JP2871772B2 - Superconducting circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] With respect to a superconducting device, an object of the present invention is to provide a superconducting circuit capable of counting up to a larger number of pulses with the same inductance as that of a conventional device. When a predetermined current pulse is input to the injection terminal and the second inductance of the two-junction quantum interference device while forming a conduction loop and having a second inductance magnetically coupled to the two-junction quantum interference device. In the superconducting circuit in which one flux quantum is stored in the superconducting loop every time one current pulse is input, and the stored flux quantum is proportional to the number of current pulses, the current pulse flowing through the second inductance is A resistor is inserted in series in the passage, and a correction nonlinear line including a Josephson junction is provided at a stage subsequent to the injection terminal of the two-junction quantum interference device. It is configured to insert a shaped circuit.

[Industrial applications]

The present invention relates to a two-junction quantum interference device and a superconducting circuit having a superconducting inductance.
The present invention relates to a superconducting circuit used for a high-sensitivity magnetic field sensor using a conducting quantum interfernce device (superconducting quantum interference device).

In fields such as biomagnetic measurement, SQUIDs (hereinafter referred to as squids) are used as indispensable devices. In this field, many squids are arranged to measure the magnetic field distribution of organs such as the brain and heart in a short time. A multi-channel squid system is desired and is being developed.

[Conventional technology]

When a conventional superconducting circuit is used for a high-sensitivity magnetic field sensor, it is excellent to use a current pulse from a multi-channel squid system as described above.To realize such a multi-channel squid, Digital squid is considered useful (for example, as technical literature, see D. Drung, Cryogenics vol. 26 pp. 623-627.1986 and Fuji
maki et al. IEEE Electron Devices vol.35, No.12 (198
8) See pp. 2412-2418). This is because, while many conventional squids are so-called dc-SQUIDs and operated analogly, digital squids output pulses, so the output S / N can be increased and digital processing can be performed as it is. There is an advantage that there is.

Also, the feedback circuit was realized by a superconducting circuit,
The present applicant has previously proposed a so-called one-chip squid (see Japanese Patent Application No. 62-177515), in which a feedback circuit is integrated on the same chip as a squid sensor. And has the advantage of reducing crosstalk between channels.

As a conventional superconducting circuit of this type, for example, there is a circuit as shown in FIG. In the figure, 1 is 2
It is a junction quantum interference device and is constituted by Josephson junctions J ₁ and J ₂ and a first inductance L ₀ . L _S is a superconducting inductance, and a superconducting loop 2 is formed by the superconducting inductance L _S and the two-junction quantum interference device 1.
The second inductance L _X is magnetically coupled (coupling coefficient is M) to the two-junction quantum interference device 1, and a current pulse from the digital squid 3 is applied to the second inductance L _X and the two-junction quantum interference device 1. I ₁ and I ₂ are input, respectively. Although the current pulses I ₁ and I ₂ may be equal, for example, 5
It may be input at a ratio of about twice.

In this superconducting circuit, when current pulses I ₁ and I ₂ are input, each time one pulse is input, one flux quantum is applied to a superconducting loop 2 including a two-junction quantum interference device 1 and a superconducting inductance L _S. Operate to add. Then, a positive magnetic flux quantum is added to a positive current pulse, and a negative magnetic flux quantum is added to a negative current pulse. As a result, the stored magnetic flux is proportional to the number of input pulses. This circuit is equivalent to the operation of the (up-down) counter.

