JPH0259653B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention switches a Josephson element using a magnetic field generated from a magneto-optical conversion element (hereinafter referred to as a photo-induced magnetic field) or a magnetic field formed by a current induced in a coil wound around this element. This relates to a Josephson element drive device, in particular, by using an optical fiber cable with low thermal conduction loss and a wide transmission band for transmitting the signal light irradiated to the magneto-optical conversion element, the high-speed response of the Josephson element is achieved. The aim is to significantly reduce the power consumption required to maintain the device at a low temperature without damaging it.

The Josephson device has a structure in which an insulating layer of several nanometers is sandwiched between superconductors, and its small size makes it excellent as a component of integrated circuits with large integration density.It also consumes little power, meaning it generates no heat. Since the amount is small, it is also convenient for increasing the degree of integration. Furthermore, the Josephson element is a functional element that can perform high-speed switching in a very weak magnetic field (approximately 2×10 ^-7 Gauss).

Therefore, if it were possible to create something on the same scale as today's ultra-large computers using only this element, the size of the computer body would be only 10 mm on a side.
It is predicted to have a cube size of about cm, consume only about 10W of power, and be more than 10 times faster at calculation speeds. However, since this device is housed in a refrigerator using liquid helium and is cooled below the temperature at which it becomes a superconductor, in order to switch the device, the signal transmission cable must be kept at room temperature. Wiring must be done from the input and output terminals located at the terminal. The use of such cables has the disadvantage that heat flows in through the wires, and this heat increases the amount of cryogen consumed.

Furthermore, since the input terminal and Josephson element are electromagnetically coupled, there is a problem in that induced noise from the outside is transmitted through the cable and is likely to cause malfunction.

Therefore, an object of the present invention is to use a magneto-optical conversion element and to use an optical fiber cable with low thermal conduction loss for transmitting the signal light irradiated to the above-mentioned magneto-optical conversion element, in order to eliminate such drawbacks. Therefore, it is an object of the present invention to provide a Josephson element driving device that can prevent induction noise from entering and can significantly reduce power consumption for helium liquefaction.

In order to achieve such an object, the present invention provides a driving device for a Josephson element in which a pair of superconducting lines are opposed to each other with an insulating layer interposed therebetween, in which a magneto-optical conversion element is irradiated with light to generate a magnetic field, and this magnetic field generates a magnetic field. This device is characterized in that the above-mentioned Josephson device is switched.

The present invention will be described in detail below with reference to the drawings.

First, FIG. 1 shows the basic structure of a tunnel junction Josephson device considered as a computer device.

In Figure 1, there is a number of tunnel junctions between the counter electrode 1 and the base electrode 2 made of superconductors.
A thin oxide film 3 on the order of nm is arranged. Even if there is no potential difference between the two superconductors 1 and 2, a device current 5 of about 1 mA flows through the oxide film 3 due to the tunnel effect. Next, when an input signal current 6 of about 0.5 mA is applied to the input signal line 4, a magnetic field is applied to the joint 3 of the electrodes by this current, and a superconducting material of about 2.5 mV is applied between the electrodes 1 and 2. A gap voltage determined by the voltage is generated. This voltage is a binary number “1”
Furthermore, by making the above-mentioned zero voltage state correspond to "0", the Josephson element is used as a logic element.

The present invention provides (1) a device that generates a current flowing through an input signal line 4 by a combination of a magneto-optical conversion element and a pickup coil, and drives a Josephson element with this current; and (2) a device that drives a Josephson element with the current. The present invention relates to a device for driving a Josephson element by directly applying a magnetic field generated from the junction part 3 to the junction part 3.

Photo-induced magnetization will be explained below.

By exciting magnetic ions in a material with light, it is possible to change the magnetization of the material.
This is called photo-induced magnetization, and the magnetic field generated from light-induced magnetization will be called photo-induced magnetic field.

The time change m(t) in the intensity of photo-induced magnetization is the magnetic moment of the magnetic ion in the ground electronic state.
mg, and when the excited state is me, it can be expressed as m(t)=n(t)(me(t)−mg(t)) (1). Here, n(t) is the time change in the number of excited magnetic ions.

As photo-induced magnetized substances, ruby (molecular formula: Al ₂ O ₃ :Cr), which is a paramagnetic substance, and erbium orthochromite, ErCrO ₃ , which belongs to a weakly ferromagnetic substance, are known.

FIG. 2 shows a block diagram of the optically induced magnetization measurement system for ErCrO ₃ . An external magnetic field Ha is applied parallel to the C axis of ErCrO ₃ single crystal 10 placed in liquid helium 9 in container 8 . ErCrO ₃ single crystal 1 was used to detect the induced magnetic field in the C-axis direction.
Wind the pick-up coil 11 around 0.

The voltage v∝−dφ/dt [V] induced in this coil 11 is led to a two-phenomenon oscilloscope 13 via a coaxial cable 12 (Zin=50Ω). On the other hand, the excitation laser beam L1 is made incident on the beam splitter 14, a part L2 of the excitation laser beam L1 is made incident on the magneto-optical conversion element 10, and the other part L3 is received by the biplanar phototube 15. The excitation light and the photo-induced magnetization signal voltage are observed simultaneously with a two-phenomenon oscilloscope 13.

