JPS6317350B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing magnetic flux traps in Josephson effect elements.

In recent years, the Josephson effect device, which consists of a pair of superconducting thin films sandwiching the top and bottom of an insulating layer as a tunnel barrier layer, has attracted attention as a low-power, ultra-high-speed calculation device, and research into integrating this device has been active. .

As the integration of Josephson effect elements progresses, a major problem arises: magnetic flux trap.

Magnetic flux trap refers to the stochastic trapping of magnetic flux in a pair of upper and lower superconducting thin film layers in the structure of the Josephson effect element described above. When this magnetic flux trap occurs, the original Josephson of the Josephson effect element This significantly impairs the effectiveness, significantly narrows the operating range of the integrated Josephson circuit, or causes malfunction.

In principle, this magnetic flux trap can be removed by once raising the temperature of the Josephson effect element in which the magnetic flux is trapped above the superconducting transition temperature and returning it to a normal conduction state.

However, conventionally, one of the following two methods has been used to remove the Josephson effect element according to the above principle.

[Pull-up method] The Josephson effect element or its integrated circuit, which has been immersed in a cooling medium such as liquid helium and has undergone superconducting transition, is once pulled out of the cooling medium,
Returns to normal conduction state due to natural temperature rise.

[Resistance heating method] Resistance heating is applied to the holder holding the Josephson effect element or its integrated circuit, thereby indirectly heating the Josephson effect element or the integrated circuit as a whole, and causing a transition to a normal conduction state.

However, the above-mentioned method is a mechanical method in which the Josephson effect element or its entire integrated circuit must be pulled out of the cooling medium each time, and therefore has the drawback of poor workability and being time-consuming.

In addition, in the above method, since it is an indirect method via a holder, the amount of heat from the heating heat source is wasted, and the surrounding cooling medium is also heated, so if the cooling medium is, for example, expensive liquid helium, etc. , the cost losses would be immeasurable.

The present invention has been made in view of the above-mentioned circumstances, and provides a method that does not have the drawbacks of the conventional examples described above, that is, a Josephson effect element that is easy to work with, is inexpensive, and can reliably remove magnetic flux traps. The main purpose of this work was to provide a method for removing magnetic flux traps.

In the present invention, the principle of removing the magnetic flux trap is the same as in the conventional example, and the magnetic flux trap is removed by once returning the Josephson effect element to a normal conduction state, but this method is superior. It is.

One feature of the present invention is that light energy is used as a heat source to return the Josephson effect element to a normal conduction state. This leads to the location of the Josephson effect element. In particular, if the light energy is radiated just above the surface of the Josephson effect element to be treated, the effect on the surrounding cooling medium can be kept to a minimum and no unnecessary evaporation occurs. Of course, it is an important requirement that the Josephson effect element to be processed be able to be processed in a non-contact manner. It is desirable that the light energy be one that guarantees good absorption at the relevant junction of the Josefson effect element and has good heat conversion efficiency, but it is generally not one with properties that are strictly specified, and can be anything from the far infrared light region to the visible range. Light or laser light covering an optical range can be used.
However, laser light is convenient in terms of focusing, energy efficiency, and controllability.

The accompanying drawings show an example of an apparatus for carrying out the invention, and to describe an embodiment of the invention in this apparatus, Josephson effect elements or their integrated circuits typically carry integrated circuits of this type. It is formed on a substrate or chip 1 and supported in a suitable holder part 2 in a cooling container 5.

A cooling medium 4 such as liquid helium is filled in the cooling container 5 to maintain the Josephson effect element integrated circuit at a temperature below the superconducting transition temperature.

