JPH0640124B2 - Digital squid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] Regarding a digital squid used for measuring a minute magnetic flux or current, a feedback circuit of the digital squid can be configured only by a superconducting circuit including a Josephson junction and a superconducting inductance. With the aim of enabling digital squid to have multiple channels and a single chip, for example, to obtain a large number of information at a plurality of locations at the same time in a magnetocardiograph and to enable detailed diagnosis at high speed. It has a Josephson circuit for converting a negative output pulse into a positive or negative flux quantum, and a superconducting loop including the Josephson circuit and magnetically coupled to the squid. And the loop current flowing in the superconducting loop is fed back to the squid. To do.

[Industrial application field]

The present invention relates to a digital squid used for highly sensitive magnetic flux and current measurement.

A superconducting loop that includes two or more Josephson junctions and a superconducting inductance is squeeze in a narrow sense.
cting Quantum Interference Device)
When there are two Josephson junctions, it is also called a two-junction quantum interferometer. At present, this is an element that can detect magnetic flux (current) with the highest sensitivity. On the other hand, squid in the title is a general term that has a function as a magnetometer including circuits such as detection system and feedback system. The system.

Therefore, the squid in the narrow sense is indicated as SQUID here.

The analog squid, which is commercially available, has already been used in a medical device for obtaining a magnetocardiogram, a magnetoencephalogram, etc. by detecting a magnetic flux generated by a minute electric current flowing in the nervous system of the body.

In place of the electrocardiogram etc. which have been used to detect voltage, these obtain similar or finer body information by detecting magnetic flux.

In addition, SQUID has not been successfully detected yet, but it has been applied to the detection of gravitational waves.

The analog squid applies a dc bias to the SQUID, and an output can be obtained with a voltage or current proportional to the input signal.
In contrast, the digital squee
Then, it is possible to output with the number of pulses that is proportional to the input signal and has the same period as the bias ac.

[Conventional technology]

Conventional analog squid requires many room-temperature devices such as lock-in amplifiers, so it was difficult to increase the number of channels needed to simultaneously measure the magnetic flux of each part of the body and create a map.

Since the digital squeeze outputs the output in pulses as described above, it is possible to feed back by a superconducting circuit composed of Josephson junctions, etc., and a feedback circuit can be made on the chip, so that it is easy to make multiple channels. It was

FIGS. 4 (1) to 4 (3) are a circuit diagram, an operation explanatory diagram and an output pulse diagram of a digital squid according to a conventional example, respectively.

This digital squeeze has been proposed by the present inventor, and seeks the measured magnetic flux Φ _C by feeding back a current _If that is proportional to the difference between the number of positive output pulses and the number of negative output pulses. It is the one.

The measured magnetic flux Φ _C is Φ _C = (L ₁ + L ₂ ) × I _C. It is represented by. Here, I _c is the current to be measured, L ₁ is the pickup coil, L ₂ is the magnetic coupling to SQUID, and both are superconducting inductances that form the superconducting loop to be measured.

In this method, the amount of feedback is the difference between the positive and negative pulse numbers, and the absolute value of each is not necessary.

Next, the operation of this digital squeeze will be described.

In the figure, a superconducting loop including _two Josephson junctions J ₁ and J ₂ and a superconducting inductance L is called a two-junction SQUID.

For a pickup coil with superconducting inductance L ₁ ,
When the measured magnetic flux Φ _c intersects, the measured current I _C flows in the measured superconducting loop so as to cancel this magnetic flux, and this is 2
A magnetic field is created in the junction SQUID, the characteristics of the two-junction SQUID are changed, and the magnetic flux is obtained from the change.

A first ac waveform generator 1 injects a first ac current into the injection (bias) terminal of the two-junction SQUID.

Superconducting inductance L magnetically coupled to a two-junction SQUID
_A second ac current having a frequency lower than that of the first ac current is supplied to the _third AC waveform generator 2.

FIG. 3 (2) is a diagram showing the threshold characteristics of the two-junction SQUID, in which the vertical axis represents the injection current I _G and the horizontal axis represents the measured current I _C.

