JP2856463B2 - Magnetic flux trap release device in superconducting circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary of the Invention] The present invention relates to a magnetic flux trap release device in a superconducting circuit for releasing magnetic flux trapped in a superconducting circuit such as a SQUID (superconducting quantum interference device) or a Josephson circuit. The body is placed in close proximity to the superconducting circuit, but there is no need to consider the shape of the superconducting circuit individually. With a single resistor arrangement, any superconducting circuit can release the magnetic flux trap. An object of the present invention is to provide a device having a simple configuration, in which a resistor circuit is arranged close to a superconductor or a superconductor circuit including a superconductor and a Josephson junction, and the resistor circuit includes a plurality of resistance units. The resistance units are arranged so that two or more resistance units extend in close proximity to each other, and a power source is connected to the resistance units so that the sum of the currents flowing through the adjacent resistance units is zero.

[Industrial applications]

The present invention relates to a magnetic flux trap release device in a superconducting circuit for releasing a magnetic flux trapped in a superconducting circuit such as a SQUID (superconducting quantum interference device) or a Josephson circuit.

The superconductor changes its state from a normal conduction state to a superconducting state by cooling to a temperature lower than a critical temperature. At this time, when cooled in a magnetic field such as geomagnetism, the magnetic flux is confined to the superconductor in a state of interlinking. This is called a magnetic flux trap. If the superconductor is made into a thin film and the thin film is made to be in a superconducting state strictly from one end to the other end, a magnetic flux trap should not occur, but the cooling is not performed uniformly and the superconducting part is a normal conducting part. When a state occurs that surrounds the superconducting portion, external magnetic flux such as geomagnetism that has passed through the normal conducting portion is surrounded by the superconducting portion. You will be trapped. This trapped magnetic flux causes the operation of superconducting circuits such as SQUID magnetic field sensors and Josephson logic / memory circuits to malfunction. Therefore, it is necessary to eliminate the magnetic flux trap.

[Conventional technology]

Conventional methods for preventing magnetic flux traps include using a magnetic shield or a superconducting shield made of a material such as permalloy to reduce the terrestrial magnetism around the superconducting circuit to be superconducted, A method of drilling a hole in a specific place in the superconducting ground plane (so-called mote), passing current through a resistor, temporarily raising the temperature of the superconducting circuit above the critical temperature, and then lowering the temperature again. It was done. In mode method, a hole in the ground plane surrounding the circuit portion which flux traps not want occurs, the area of the circuit portion S, the circuit unit in the S × B <φ _0/2 the flux density as B is The magnetic flux is not allowed to remain (the magnetic flux in the superconducting loop is limited to an integral multiple of the magnetic flux quantum φ ₀ ), and the magnetic flux exits to the surrounding holes (the magnetic flux is trapped in the holes). We expect that trapping, which is a stochastic phenomenon, will not occur when the temperature is increased or decreased.

However, in the former method or the method combining the two, there is a limit that can be shielded. For example, in a large superconducting circuit such as a SQUID used as a magnetic field sensor coupled with a multi-turn input coil, eliminating a flux trap Can not. In addition, since the magnetic field generated by the current flowing through the resistor again causes a magnetic flux trap, if the resistor is sufficiently separated from the superconducting circuit, or if the resistor is placed in close proximity to the superconducting circuit, it is symmetrical to the superconducting circuit. It is necessary to reduce the total magnetic flux linked to the superconducting circuit by the magnetic field generated in the position to zero. In the former case, it is not easy to give a temperature distribution on the superconducting circuit as compared with the case where the resistors are close to each other, and the superconducting circuit is warmed in a spatially uniform state. Thus, when cooled again, there is a high probability that the entire superconducting circuit transitions to superconducting almost simultaneously, thus causing a flux trap. In the latter, look at the shape of each superconducting circuit,
The arrangement of the resistors must be determined, which is inconvenient.