The lead-out line 8 to the outside of the Josephson effect element or its integrated circuit is supported by a suitable holding element 9, which in this case also supports the optical waveguide or optical fiber 3. The material of the optical waveguide must have as little loss as possible due to absorption and transmission, therefore have little effect on the surrounding cooling medium, and must be able to withstand temperature changes between extremely low temperatures and room temperature. In this respect, there are quite a lot of optical waveguides made of materials that can be used without any shortage in practice. However, among others,
Optical fibers whose core and cladding are both made of quartz are most desirable. This is because it has low optical loss, therefore generates little heat itself, and can be easily obtained in any desired diameter, from minute diameters to relatively large diameter bundles.

One end of the optical waveguide 3 extends outward from the cooling container 5 and serves as an input end for light from a suitable light source 6. On the other hand, the other end enters into the cooling container 5 and constitutes an output end of optical energy directly above the Josephson effect element substrate 1.

The light emitting source or light energy source 6 may be a natural light source or a discharge energy conversion type light source, but is preferably a suitable laser oscillation source. It is possible to obtain large output with a small size, has good directivity due to good coherency, and has a reduced diameter.
This is because the enlargement operation can be easily performed.

In the present applicant's practical example, a continuous wave He-Ne laser oscillator is used as the optical energy source 6, and its output beam is introduced into the input end of the quartz optical fiber 3 via the focusing lens 7. I have to.

To describe the actual situation in the above practical example,
A Josephson effect element whose lower electrode is a Pb/Au/In alloy film, whose tunnel barrier film is a Pb oxide, and whose upper electrode is a Pb/Au alloy film is immersed in a cooling container using liquid helium as a cooling medium. output
Laser light from a 20 mW continuous wave He--Ne laser oscillator 6 was introduced into a quartz optical fiber with a core diameter of 80 μm, which can be used even in liquid helium, and was irradiated to the Josephson effect element directly above the Josephson effect element. The light irradiation diameter on the Josephson effect element substrate at this output end was about 500 μm, and the amount of light on the Josephson effect element surface was about 3.8 W/cm ² .

Under these conditions, the Josephson effect device transitioned from a superconducting state to a normal conducting state within a few seconds after being irradiated with light. Also, according to the principle mentioned above,
It was confirmed that the magnetic flux trap had been removed.

When applying this method to the removal of magnetic flux traps in integrated circuits of actual Josephson effect elements, by appropriately setting the output beam diameter, it is possible to Only a group of effect elements may be selectively processed, or conversely, the beam diameter may be widened and the entire integrated circuit on the substrate 1 may be processed at once. Mechanical systems for fine adjustment of optical waveguides can be made with sufficient precision using existing techniques, and can even be done on the μm scale. Of course, a light beam scanning method can also be considered. In the case of such scanning, the optical waveguide may be mechanically swung, but it is preferable to sway the light beam using an electrical signal via an optical system or an electro-optical system at the output end.

As described above, according to the present invention, it is possible to provide a method for removing magnetic flux traps in Josephson effect elements that successfully eliminates all the unreasonable and uneconomical drawbacks of the conventional methods, and also improves workability. It is good, the working time is extremely short, and the negative effects on the cooling medium can be kept to a minimum.
Maintenance and management of the Josephson effect element circuit network in its mounted state can be performed, and in the future it will make a major contribution to the realization of ultra-high-speed computers using this type of Josephson effect element.

[Brief explanation of the drawing]

The drawing is a schematic diagram of an example of an apparatus used to carry out the magnetic flux trap removal method of the present invention. In the figure, 1 is a substrate carrying the Josephson effect element or its integrated circuit, 3 is an optical waveguide, 4 is a cooling medium, 5 is a cooling container, and 6 is a light energy source.

Claims (1)

[Scope of Claims] 1. A method for removing magnetic flux traps in a Josephson effect element cooled to below the superconducting transition temperature in a cooling container, the method comprising: A Josephson effect element, characterized in that the light energy is guided into the cooling container through an optical waveguide and irradiated onto the surface of the Josephson effect element, and the optical energy is used as a heating source to transition the Josephson effect element to a normal conduction state. How to remove magnetic flux traps.