The voltage of the SQUID is 0 when the operating point is inside the curve, and the Josephson junction is in the voltage state when the operating point is outside the curve.
Is a finite voltage.

Now, make the waveform of the second ac current a triangular wave as shown in the figure,
Its positive half-cycle is t ₁ to t ₂ , and its negative half-cycle is t ₂ to t _3.
And

First, in the vicinity of t ₁ , I _C is near point A (OA corresponds to the difference between the current obtained by converting the measured magnetic flux Φ _C and the feedback current _If ), and even if the injection current I _G is entered here, the operating point Does not exceed the threshold and SQUID does not switch.

When I _C increases with the lapse of time from t _1, from a certain point, the operating point exceeds the threshold value when the injection current I _G enters, and the SQUID switches to output a voltage. Injection current I
_{When G} drops, its output voltage becomes zero. That is, one output pulse appears for each injection current pulse.

Since the threshold characteristic is asymmetrical, when I _C is on the positive side, the output is switched to the positive side and not to the negative side.

On the contrary, in the half cycle of t ₂ to t ₃ , the output switches to the negative side,
Do not switch to the positive side.

When the center of the triangular wave is at the origin and I _C = 0, the threshold characteristic is symmetrical with respect to the origin, so that the output has the same number of switches to the positive side and the number of switches to the negative side. However, I _C ≠
When it is 0, the threshold characteristic is left-right unbalanced,
When I _C is positive, there are many positive output pulses, and when I _C is negative, there are many negative output pulses.

Therefore, the difference between the positive and negative output pulse numbers corresponds to the input current I _C.

The measurement system actually uses a null method (zero method) by adding a feedback circuit as shown in Fig. 3 (1).

The output is extracted from the injection terminal B of the two-junction SQUID, and the output pulse shown in Fig. 4 (3) appears here.

This is put into the up / down counter 3, integrated by the integrator 4, converted into the feedback current _If which is an analog amount by the D / A converter 5, and returned to the superconducting inductance L _f magnetically coupled to the two-junction SQUID.

The sign of the returning method is such that the current flows in the negative direction on the I _c axis when there are many positive output pulses. If the opposite is the case, reverse.

The output may take I _f or take the digital pulse number corresponding to I _C from the middle of the feedback circuit.

[Problems to be solved by the invention]

In the conventional digital squeeze, it is difficult to fabricate a D / A converter in a feedback circuit consisting of a superconducting circuit as the number of bits increases. This is because it is difficult for a Josephson circuit to obtain highly accurate constant-current characteristics with a transistor in a D / A converter such as a semiconductor circuit.

On the other hand, an object of the present invention is to provide a digital squeeze feedback circuit, that is, a D / D of about 12 to 15 bits
By realizing the A conversion function and the up-down counter and integration function of the previous stage with a superconducting circuit composed of Josephson junctions, etc., to facilitate its integration and enable digital squid to have multiple channels. is there.

[Means for solving problems]

To solve the above-mentioned problems, a superconducting inductance and a squid including two or more Josephson junctions, an inductance that allows a current to be measured to flow and is magnetically coupled to the squid,
An AC waveform generator that applies a bias current of an AC waveform to the squid, the digital squid being configured to inject an AC waveform current into the squid and take out an output in a pulse train, It has a Josephson circuit for converting a negative output pulse into a positive or negative flux quantum, and a superconducting loop including the Josephson circuit and magnetically coupled to the squid. Is stored in the superconducting loop and thereby the loop current flowing in the superconducting loop is fed back to the squid.

[Action]

The function required for a digital squeeze feedback circuit is to find the difference in the number of positive or negative pulses generated from the squeeze and to create a feedback current proportional to the integral value of this pulse number difference.

This feedback circuit consists of a superconducting Josephson circuit.

FIG. 1 is a block diagram of a digital squeeze illustrating the concept of the present invention.

In the figure, the squid 12 is injected with a bias current from the AC waveform generator 11 and outputs a positive or negative pulse train.
The superconducting loop 14 including the Josephson circuit 13 is made, and this Josephson circuit 13 receives the output pulse from the squid 12, and puts or takes out one quantum magnetic flux Φ ₀ into the superconducting loop 14 per pulse. To do so.