Figure 3 shows the dependence of the observed photoinduced magnetization signal voltage on the external magnetic field. In this case, a ruby laser (wavelength: 6943 Å, excitation power: approx.
4 mJ) to excite the R absorption band of ErCrO ₃ . The excitation laser pulse waveform is shown at the bottom of the figure. The induced voltage changes depending on the strength of the external magnetic field, but when Ha is 940 Oe or more, the waveform resembles an excitation light pulse, and the peak value of the induced voltage is about 1.8 V when Ha = 1.2 kOe. Note that the measurement conditions in this case are that the excitation power is approximately 4 mJ and the pick-up coil is 1.
Number of turns of 1: 7 turns, input impedance of oscilloscope 13 Zin = 50Ω, thickness of ₃ ErCrO elements
200μ, area 4mm x 4mm, and temperature of ErCrO ₃ is
It is 4.2K.

Next, the current value that can be extracted by such photo-induced magnetization will be described.

In liquid helium 9, pick-up coil 1
Since superconducting wire can be used as 1,
The DC resistance becomes zero. By shortening the wiring of the pick-up coil 11 so that the inductance and capacitance are small and making them equal, it is possible to lower the impedance, and it is possible to make the value Zin = about 1Ω. . As a result, the current flowing due to photo-induced magnetization is i=v/Zin=1.8A (1800mA) (2). Since the control current ic of the Josefson element is about 0.5mA as mentioned above, the above-mentioned current i
= 1800mA is 3600 times the IC. Furthermore, if the number of turns of the pick-up coil 11 is 10 turns, the thickness of the magneto-optical conversion element ErCrO ₃ 10 is 400μ, and the area is 1cm ² , then
The current value increases by about 6×10 ⁴ times, and in order to flow a current of about 0.5 mA, the excitation power can be reduced to 0.67×10 ⁻⁷ J.

FIG. 4 shows an example of a device for driving a Josephson element with a current induced in the pick-up coil 11 when ErCrO ₃ is used as the photo-induced magnetization material.

In FIG. 4, 16 is a permanent magnet for applying an optimum magnetic field to the magneto-optical conversion element 10. In FIG. The yoke 17 is for efficiently guiding the magnetic field generated from the permanent magnet 16 to the magneto-optical conversion element 10. In addition, when using near the Josephson element, in order to prevent unnecessary magnetic field from being applied to the Josephson element, the magnetic shielding device 1 is installed.
It is necessary to provide 8.

In addition, the photo-induced magnetization of ruby, which is known as a photo-induced magnetized substance, is caused by the fact that when excited with the C-axis of the crystal parallel to the direction of light propagation, the excitation light is directed to the right as shown in Figure 5A. When circularly polarized light (δ+) is used, the light is generated in the opposite direction to the traveling direction of the light, and when excited with left circularly polarized light (δ−), as shown in Figure 5B, it is generated in the traveling direction of the light, but as shown in Figure 5C. As shown, no magnetization occurs when excited with linearly polarized or unpolarized light. Note that this photo-induced magnetization occurs even when the external magnetic field is zero.

Regarding the magneto-optical conversion element, please refer to Japanese Patent Application No. 53-54025 (Japanese Patent Publication No. 59643-1983) related to the applicant's earlier application.
(No.) is described in detail.

Furthermore, as shown in FIG. 6, the strength of induced magnetization varies depending on the Cr concentration in ruby, and reaches its maximum value when the Cr concentration is approximately 1.6 weight percent.

When a 1.6 weight percent sample with an area of 0.27 cm ² and a thickness of 2 mm is excited with approximately 200 mJ of circularly polarized light, a magnetic field of approximately 0.25 Gauss is generated. This value is the minimum value when driving the Josephson element with a magnetic field Φ = 2.07 ×
Since this corresponds to about 10 ⁶ times 10 ₋₇ Gauss, the excitation light intensity can be made 10 ⁻⁶ times weaker, and in this case, the excitation power of about 2×10 ⁻⁷ joules is sufficient.

The above is the result of an experiment conducted at room temperature by exciting the _R1 absorption line of a ruby crystal with circularly polarized ruby laser light (6943 Å).In order to generate magnetization at liquid helium temperature, It is necessary to excite it with light at an absorption wavelength of .

Next, a seventh embodiment will be described in which the Josephson element is directly driven by the magnetic field generated from the magneto-optical conversion element.
As shown in the figure.

Here, the operating principle of the Josephson element is exactly the same as that described with reference to FIG. Use. In this case, the size of the photo-induced magnetization crystal and the intensity of the laser beam shall be optimally set with reference to the installation location of the element and FIG. 3, etc.
Note that it is also possible to drive a plurality of Josephson elements by constructing a magnetic circuit using a material with high magnetic permeability in this manner.

