JPH08162683A - Method and apparatus for transmitting information using josephson transmission line - Google Patents

Abstract

PURPOSE: To effectively increase the propagation speed of a soliton corresponding to a signal when information is transmitted by the soliton by using a Josephson transmission line. CONSTITUTION: A soliton corresponding to a transmission signal is transmitted to a Josephson transmission line 10 by a soliton signal transmitter 14, the soliton having smaller propagation speed than that of the soliton corresponding to the transmission signal is used as a background signal soliton, which is ordinarily and periodically transmitted. The propagating speed of the soliton of the side corresponding to the transmission signal is effectively enhanced by the shift of the phase when the background signal soliton passes on the line 10. At the reception side, a soliton signal identifying circuit 16 identifies by the sum of the square of the differentiation of sign=Galtonian solution of a phase difference signal and twice differentiation, and only the soliton corresponding to the transmission signal is detected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for transmitting information using soliton waves, and more particularly to a method and apparatus for transmitting information using Josephson junctions using superconductors.

[0002]

2. Description of the Related Art A Josephson device in which two superconductors are weakly coupled to each other by sandwiching a thin insulating film has attracted attention, and application to various uses has been attempted. Josephson devices have already been put to practical use as SQUID magnetometers, voltage standards, electromagnetic wave detectors, and the like, and their application to logic devices and memory devices has been studied by taking advantage of their high-speed switching characteristics and low power consumption characteristics. There is.

By the way, in the Josephson junction, sine =
It is conventionally known that a soliton wave represented by the Gordon equation can be obtained. Therefore, in a Josephson computer using a Josephson element, a Josephson transmission line is used for connection between each logic element and a storage element, and information is transmitted by propagating solitons on this Josephson transmission line, and It has been studied to use the Son transmission line itself as a logic element or a storage element to perform a switching operation by a soliton.

Here, the Josephson transmission line will be described with reference to FIG. The Josephson transmission line 90 illustrated in the figure sandwiches a thin insulating layer 93 having a thickness of a, a length of L and a width of W, and sandwiches a first superconductor 91 and a second superconductor 91 on both sides thereof. This is a configuration in which the conductor 92 is arranged. Here, the thickness a of the insulating layer 93 is sufficiently small (for example, 100 nm or less), and the insulating layer 93 is a thin film. As a result, both superconductors 91 and 92 are weakly coupled to each other via the insulating layer 93 to form a Josephson junction. For the sake of explanation, the direction in which the Josephson transmission line 90 extends (the soliton propagation direction) is the x direction, the direction perpendicular to the Josephson junction surface is the z direction, and the width direction of the Josephson transmission line 90 is the y direction. Note that, as long as the first superconductor 91 and the second superconductor 92 are weakly coupled to each other and the Josephson effect can be exhibited, another layer can be used instead of the insulating layer 93.

The phase of the wave function of a Cooper pair in a superconductor can be controlled by applying an electromagnetic field. Therefore, on one end P side of the Josephson transmission line 90, for example, by applying a pulse-like potential difference between the superconductors 91 and 92, the superconductors 91 and 9 are connected.
A soliton can be formed by instantaneously changing the phase of 2. Here, when the magnetic field B is applied in the y direction in the drawing, the Lorentz force acts on the formed soliton, the soliton propagates along the Josephson transmission line 90, and reaches the other end Q.

As is well known, solitons behave as particles due to the balance between diffusion and dispersion. In addition, it is known from many mathematical studies that solitons are quantized, even though they are classical mechanical entities, and that they exhibit fermion-like behavior. Therefore, the plurality of solitons injected with the quantized momentum preserve their respective particle properties before and after the collision. In addition, at the time of collision, a phase shift is induced as an event peculiar to the nonlinear phenomenon. Therefore, by assigning the presence or absence of the soliton to "1" and "0" of the binary signal respectively, it becomes possible to transmit information through the Josephson transmission line. Moreover, it is possible to construct a logic element or a memory element using the Josephson transmission line by utilizing the fact that the soliton preserves each particle property before and after collision and is a non-linear phenomenon.

[0007]

When Josephson elements are connected by a Josephson transmission line and signals are transmitted between these elements using solitons, the Josephson elements generally operate at a very high speed. It is desirable to increase the propagation speed of the signal soliton on the Josephson transmission line as much as possible to reduce the delay time of the signal.

An object of the present invention is to provide a method and an apparatus capable of effectively increasing the propagation speed of a soliton corresponding to a signal to be transmitted when information is transmitted using a Josephson transmission line. It is in.

[0009]

The information transmission method of the present invention uses a Josephson transmission line having a first superconductor and a second superconductor weakly coupled to each other.
Phase of the Cooper pair wavefunction and the second
In the information transmission method of transmitting a signal from one end side to the other end side of the Josephson transmission line by excitation based on the difference between the phase of the wave function of the Cooper pair in the superconductor and the excitation corresponding to the signal. Excitation having a propagation velocity smaller than the propagation velocity is used as a background signal, and the background signal is generated and steadily propagated from the one end side to the other end side.

