JPH0640593B2 - Superconducting quantum interferometer gate - Google Patents

Description

The present invention relates to a superconducting quantum interferometer gate.

(Prior Art) Conventionally, in a Josephson junction digital circuit, a superconducting quantum interferometer gate including a Josephson junction in a superconducting loop is used as a logic circuit or a memory circuit.

FIG. 2 is a perspective view of a first example of a conventional superconducting quantum interferometer gate.

In FIG. 2, 1 is a lower electrode, 2 is an upper electrode, 3 is an insulating film,
4 is a control film, 5 is a Josephson junction, 6 is a superconducting loop, and 7 are unit vectors perpendicular to each other. 8 is a damping resistor.

Such a superconducting quantum interferometer gate has a gate current I _g
It is biased by and is switched to a voltage state by the current I _in flowing through the control line 4. The inductance of the damping resistor 8 for removing the resonance mode of the interferometer gate is
It is designed to be sufficiently smaller than the inductance of the superconducting loop 6. Below the superconducting loop 6, a ground plane 9 made of a superconducting thin film is provided via an insulating film 10.
The dependence of the external magnetic field and the control current I _in of an interferometer type circuit is represented by the following equation: In equation (1), Δθ is the phase difference of the superconducting order parameter through the Josephson junction 5 in the superconducting loop, Φ _o is the flux quantum, Φ _ex is the external magnetic flux, and Φ _s is the shield that flows in the superconducting loop. The magnetic flux generated by the current, I _in is the input signal current to the gate, and M is the mutual inductance between the superconducting loop 6 and the input coil.

As is clear from the equation (1), the magnetic flux I _in M generated by the input signal and operating the circuit is completely equivalent to the external magnetic flux Φ _ex , and the interferometer circuit similarly reacts to both. Therefore, variations in the presence or [Phi _ex external magnetic flux [Phi _ex becomes to cause cause malfunction of the interferometric type gate. The external magnetic flux Φ _ex is represented by the formula: The first term on the right side of Eq. (2) is due to the magnetic field in the background. Is a unit vector perpendicular to the superconducting loop 6, Is the magnetic field that penetrates the interferometer, which is a function of space,
The integral ∫ dA ₁ is inside the superconducting loop On a plane perpendicular to. The second term on the right side of equation (2) is due to the magnetic flux trapped in the superconducting ground plane 9, where N is the number of magnetic flux trapped through the interferometer, and K is the superconducting ground plane 9 and the superconducting loop. Is a coupling coefficient between 6 and 6. A
The size of ₁ is usually about 100 μm ² . Assuming an external magnetic field B1μG in an experimental environment using a triple magnetic shield, a superconducting ground plane, etc., the first term in equation (2) is 5 × 10 ^-6 Φ
_Although it is about _o , the second term is 0.5Φ _{o when} N = 1 and K = 0.5. Therefore, if there is a magnetic flux trapped in the superconducting ground plane 9, most of the influence of the external magnetic field is due to this trapped magnetic flux. In actual operation of the quantum interferometer gate, this magnetic flux trapping phenomenon occurs very frequently, causing malfunction of the superconducting quantum interferometer gate, which is one of the major obstacles to normal operation of the superconducting quantum interferometer gate. Is believed to be. In addition, the influence of the background magnetic field that has an almost even distribution other than traps is also greatly affected by superconductivity when the above magnetic shielding is not used or when magnetic substances are inadvertently mixed into the environment of the device. This may cause malfunction of the quantum interferometer gate.

FIG. 3 is a perspective view of a second example of a conventional superconducting quantum interferometer gate.

This superconducting quantum interferometer gate is formed in a three-dimensional shape so as not to be affected by the magnetic flux trapped in the superconducting ground plane 9. In FIG. 3, 1 is a lower electrode, 2 is an upper electrode, 3 is an insulating film, 4 is a control line, 5 is a Josephson junction, 6 is a superconducting loop, and 7 is a unit vector. Reference numeral 9 is a superconducting ground plane installed below the interferometer via an insulating film 10.

For such an interferometer gate, the magnetic flux trapped on the ground plane is parallel to the interferometer loop, so that it is not coupled to the interferometer and has no effect. But,
This interferometer gate is affected by the magnetic field in the direction horizontal to the superconducting ground plane 9, and the external magnetic flux is Becomes

Is a unit vector perpendicular to the superconducting loop 6 and ∫dA
₂ is Is integrated about the inside of the superconducting loop perpendicular to. Further, in such an interferometer gate, there is a drawback that the interferometer gate cannot be shunted with a resistor having an inductance sufficiently smaller than the interferometer inductance for removing the resonance mode.

To summarize the above conventional example, the planar interferometer gate as shown in FIG. 2 could be provided with a shunt resistance, but the influence of the magnetic flux trapped on the ground plane and the distribution in the direction perpendicular to the ground plane. It had the drawback of being affected by the background magnetic field. On the other hand, the three-dimensional interferometer gate as shown in FIG. 3 is not affected by the trap magnetic flux, but is affected by the background magnetic field horizontal to the ground plane, and at the same time, the shunt resistor cannot be installed. was there.