That is, one positive magnetic flux quantum is inserted for one positive pulse, and one negative magnetic flux quantum is inserted for one negative pulse (or one positive magnetic flux quantum is output).

Since it is a superconducting loop, the magnetic flux stored here is permanently stored and does not disappear unless it is put in or taken out. In addition, this operation includes the integral function because it adds or decreases the magnetic flux quantum one by one.

As the feedback current, the loop current I _L flowing in the superconducting loop 14 generated by storing the magnetic flux quantum is used, and this may be magnetically coupled to the squid 12.

〔Example〕

FIG. 2 is a circuit diagram of a digital squeeze for explaining an embodiment of the present invention.

In the figure, the first SQUID 12 is composed of Josephson junctions J ₁ and J ₂ and a two-junction SQUID including a superconducting inductance L.
And magnetically coupled to this, and the measured current I _C = Φ _C / (L
₁ + L ₂ ) has a superconducting inductance L ₂ and an injection terminal B also serving as an output terminal is provided at an asymmetrical position of the first SQUID 12, where a bias current amplitude-modulated by the promotion modulation waveform generator 11A is provided. injecting I _G.

In the first SQUID 12, an output pulse depending on the difference between the measured magnetic flux Φ _C and the fed back magnetic flux is generated. That is, if the fed-back magnetic flux is positive, the number of output pulses on the negative side is larger than that on the positive side, and if the fed-back magnetic flux is negative, the number of output pulses on the positive side is larger than that on the negative side.

This output pulse is shunted to the resistors R ₁ and R ₂ , and the magnetic field coupling current I _C passing through the resistor R ₁ is magnetically coupled to the Josephson circuit 13 by the superconducting inductance L _{5 and} injected through the resistor R _2. The current I _g is injected into the Josephson circuit 13.

In this way, the current pulse is added to the Josephson circuit 13 that has the function of converting it into magnetic flux quanta.

In this example, the Josephson circuit 13 is a Josephson junction.
A second SQUID including a two-junction SQUID including J ₃ and J ₄ and a superconducting inductance L _4, and a superconducting loop 14 including the second SQUID and the superconducting inductance L ₃ is provided.

The resistance r is a damping resistance so that the change of the loop current when the flux quantum is put into or out of the superconducting loop 14 by the second SQUID does not overshoot.

Now, the operation of converting a current pulse into a magnetic flux quantum by utilizing the threshold characteristic of the second SQUID will be described with reference to FIG.

FIG. 3 is a diagram showing threshold characteristics of the second SQUID.
The vertical axis represents the injection current I _g , and the horizontal axis represents the magnetic field coupling current I _C.

In the figure, the threshold characteristics shown by the solid line are SQUIDs that switch to the voltage state when the operating point crosses from the origin side, but the threshold characteristics shown by the dotted line where the threshold characteristics overlap each other. Even if the voltage exceeds the threshold, it does not transit to the voltage state, but it enters into the adjacent threshold characteristic.

When no current is flowing in the superconducting loop 14, the second SQ
The UID has an operating point of 0.

When a positive pulse is applied to the second SQUID, it is shunted by the resistors R ₁ and R ₂ and the operating point moves obliquely. That is, the operating point is 0
It moves as → A → 0, but one positive flux quantum is stored in the superconducting loop 14 by crossing twice the threshold value of the mode transition represented by the dotted line.

Next, the operation during this period will be described in detail.

When a positive pulse is applied to the second SQUID, first, when the pulse rises, and 0 → A, the threshold value (right side) of the dotted line is crossed once, the Josephson junction J ₃ of the second SQUID is
Becomes a voltage state momentarily, and immediately returns to 0 voltage. As a result, one flux quantum Φ ₀ enters the second SQUID.

Next, at the trailing edge of the pulse A → 0, once the threshold value (left side) of the dotted line is crossed, the Josephson junction J ₄ temporarily switches and transiently outputs a voltage pulse, and as a result, the second pulse
Sends out one magnetic flux quantum Φ ₀ in the SQUID to the superconducting inductance L ₃ on the right side.