Regarding the removal of the residual quantum magnetic flux from the superconducting thin film, there is a method disclosed in Japanese Patent Publication No. 1-451212. Since the residual quantum flux cannot be completely removed by the current heat flash method, a small area of the superconducting thin film being cooled is heated to normal conductivity by heating with a laser beam, and the heated area is directed to the edge of the thin film. Moved and included in the heating area. Quantum magnetic flux is discharged from the thin film.

In this embodiment, a scanning device that rotates and moves up and down with respect to a cylindrical superconducting thin film is provided, and an SQ attached to the scanning device is provided.
The UID detects in advance where the magnetic flux trap is on the thin film, stores it, and irradiates an area with the magnetic flux trap with a laser beam irradiation unit attached to the scanning device and heats it.
It becomes normal conducting and guides the region to the open end of the cylinder. SQUID
Inspection of the magnetic flux trap is carried out, and the above processing is repeated until it disappears.

However, this method requires a scanning mechanism, a laser irradiation mechanism, and the like, and the shape of the superconducting thin film is limited by rotation and linear scanning.

[Problems to be solved by the invention]

According to the conventional method of removing the magnetic flux trap, it is difficult to reliably remove the magnetic flux in the above case. In the above method, it is difficult to remove the large superconducting circuit in relation to the size (S) of the superconducting circuit. This causes a magnetic flux trap, and if it is separated therefrom, it is difficult to eliminate the magnetic flux trap. If the magnetic flux trap is brought close, it is troublesome to reduce the combined magnetic field to zero in a superconducting circuit.

The present invention has been made in order to improve such a point, and the resistor is arranged close to the superconducting circuit. However, it is not necessary to individually examine the shape of the superconducting circuit. It is an object of the present invention to provide a device having a relatively simple configuration that can release a magnetic flux trap in any superconducting circuit in an arrangement.

[Means for solving the problem]

As shown in FIG. 1, in the present invention, a resistance circuit 20 is provided near the superconducting circuit 10, and the superconducting circuit is heated by the resistance circuit to change from superconducting to normal conducting to superconducting.

The superconducting circuit 10 may be any including a superconductor or Josephson junction, and the figure shows a SQUID consisting of two Josephson junctions, a superconducting inductor 14, and a superconducting wire connecting them. Reference numeral 16 denotes a resistor, and the output is a voltage drop generated in the resistor.

The resistance circuit 20 includes a plurality of resistance units 22a, 22b,... The resistance units are arranged so that two or more resistance units extend in close proximity. These resistance units are connected to a power supply to supply a current and generate heat, but the sum of the currents flowing through the adjacent resistance units is zero. For example, the resistance units 22a and 22b are adjacent and extend side by side, but they are connected at one end to a power supply 24a, and the other end is connected by a superconducting wire 26 to form a closed circuit. The current is drawn from one end of 22a. That is, I
₂₁ + I ₂₂ = 0. The resistor 22c, 22 d, ...... 22i is extending alongside close, current I ₁₁ and connect the power 24b to these,
I ₁₂ ,..., I _ig flow, but I ₁₁ + I _12, ..., I _ig = 0.

The connection in units of resistance may be any of series, parallel, and series-parallel. The resistance units 22a and 22b are in series, and the resistance units 22c to 22i are in parallel.

The resistance units are unevenly distributed or the current flowing is uneven so that the temperature distribution of the superconducting circuit heated by the resistance circuit has one or more peaks and valleys. When a plurality of peaks and valleys are provided, the current flowing through each resistance unit is changed so that these positions change with time and move from one end to the other end of the superconducting circuit.

[Action]

As described above, in the present invention, the resistor circuit is energized to generate heat,
Heating a superconducting circuit placed in close proximity. Superconducting circuit is liquid
It is cooled by He or the like and is in a superconducting state. However, when the temperature rises by the above-mentioned heating and exceeds a critical temperature, the superconductor transits to normal conduction. Thereafter, when the current is reduced, the temperature decreases, and when the temperature becomes lower than the critical temperature, the state transits to superconductivity again. Since the current flows so that the sum of the spatial current vectors becomes almost zero, the further away from the resistance circuit, the more the criticality created by the current flowing through each resistance unit cancels out. The conducting circuit will transition to superconducting with little or no perception of the magnetic field created by the resistive circuit, thus allowing the transition between normal and superconducting to occur without any additional flux trapping.

