bias current, or when the bias current changes due to fluctuations in the temperature of the squid, the occurrence probability decreases or increases simultaneously at both operating points.

Using this phenomenon, it is possible to distinguish a change in critical current value due to noise or temperature fluctuation in bias current and a change in applied magnetic field from a change in pulse generation probability at each operating point. The actual control method of the drive detection device is
This is a null method in which a pulse at each operating point is detected, an operation is performed using those values, an output corresponding to a change in the applied magnetic field and an output corresponding to noise are obtained, and negative feedback is applied to each. Negative feedback to the squid with respect to the applied magnetic field is performed from a feedback coil placed near the squid by converting the obtained output into a current. Therefore, the change in the applied magnetic field can be known by monitoring the negative feedback current flowing through the feedback coil. Negative feedback on the noise component is performed by superimposing the output of the noise component on the bias current.

[0017]

Next, the embodiments will be described in detail. FIG. 3 shows a block diagram of the squid and the drive detection device. An alternating current as shown at 37 in FIG. 2 from the bias current source 1 and a magnetic field modulated wave as shown at 39 in FIG. 2 from the magnetic field modulated wave source 3 via the feedback coil 4 are applied to the squid 2. The pulse voltage generated from the squid is
The signal is amplified to about TTL level by the amplifier 5. This is to make the voltage easy to handle in the semiconductor circuit, but is not always necessary.

In the next stage, the amplified output pulse is supplied as shown in FIG.
8 is a circuit for determining which of the operating points A and B is the pulse generated. The pulse discrimination is performed by taking the signal of the magnetic field modulated wave source 3 into the multiplexer 6 and switching at the rise to each operating point. The pulse generated at the operating point A is output to the single shot pulse generator 7, and the pulse generated at the operating point B is output to the single shot pulse generator 7 '. The outputs of the single shot pulse generators 7, 7 'are monitored by a frequency counter 23. This single shot pulse generator 7,
7 'has the function of always correcting the pulse height and pulse width to the same size.

The outputs of the single-shot pulse generators 7 and 7 'are converted into analog signals by the integrators 8 and 8', and the integration corresponding to when the voltage transition probability of the squid is 1/4 of the frequency of the bias current at each operating point. The voltages which cancel the outputs of the adders 8 and 8 'are supplied from the variable voltage sources 9 and 9', and the adders 10 and 9 '
The operation at 10 'is performed with the code shown at the input of the adder. The output of each adder 10, 10 'is connected to terminals 11, 1
It can be monitored at 1 '. Adders 10, 10 '
Is calculated with the sign indicated at the input terminal by the adder 12 to obtain an output corresponding to the magnetic field applied to the squid.

This signal is sent to the integrator 1 via the switch 13.
At 4, the signal is made a first-order lag, the current is converted by the current-voltage converter 16 through the adder 15, and non-feedback is applied through the adder 21 so as to cancel the applied magnetic field from the feedback coil. The output corresponding to the applied magnetic field can be known by monitoring the terminal 18. The variable voltage source 17 adjusts the offset of the operating point. Next, adder 1
When the outputs of 0 and 10 'are operated by the sign indicated at the input terminal by the adder 19, the output of the adder 19 outputs a temperature change and a bias current noise component. Switch 2 to this component
Non-feedback is applied to the bias current by the adder 21 'via "0". This noise component can be monitored at terminal 22.

[0021]

The sensitivity of the squid is improved because noise superimposed on the bias current or noise due to temperature change can be distinguished from the signal. Since the temperature can be compensated, the allowable range of the stable temperature environment required for the squid is widened, which is advantageous for direct cooling by a refrigerator or the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing current / voltage characteristics of a tunnel type two junction squid.

FIG. 2 is a diagram showing a relationship between a bias current waveform, a magnetic field dependence of a critical current, a magnetic field modulation waveform, and an output pulse.

FIG. 3 is a circuit diagram of one embodiment of the present invention.

FIG. 4 is a circuit diagram of a conventional example.

FIG. 5A is a bias current waveform diagram of a conventional example.
(B) is a magnetic field dependence diagram of the critical current of the tunnel type two junction squid. (C) is an output pulse diagram of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Bias current source 2 Squid 3 Magnetic field modulation wave source 4 Feedback coil 6 Multiplexer 7 Single shot pulser 9 Variable voltage source 10 Adder 12 Adder 16 Current-voltage converter

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshiaki Takai, Furo-cho, Chikusa-ku, Nagoya-shi, Aichi Prefecture (Nanbanashi) Inside Nagoya University (72) Inventor Akira Fujimaki, Furo-cho, Chikusa-ku, Nagoya-shi, Aichi (Banchi Nashi) (56) References JP-A-63-149914 (JP, A) JP-A-63-290979 (JP, A) JP-A-64-21378 (JP, A) JP-A-64-21379 (JP, A) A) JP-A-1-307681 (JP, A) JP-A-2-257076 (JP, A) JP-A-3-20684 (JP, A) JP-A-3-1977887 (JP, A) JP-A-3 -238378 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01R 33/00-33/18

Claims (3)

(57) [Claims] 1. A pulse output type squid having two Josephson junctions and a bias current injection terminal in a superconducting closed loop has a magnetic field offset and has two or more operating points on a threshold curve of the squid. A magnetic field modulated wave source to be applied, a device for calculating a change in the generation probability of output pulses from two or more operating points to detect noise of a bias current and / or a temperature change of a squid, and two or more operations for a change in an applied magnetic field A pulse output type squid having a device for calculating a change in a pulse generation probability at a point. 2. The pulse output type squid according to claim 1, wherein there are two operating points, and the magnetic field modulation waveform from the magnetic field modulation wave source is an AC waveform. 3. The pulse output type squid according to claim 2, further comprising a multiplexer for receiving a pulse voltage generated by the squid and an AC waveform from a magnetic field modulated wave source, and discriminating output pulses generated at each operating point.