In addition to those described above, other rare earth orthochromites and rare earth orthoferrites, which have similar electronic structures and magnetic properties and are optically transparent, can also be used as the magneto-optical conversion element 10.

In addition, in order to reduce the reflection loss of the signal light incident on the magneto-optical conversion element 10, the light incident surface of the element 10 is coated with a non-reflection coating, and the back surface is coated with a total reflection coating, so that the light reflected on the back surface is coated with a total reflection coating. The amount of cryogen consumption can be reduced by guiding unnecessary light generated from the cryogen and elements to the outside of the refrigerator using an optical fiber cable or the like.

FIG. 8 shows a schematic configuration of the Josephson drive device of the present invention using a magneto-optical conversion element. Here, 2
Reference numeral 1 denotes a light source composed of, for example, a semiconductor laser, a light emitting diode, a dye laser, etc., which converts a signal current into a corresponding signal light. Light generated from this light source 21 is guided to a magneto-optical conversion element 10 in the main body of the apparatus, which is set in liquid helium 9 in a container 8, via an optical fiber cable 22 with poor thermal conductivity. This element 10 generates a magnetic field corresponding to the signal light to drive the Josephson element 23 as shown in FIGS. 4 and 7. Note that the container 8 is the liquid nitrogen 2 in the other container 24.
It shall be cooled according to step 5.

In addition, as shown in Fig. 7, in a device that directly drives a Josephson element with a photo-induced magnetic field,
By deflecting and scanning the light using a light polarizer as shown in FIG. 9, or by addressing, it is possible to sequentially drive a large number of Josephson elements in a time-division manner.

Here, 26 is an optical modulator that converts incident light L ₂ into circularly polarized light 28 using a signal voltage from an optical modulation signal generator 27 . This circularly polarized light 28 is further deflected by an optical deflector 30 driven by an optical deflection signal generator 29, and then converted into parallel light by a lens 31 and irradiated onto the photoinduced magnetization crystal 10. This crystal 10
A large number of Josephson elements 33 can be driven by generating a photo-induced magnetic field 32 from the photo-induced magnetic field 32 .

As is clear from the above description, according to the present invention, the following effects can be obtained. . That is,
Since signals are transmitted from the input terminal at room temperature to the main body of the device using optical fiber cables, which have thermal conductivity two orders of magnitude lower than copper wires, the consumption of cryogen can be significantly reduced. Further, by using a multi-branch optical fiber cable, it is possible to easily switch a large number of Josephson elements at once.

Furthermore, since the optical signal does not interfere with electrical noise, it is difficult for electrical noise to enter from the outside, and there is an advantage that malfunction of the Josephson device can be reduced.

Additionally, the device shown in FIG. 3, which converts a photo-induced magnetic field into an electrical signal using a pickup coil, can also be used for switching a current injection type Josephson device in which a current is directly applied to the device. Moreover, by using superconducting wire for the pickup coil and control wire, it is possible to prevent heat generation.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing an example of the structure of a tunnel junction Josephson device, Fig. 2 is a block diagram showing an example of a photoinduced magnetization measurement system, and Fig. 3 is a characteristic showing the magnetic field dependence of photoinduced signal voltage. 4 is a perspective view showing a case where a Josephson element is driven by a current generated from a photo-induced magnetized substance as an example of the driving device of the present invention, and FIG. 5 A to C are polarized light in the direction of photo-induced magnetization of ruby. Fig. 6 is a characteristic curve diagram showing the dependence of photo-induced magnetization of ruby on Cr concentration; Fig. 7 is a perspective view showing an example of the driving device of the present invention that directly drives a Josephson element by a photo-induced magnetic field. 8 is a schematic diagram showing an example of the driving device of the present invention, and FIG. 9 is a block diagram showing another example of the driving device of the present invention using an optical deflector. DESCRIPTION OF SYMBOLS 1... Counter electrode, 2... Base electrode, 3... Oxide film, 4... Input signal line, 5... Element current, 6...
Input signal current, 7...Ground plane, 8,2
4... Container, 9... Liquid helium, 10... Magneto-optical conversion element, 11... Pick-up coil, 12
... Coaxial cable, 13 ... Oscilloscope, 1
4...beam splitter, 15...piplanar phototube, 16...permanent magnet, 17...yoke,
18... Magnetic shielding device, 19... Permalloy, 20, 28... Circularly polarized light, 21... Light source, 22... Optical fiber cable, 23, 33
... Josephson element, 25 ... Liquid nitrogen, 26
... Optical modulator, 27 ... Optical modulation signal generator, 29
...Optical deflection signal generator, 30... Optical deflector, 31
...Lens, 32...Light-induced magnetic field, L ₁ , L ₂ , L ₃
... Laser light for excitation.

Claims (1)

[Claims] 1. In a driving device for a Josephson device, which includes a pair of superconducting lines facing each other with an insulating layer interposed therebetween, a magneto-optical conversion element placed close to the Josephson device is irradiated with light through an optical fiber cable to generate a magnetic field, A Josephson element driving device characterized in that the Josephson element is switched by this magnetic field.