When the superconducting circuit is small, the transition between normal conduction and superconductivity is performed by heating the resistor in this way, thereby releasing the magnetic flux trap. However, when the superconducting circuit is large, even if transition between normal conduction and superconductivity is performed, the probability that a magnetic flux trap remains somewhere increases. For this, non-uniform heating is performed.

As shown in FIG. 2 (a), the resistance circuit 20 has its resistance unit.
When the 22a, 22b,... Are densely arranged at one end and coarsely arranged toward the other end, the temperature rising state changes even when a current of the same value is applied to each resistance unit, as shown in FIG. 2 (b). The temperature T of the superconducting circuit is higher at one end and lowers toward the other end. By performing such non-uniform heating, once as shown in FIG. 2 (b), the entire region of the superconducting circuit is brought to a temperature higher than the critical temperature Tc so as to be in a normal conduction state. ), (D),
(E), it starts to return to superconductivity from the other end, spreads to one end, and eventually one end also returns to superconductivity. In this way, the magnetic flux trap and the geomagnetism are pushed from the other end to one end, and are discharged from the one end to the outside of the superconducting circuit. Thus, even in a large superconducting circuit, a magnetic flux trap is hardly generated.

In the case of an even larger superconducting circuit, a single peak or valley will not create a sufficiently large temperature gradient, so in this case the temperature gradient is made periodic as shown in FIG. For example, a plurality of temperature gradients may be created.
In this example, two sets of resistors for forming three peaks and valleys are prepared by changing the value of the current flowing in each resistance unit of the resistance circuit 20 so that the positions of the peaks and valleys of the temperature distributions formed by the respective circuits are shifted by half a cycle. Keep it. First, the current of the first set (φ ₁ ) of the resistor is increased (FIG. (B)) and then reduced, (c)
The trapped magnetic flux gathers at the position shown in the figure. Next, the second set (φ
₂ ) A current is passed through (d), and the trapped magnetic flux collected as shown in FIG. By repeating the above current supply φ ₁ → φ ₂ , all trapped magnetic flux can be pushed to the left end.

Flux traps and geomagnetism such phi _t accumulates the peak portion of the temperature T, is discharged from the superconducting part of the valley, since the waveform of the temperature T proceeds to left, also travels to the left it as the magnetic flux phi _t, eventually left From the superconducting circuit.

The current waveform may be a sine wave, a trapezoidal wave, or any other appropriate waveform such as a waveform having a continuously decreasing portion, and need not be a periodic waveform. The currents flowing in adjacent resistance elements may or may not overlap. Power supply is 2 phase, 3
The temperature distribution may be a sawtooth wave as shown in FIG. 3 or a symmetrical triangular wave as shown in FIG. In FIG. 4, the resistance unit group is driven in three phases.

Instead of advancing the distribution of the temperature T from one end of the superconducting circuit to the other, it may be driven from the center to the periphery.

〔Example〕

An embodiment of the present invention is shown in FIG. The superconducting circuit 10 and the resistance circuit 20 are formed on substrates 10a and 20a, respectively. In (a), the substrate 10a is stacked on the substrate 20a so that the resistance circuit 20 is located between these substrates. In (b), the substrates 10a and 20a are stacked back to back, and the resistance circuit 20 The superconducting circuit 20 is on these lower surfaces.