After all, when the operating point moves from 0 → A → 0, one flux quantum Φ ₀ passes through the Josephson junctions J ₃ and J ₄ and crosses the threshold of the dotted line one by one to the superconducting inductance L ₃ . enter.

Similarly, when a negative pulse is applied to the second SQUID, the threshold characteristic shifts to the third quadrant, the operating point moves from 0 → B → 0, and one negative flux quantum is generated in the superconducting loop. Stored in 14.

When storing a magnetic flux in the superconducting loop 14, resulting loop current I _L flows, the operating point from the current applied to the I _g, for example 0 '→ A' → 0 ' , 0' → B '→ 0' etc. Becomes

The loop current I _L is I _L = nΦ ₀ / L ₃ .., where n is the number of accumulated magnetic flux quanta Φ ₀ . , The injection current into the second SQUID becomes I _g + I _L , and the origin of the coordinates deviates from 0 to 0 ′.

If 0 ′ is greatly deviated from 0, the magnetic flux quantum cannot be taken in or out, or the second SQUID makes a malfunction such as transition to a voltage state. However, if the inductance L _{3 of the} superconducting loop is made sufficiently large, the loop current I _L becomes smaller, 0 '
Does not deviate significantly from zero.

In this way, magnetic flux quanta corresponding to the difference in the number of output pulses are stored in the superconducting loop 14, and a loop current I _{L that} is almost proportional to this flows. This loop current I _L is magnetically coupled to the first SQUID 12 for feedback.

By this feedback, the measured magnetic flux Φ _C
A magnetic flux coupled to SQUID12, the feedback current I _L and the magnetic flux coupled to the first SQUID12 cancel substantially.

As for the output, this feedback current may be read or extracted from the output pulse as in the conventional example.

〔The invention's effect〕

As described in detail above, according to the present invention, the feedback circuit of the digital squid can be constituted only by the superconducting circuit composed of the Josephson junction and the superconducting inductance. Etc. becomes easy.

Therefore, a large number of pieces of information at a plurality of places can be obtained at the same time in a magnetocardiograph, and a detailed diagnosis can be performed at high speed.

[Brief description of drawings]

FIG. 1 is a block diagram of a digital squeeze for explaining the concept of the present invention, FIG. 2 is a circuit diagram of the digital squeeze for explaining one embodiment of the present invention, and FIG. 3 is a threshold value of a second SQUID. FIGS. 4 (1) to 4 (3) are diagrams showing the characteristics, respectively, a circuit diagram, an operation explanatory diagram and an output pulse diagram of a conventional digital squeeze. In the figure, 11 is an AC waveform generator, 12 is the first SQUID, J ₁ and J ₂ are Josephson junctions, L is a superconducting inductance, 13 is a Josephson circuit, and the second SQUIDs, J ₃ and J ₄ are Josephson junction, L ₄ is a superconducting inductance, 14 is a superconducting loop, L ₃ is a feedback superconducting inductance, I _C is the magnetic field coupling current to the second SQUID of the output, Φ _c is the measured magnetic flux, L ₁ superconducting inductance pickup coil, L ₂ is superconducting inductance of the measured superconducting loop, L ₅ is superconducting inductance magnetic field coupled to the second SQUID, B is injection terminal, I _G is the first SQUID the injection current, I _g of current injected into the second SQUID, I _L is the loop current feedback, R _1, R ₂ is shunt resistor.

Claims (1)

[Claims] 1. A squid including a superconducting inductance and two or more Josephson junctions, an inductance for applying a current to be measured and magnetically coupling to the squid, and an AC waveform generating for applying an AC waveform bias current to the squid. And a digital circuit configured to inject a current of an alternating waveform into the squid and take out the output as a pulse train.
A squid having a Josephson circuit for converting a positive or negative output pulse into a positive or negative flux quantum, respectively, and a superconducting loop including the Josephson circuit and magnetically coupled to the squid, A digital squid characterized in that it stores the magnetic flux quantum in a superconducting loop and thereby feeds back the loop current flowing in the superconducting loop to the squid.