The information transmission device of the present invention is provided with a Josephson transmission line having a first superconductor and a second superconductor weakly coupled to each other, and is provided at one end of the Josephson transmission line. Generating excitation in the Josephson transmission line based on the difference between the phase of the wave function of the Cooper pair in one superconductor and the phase of the wave function of the Cooper pair in the second superconductor; It has a transmitting means for transmitting to the other end side of the transmission line, and a receiving means provided on the other end side for detecting the excitation, and the transmitting means has a relatively large propagation speed according to a transmission signal. Along with transmitting the excitation, it constantly transmits the excitation with a relatively small propagation velocity,
Only when the receiving means receives the excitation having a relatively high propagation speed, it is detected as a received signal.

In the present invention, it is preferable to use solitons as the excitation formed in the Josephson transmission line. Further, in the Josephson transmission line, the insulating thin film may be sandwiched between the first superconductor and the second superconductor.

[0012]

According to the present invention, when two solitons having different speeds propagate in the Josephson transmission line and the faster soliton overtakes the slower soliton, a phase shift occurs, and the faster soliton travels in the propagation direction. The phenomenon of shifting the position in a more advanced direction is used. That is, every time such overtaking occurs, the faster soliton shifts to a position advanced from the position based on the original velocity, so that the effective propagation velocity of the faster soliton increases. Less than,
The improvement of the effective propagation speed of the soliton corresponding to the signal in the present invention will be described in more detail.

Regarding the Josephson transmission line 90 shown in FIG. 1 described above, Miki Watatsu, Iwanami Lecture, Modern Physics,
Consider the propagation of solitons according to "Nonlinear Waves" (Iwanami Shoten, 1992).

In each superconductor 91, 92, the wave function of the Cooper pair according to the Landau-Ginzburg equation is
It is assumed that they are given by Ψ ₁ and Ψ ₂ , respectively. It is also assumed that the wave functions Ψ ₁ and Ψ ₂ have phases Φ ₁ and Φ ₂ , respectively. At this time, the phase difference between them is Φ = Φ ₁ −Φ ₂ . Current j (z, t) and voltage V in the z direction at time t
(x, t) is the Josephson equation

[0015]

[Equation 1] Given in. Where j _o is the reference value of current, e is the unit electron charge,

[0016]

[Outside 1] Is h / 2π (h is Planck's constant).

By the way, due to the Meissner effect, magnetic flux cannot enter the inside of each superconductor 91, 92. However, magnetic flux is allowed to penetrate into the insulating layer 93, which allows the phase difference Φ to be changed spatially and temporally. For the magnetic field B _y (x, t) in the y direction at the position z, the Maxwell equation

[0018]

[Equation 2] Than,

[0019]

(Equation 3) Becomes With respect to the electric field E _z (x, t) in the z direction, E _z (x, t)
= V (x, t) / a holds,

[0020]

[Equation 4] Is obtained. Substituting equation (2) into this,

[0021]

(Equation 5) Is obtained. On the other hand, Maxwell's equation

[0022]

(Equation 6) (Where μ _o is the magnetic permeability of the vacuum and ε is the dielectric constant of the insulating layer)
Than,

[0023]

(Equation 7) Is obtained, and by making it dimensionless appropriately,

[0024]

(Equation 8) Get. This equation (9) is the sine-Gordon equation well known in the field of mathematical physics.

It is known that the sine-Gordon equation can be obtained exactly. For example, Hideo Tsuru, Miki Wada,
Journal of the Physical Society of Japan, 1983, No. 53, 290
The derivation of the solution is described in detail on page 8 or in Chapter 9 of Shunichi Tanaka, Etsuro Date, "KdV equation" (Kinokuniya Shoten, 1979). By solving the equation (9), the phase difference Φ at any position in the x direction at any time can be obtained.

As described above, the solitons have the isolation property and cause a phase shift when they collide with each other. Hereinafter, a phase shift when a soliton having a different propagation velocity is overtaken will be described. The propagation velocity of solitons in the Josephson transmission line can be changed for each individual soliton depending on the soliton generation conditions.

FIG. 2 is a diagram showing the relationship between the isolation of two solitons 21 and 22 and the collision. The position of each soliton 21 and 22 in the x direction and the parameter u are shown for each time t. There is. The parameter u shown on the vertical axis is introduced to facilitate understanding of the phase shift due to collision, and is represented by the sum of the square of the derivative of the sine-Gordon solution and the second derivative. That is,

[0028]

[Equation 9] Is. It is known that the sum u of the square of the derivative of the sine-Gordon solution and the second derivative follows the KdV (Cortévegues-de Vries) equation, and the larger the amplitude of the solution of the KdV equation, It is known that its propagation speed is high. Therefore, of the solitons 21 and 22 shown in FIG. 2, the soliton 21 having a larger height in the u-axis direction has a higher propagation speed than the soliton 22 having a smaller height.

FIG. 3 is a graph showing the relationship between the position of each soliton 21 and 22 and time.
The trajectory of the position of the center of gravity of No. 2 is shown by the solid line. Then, in the area 23 where the trajectories of both solitons 21 and 22 intersect, the collision (overtaking) of these solitons 21 and 22
Is happening. As is clear from FIG. 3, the collision area 2
Before and after 3, the phase of the soliton 21 with the higher speed is shifted to the left, and the center of gravity of the soliton 21 with the higher speed is deviated toward the traveling direction side compared to the position of the center of gravity when the collision did not occur. .