The resistance units can be arranged in thin films on the substrate 20a by integrated circuit technology. The resistance circuit 20 may be composed of only a resistor, or may include a resistor and a superconductor connecting the resistor or an insulator for insulating between the resistors. From the viewpoint of reducing the magnetic field acting on the superconducting circuit to zero, one of the current going and the returning may be a superconductor. A substrate 10a including a superconducting circuit is arranged close to the resistance circuit. It is preferable to place them at an appropriate distance so that the magnetic field generated by the current flowing through the resistor is sufficiently canceled. However, with one conductor, the magnetic field generated by the current flowing through the conductor only decreases in proportion to the reciprocal of the distance r. Since the magnetic field generated by the current decreases in proportion to 1 / r ³ , the separation distance may be small, and depending on the arrangement of the resistors, may be about the thickness of the substrate (for example, about 0.3 mm). Therefore, the substrate 10a may be disposed so as to be in close contact with the substrate 20a. It is also possible to arrange them at appropriate intervals. As shown in FIG. 6, one substrate 10b
A superconducting circuit and a resistor circuit may be arranged on the front and back of the above. As the arrangement of the resistance units, various combinations such as juxtaposition of two resistance units, juxtaposition of four resistance units, and stacking are possible as shown in FIGS. 7 (a) and 7 (b). Seventh
Field in FIG. (B) is attenuated in proportion to 1 / r ^5.

In the drawings, the line insulator, the interlayer insulating layer, the surface insulating layer, and the like are omitted, but they are of course provided as appropriate.

Removing the flux trap also removes the permanent current. When an external magnetic flux tries to interlink with the superconductor, a current flows to cancel it. Generally, a superconducting circuit is provided on a ground plane, and the canceling current flows to the ground plane.

〔The invention's effect〕

As described above, according to the present invention, the magnetic field generated by the current flowing through the resistor can be canceled, and the magnetic flux trap of the superconducting circuit can be released by the transition between superconductivity and normal conduction. Since the magnetic field is canceled by the arrangement of the resistors, the flux trap can be released by one resistor circuit in any circuit regardless of the shape of the individual superconducting circuit.

Further, since the transition is performed by making the temperature distribution on the superconducting circuit non-uniform, or the temperature distribution is moved with a peak or valley, the magnetic trap can be reliably released.

[Brief description of the drawings]

FIG. 1 is a principle diagram of the present invention, FIGS. 2 to 4 are explanatory diagrams of temperature distribution examples 1 to 3, and FIGS. 5 to 7 are explanatory diagrams showing first to third embodiments of the present invention. . In FIG. 1, 10 is a superconducting circuit, 20 is a resistance circuit, 22a, 22b,.
Is a resistance unit, and 24a and 24b are power supplies.

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 39/00 H01L 39/02-39/04 H01L 39/22-39/24 H01L 39/14-39/16 G01R 33 / 035

Claims (5)

(57) [Claims] 1. A resistance circuit is disposed in close proximity to a superconductor or a superconductor circuit including a superconductor and a Josephson junction. The resistance circuit includes a plurality of resistance units and flows through the plurality of resistance units. Two or more resistance units are arranged in close proximity so that a magnetic field acting on the superconducting circuit as a current is zero, and the power supply is connected so that the sum of the currents flowing through these close resistance units is zero. A magnetic flux trap release device in a superconducting circuit, which is connected to a unit. 2. The temperature distribution at the time of heating the superconducting circuit by the resistance circuit by changing the arrangement density of the resistance unit of the resistance circuit with respect to the superconducting circuit and / or changing the value of the current flowing in the resistance unit depending on the location. 2. The device according to claim 1, wherein the temperature is higher at one end of the circuit and decreases toward the other end. 3. A power supply is brought close to the resistance unit so that the value of the current flowing in the resistance unit of the resistance circuit with respect to the superconducting circuit varies depending on the location and time, and the temperature distribution when the superconducting circuit is heated by the resistance circuit is 2. A superconducting circuit having a plurality of peaks and valleys between one end and the other end thereof and moving toward the other end.
A device for releasing a magnetic flux trap in a superconducting circuit as described in the above. 4. The semiconductor device according to claim 1, wherein the resistor circuit is formed on a first substrate, and the superconducting circuit is formed on a second substrate, and the first and second substrates are arranged so as to overlap with each other. 4. The device for canceling a magnetic flux trap in a superconducting circuit according to any one of items 1 to 3. 5. A circuit according to claim 1, wherein a resistance circuit is formed on one surface of the substrate, and a superconducting circuit is formed on the other surface of the substrate.
4. The device for canceling a magnetic flux trap in a superconducting circuit according to any one of items 1 to 3.