Next, the above operation will be described on the threshold characteristics of the two-junction quantum interference device 1 in FIG. In the threshold characteristic, curves of the same shape are arranged periodically in the horizontal axis direction, and FIG. 6 shows only two curves related to the circuit operation. First, when there is an operating point in the curve, the two-junction quantum interference device 1 is in a zero voltage state. When the currents I ₁ and I ₂ are increased beyond the solid lines on the threshold curve, the state transits to the voltage state. Also, when you cross over the broken line,
Josephson junction J ₁ or J ₂ is shifted to transiently voltage state, by approximately one placed in the two-junction quantum interference device 1 loop and enters the next threshold curve (referred to mode transition). Usually, trapezoidal current waveforms are applied as the current pulses I ₁ and I ₂ . With proper design of the magnetic field junction, I ₁
It can be equal the value of I _2, for example, connected to, as shown in FIG. 5 (b) second inductance L _X other end two injection terminal of the junction quantum interference device 1 (in the figure, T point) However, the operation can be performed by applying only one current from the outside.
In any case, the current increases, the operating point in Figure threshold characteristic motion on a straight line O-P _0, the Josephson junction J ₁ is switched transiently at _one point P, the flux quantum is It enters the two-junction quantum interference device 1. In the figure, M ₁ is a threshold characteristic of a mode transition in which the Josephson junction J ₁ transiently switches, and M ₂ is a threshold characteristic of a mode transition in which the Josephson junction J ₂ transiently switches. It is.

Next, when the pulse ends, the operating point returns from P ₀ to O,
Then, when the operating point crosses P ₂ , the Josephson junction J ₂ is now transiently switched, and the flux quantum in the two-junction quantum interference device 1 is transferred to the superconducting loop 2. Thus,
Flux quanta are added each time a pulse is created. Although the figure shows a case of a positive pulse, the same applies to a case of a negative pulse. At this time, a current I _S flows through the loop corresponding to the magnetic flux Φ stored in the superconducting loop 2. This current value if superconducting inductance L _S is large, represented approximately Φ / L _S. Since the current I _S is flowing through the two-junction quantum interference device 1 through the injection terminal, as the magnetic flux Φ increases, the trajectory of the operating point from the O-P _0, moves parallel to the bottom. Then, when the magnetic flux Φ further increases and reaches O′−P ₀ ′ in the figure, the mode transition at the point P ₂ causes the transition of J ₁ instead of J ₂ to be transiently switched, and the transfer of the flux quantum is performed. Become impossible. This determines the maximum number of pulses the circuit can count.
A typical design value of the current I _S is about 5% of the sum I ₀₁ + I ₀₂ of the critical current values of the Josephson junctions J ₁ and J ₂ .

[Problems to be solved by the invention]

However, in such conventional superconducting circuit, when used as a counter device for superconducting, the maximum value that can be counted is determined by the maximum value of the I _S, in order to increase the maximum value of the count Is the superconducting inductance
L _S had to be increased.

However, in practice, the size of the inductance produced by the integrated circuit technology is limited, and there is a disadvantage that a large inductor occupies a large area.

Therefore, an object of the present invention is to provide a superconducting circuit that can count up to a larger number of pulses with the same inductance as that of the related art.

[Means for solving the problem]

The superconducting circuit according to the present invention has a superconducting loop formed by the two-junction quantum interference device and the superconducting inductance because of the above-described configuration, and has a second inductance magnetically coupled to the two-junction quantum interference device. , 2
When a predetermined current pulse is input to the injection terminal of the junction quantum interference device and the second inductance, the current pulse becomes 1
In each superconducting loop, one flux quantum is stored in the superconducting loop each time a current is input. At the same time, a correction non-linear circuit including a Josephson junction is inserted after the injection terminal of the two-junction quantum interference device.

[Action]

According to the present invention, when a current pulse is input, it passes through a correction nonlinear circuit including a Josephson junction in a predetermined range.
I ₂ flows, Josephson junctions between the correction nonlinear circuit exceeds this range becomes the chromatic voltage state, so that current flows I ₁ also towards the second inductance L _X.

Therefore, the threshold characteristic of the superconducting circuit is changed as shown in FIG.
It changes like -Q ₀ -P ₀ and can count up to a larger number of pulses with the same inductance as before.

〔Example〕

 Hereinafter, the present invention will be described with reference to the drawings.