(Object of the Invention) An object of the present invention is to prevent a superconducting quantum interferometer gate from being affected by the magnetic flux trapped in the ground plane as described above, or by an external magnetic field coupled to the interferometer other than the trap magnetic flux. To provide.

(Structure of the Invention) The superconducting quantum interferometer gate of the present invention is characterized in that the first and second superconducting thin films having a U-shaped planar shape are superposed on each other with an insulator interposed therebetween and are electrically connected by two or more Josephson junctions. To form a superconducting loop having a coupling surface for an external magnetic field in a plane perpendicular to the U-shaped plane, the superconducting loop having a U-shaped plane shape and each side of the U-shaped plane, When a magnetic field parallel to the U-shaped plane and having a uniform distribution is applied, the effects of the magnetic field on the two facing sides of the U-shaped superconducting loop are canceled each other. To do.

(Example) Next, the Example of this invention is described in detail using drawing.

FIG. 1 is a perspective view showing an embodiment of the present invention.

In FIG. 1, 1 is a lower electrode, 2 is an upper electrode, 3 is an insulating film,
5 is a Josephson junction, 6 is a superconducting loop, 7 is a unit vector perpendicular to each other 8 is a damping resistor. Note that the control line is not drawn in this figure because the drawing is complicated, but as in FIG. 2, the control line can be provided on the upper electrode via an insulator. The feature of this embodiment is that the superconducting loop 6 is formed by a double U-shaped superconducting film.

The effect of the interferometer gate having such a structure on the external magnetic field will be described below. In this embodiment, no matter where the magnetic flux is trapped in the superconducting ground plane, the direction of this magnetic flux is parallel to the interferometer loop, so this magnetic flux is not joined to the superconducting loop 6 and the superconducting loop 6 is not joined. The external magnetic field received by Is.

Is shown in FIG. Unit vector with the opposite direction to, integration ∫dA ₂ , ∫d
A ₃ and ∫ dA ₄ are respectively Over the inner plane of the superconducting loop perpendicular to. The value obtained from the equation (4) is a magnetic flux which is almost the same as that of the first term of the equation (2), and is sufficiently smaller than the magnetic flux quantum Φ _o . When there is a superconducting ground plane 9 that has a sufficiently larger spread than the superconducting loop, the local distribution of the external magnetic field near the interferometer loop is almost uniform. Exists in the plane. If ∫dA ₂ = ∫dA ₄ >> ∫dA ₃ , then equation (3) becomes Becomes But, Is in the opposite direction, and if the distribution of the magnetic field B is locally almost uniform as described above, then Φ _ex 0. Therefore, the superconducting interferometer gate of the present embodiment has no effect on the operation of the quantum interferometer gate even if there is a background magnetic field in both vertical and horizontal directions on the ground plane not depending on the trap, as well as the trap magnetic flux. It has the characteristic of not receiving. In addition, by using a double U-shaped loop as in the present invention, the interferometer is shunted as shown in FIG. 1 by a damping resistor having an inductance sufficiently smaller than that of the superconducting loop. Can be done.

Although the ground plane is provided below the quantum interferometer gate in this embodiment, this ground plane may be omitted.

(Effects of the Invention) As described above, according to the present invention, the interferometer is shunted by the damping resistor having a sufficiently small inductance without being affected by the magnetic field of the background or the magnetic flux trapped in the ground plane. A superconducting quantum interferometer gate that can also be obtained is obtained.

[Brief description of drawings]

FIG. 1 is a perspective view of an embodiment of the present invention, FIG. 2 is a perspective view of a first example of a conventional superconducting quantum interferometer gate, and FIG. 3 is a second view of a conventional superconducting quantum interferometer gate. 3 is a perspective view of an example of FIG. 1 ... lower electrode, 2 ... upper electrode, 3 ... insulating film, 4 ...
… Control line, 5 ... Josephson junction, 6 ... Superconducting loop, 7 ... Unit vector, 8 ... Damping resistance, 9 ...
… Superconducting ground plane, 10… Insulating film.

Claims (1)

[Claims] Claim: What is claimed is: 1. A first and a second superconducting thin films having a U-shaped planar shape are superposed on each other with an insulator interposed therebetween and electrically connected by two or more Josephson junctions so that the planar shape is U-shaped. Form a superconducting loop having a coupling surface for an external magnetic field in a plane that includes each side of the U-shape and is perpendicular to the U-shape plane, and is parallel to the U-shape plane and has a uniform distribution. A superconducting quantum interferometer gate, characterized in that, when a strong magnetic field is applied, the effects of the magnetic field on the two opposite sides of the U-shaped superconducting loop cancel each other out.