1 to 4 are views showing one embodiment of a superconducting circuit according to the present invention. In the description of this embodiment, the same components as those of the conventional example are denoted by the same reference numerals, and redundant description will be omitted. FIG. 1 is a basic circuit diagram of a superconducting circuit.
Inductance L _X left side in the figure to the resistance r of are inserted in series in the passage of the current pulse I ₁ flowing in the second inductor L _X. The resistor r may be inserted on the right side in the figure in the inductance L _X. On the other hand, a correction non-linear circuit 10 including a Josephson junction is inserted after the injection terminal T of the two-junction quantum interference device 1. Correcting non-linear circuit 10 is composed Josephson junction J ₃ and the resistor R as shown in FIG. 2, when designing the resistor R to an appropriate value, the current - voltage characteristic becomes the form as shown in FIG. 3 . That is, in the predetermined range the current flowing through towards the Josephson junction J _3, outside this range if becomes Josephson junction J ₃ is perforated voltage state, because the current is also towards the resistance R flows, after all, the It exhibits current-voltage characteristics as shown in FIG.

A specific circuit for supplying the output pulse from the digital squid 3 to the two-junction quantum interference device 1 is also shown in FIG.
The street, and via the resistor R _S in synchronism with the AC bias R _T resistor
Are input to the two-junction quantum interference device 1.

In the above configuration, when the current pulse from the digital slid 3 is input as a trapezoidal pulse, the characteristic of the correction nonlinear circuit 10 is as shown in FIG.
First I ₂ flows through towards the correction nonlinear circuit 10 which includes a Josephson junction J ₃ is in a predetermined range, the Josephson junction J ₃ is closed the voltage state of the correction nonlinear circuit 10 exceeds this range, the to 2/5 of the inductance L _X to flow a current I ₁ through the resistor r.

Accordingly, the operating point trajectory on threshold characteristics of the superconducting circuit is as O-Q ₀ -P ₀ shown in Figure 4. If the resistance r is actually designed to be, for example, 1/14 times as large as R, the operating point locus will be like this. Therefore, when the accumulated magnetic flux increases and the superconducting loop current I _S increases, this operating point locus moves downward in FIG. 4, but since this locus is convex upward, a larger loop any current I _S, the flux quantum is transferred. The 4 O'-Q ₀ in FIG. '-P _0' gives the maximum value of the flux quantum transfer but, loop current I _S in this case, the Josephson junction J ₁ in the present embodiment, J ₂ of the critical current Sum of values I
The value is about 20% of ₀₁ + _I02 , and can be counted up to four times larger than the conventional example. Therefore, even if the superconducting inductance for accumulating magnetic flux has the same value as the conventional one, the maximum value of the number of pulses that can be counted can be increased.

〔The invention's effect〕

ADVANTAGE OF THE INVENTION According to this invention, the threshold value characteristic of a superconducting circuit can be changed, and it is possible to count up to a larger number of pulses while keeping the superconducting inductance for superconducting storage at the same value as in the past.

[Brief description of the drawings]

1 to 4 are views showing an embodiment of a superconducting circuit according to the present invention, FIG. 1 is a view showing a basic circuit thereof, FIG. 2 is a view showing a detailed circuit thereof, FIG. Fig. 4 is a diagram showing the characteristics of the nonlinear circuit for correction, Fig. 4 is a diagram showing the threshold characteristics thereof, Figs. 5 and 6 are diagrams showing a conventional superconducting circuit, and Figs. 6B is a diagram showing the circuit, and FIG. 6 is a diagram showing the threshold characteristics. 1 ... 2 junction quantum interference device, 2 ... superconducting loop, 3 ... digital squid, 10 ... nonlinear circuit for correction, 11 ... Josephson logic gate, L _O ... first inductance, L _X ... second inductance, L _S ... superconducting inductance, J ₁ , J ₂ , J ₃ ... Josephson junction, R, r, R _S , R _T ... resistance.

Claims (1)

(57) [Claims] 1. A superconducting loop is constituted by a two-junction quantum interference device and a superconducting inductance.
The device has a second inductance magnetically coupled to the junction quantum interference device. When a predetermined current pulse is input to the injection terminal and the second inductance of the two junction quantum interference device, one current pulse is input. In the superconducting circuit in which one flux quantum is stored in the superconducting loop each time the stored flux quantum is proportional to the number of current pulses, a resistor is inserted in series with the path of the current pulse flowing through the second inductance. A superconducting circuit, wherein a non-linear circuit for correction including a Josephson junction is inserted after the injection terminal of the two-junction quantum interference device.