As described above, the two moving in the same direction
When two solitons collide, that is, when the higher-speed soliton overtakes the lower-speed soliton, the higher-speed soliton shifts its position in the traveling direction. In other words, the effective speed of the soliton having the higher speed can be increased by overtaking the soliton having the lower speed. In the present invention, while the soliton of the lower speed is constantly sent to the Josephson transmission line as the background signal, the soliton of the higher speed is used for signal transmission, so that the soliton is periodically overtaken. Since the position of the soliton used for signal transmission shifts to the traveling direction each time, the effective signal transmission time can be shortened.

[0031]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 4 is a block diagram showing the configuration of the information transmission device according to one embodiment of the present invention.

This information transmission device is for transmitting information by propagating solitons on the Josephson transmission line 10. A soliton signal transmission circuit 14 is attached to one end of the Josephson transmission line 10, and the Josephson transmission line 10 is connected to the Josephson transmission line 10. A soliton signal receiving circuit 15 is provided on the other end side of the transmission line 10.
Is attached. A soliton signal recognition circuit 16 is provided on the output side of the soliton signal reception circuit 15.

The Josephson transmission line 10 is similar to that shown in FIG. 1 with a first superconducting layer on both sides of a thin insulating layer 13 having a thickness of a, a length of L and a width of W. This is a configuration in which the body 11 and the second superconductor 12 are arranged. Here, the thickness a of the insulating layer 13 is sufficiently small (for example, 100 nm or less), and the insulating layer 13 is a thin film. As a result, both superconductors 11 and 12 are weakly coupled via the insulating layer 13 to form a Josephson junction. As in the case of FIG. 1, the direction in which the Josephson transmission line 10 extends is the x direction, the width direction is the y direction, and the direction perpendicular to the Josephson junction surface is the z direction. A magnetic field in the y direction is applied to the Josephson transmission line 10. The magnetic field in the y-direction may be an external magnetic field or may be generated by passing a bias current through the Josephson transmission line 10.

The soliton signal transmitting circuit 14 is for generating a soliton in the Josephson transmission line 10 by applying a pulsed potential difference between the superconductors 11 and 12. The soliton generated by the soliton signal transmission circuit 14 includes a soliton having a relatively high propagation speed (a soliton having a large u value, which is the sum of the square of the derivative of the sine-Gordon solution and the second derivative) and the propagation speed. There are two types: relatively small solitons (solitons with a small u value). In the following explanation, the former soliton is first
The soliton of the species and the latter soliton are the solitons of the second species. The soliton signal transmission circuit 14 generates a first type of soliton according to the input transmission signal, and constantly and repeatedly generates a second type of soliton. The propagation velocity of the soliton can be changed by changing the peak value or the waveform of the pulse voltage applied between the superconductors 11 and 12. Alternatively, solitons may be generated by changing the magnetic field.

The soliton signal receiving circuit 15 is a circuit for detecting the phase difference Φ between the wave functions Ψ ₁ and Ψ ₂ of the Cooper pairs of both superconductors 11 and 12, and the detected phase difference Φ is the soliton signal recognition. Input to the circuit 16. The soliton signal recognition circuit 16 is composed of a differential operation circuit, a squaring circuit, an adding circuit, a discriminating circuit, etc., and according to the above equation (10), the input phase signal (sine = Gordon's solution) is squared of its differentiation. Is converted to the sum u of the second differential and the converted signal is processed. At this time, the voltage value V of the converted signal
Threshold V _o prepared in advance, it is determined that this threshold V _o is a signal for large voltage wave amplitude with respect to the threshold V
_{o In the} following cases, the presence or absence of a signal is determined by determining that there is no signal.

Here, how to determine the threshold value V _o will be described with reference to FIG. In this embodiment, the propagation velocity is high (u
The presence or absence of the first type soliton 31 having the larger value corresponds to the presence or absence of the signal to be transmitted, and the second type soliton 32 having the smaller propagation velocity (the smaller u value) is the content of the information to be transmitted. It has nothing to do with On the other hand, the voltage value V of the converted signal in the soliton signal identification circuit 16 is proportional to the u value of the received soliton. Therefore, the signal amplitude for the second type soliton 32 is smaller than the threshold value V _o , and the first type soliton 31
As the signal amplitude is greater than the threshold V _o for, by setting the threshold value V _o, a first type of soliton 31,
That is, only the soliton sent as information from the soliton signal transmission circuit 14 side can be detected. The soliton detected in this way is output to the outside from the soliton signal identifying circuit 16 as a received signal.

In the present embodiment, the soliton signal transmitting circuit 14
At the same time, the transmission signal is converted into the first type soliton (signal soliton) 31, and the first type soliton 3
A second type soliton 32 (soliton used as a background signal) having a velocity smaller than 1 is constantly generated at a constant cycle. As a result, as described in the "Action" column, the propagation speed of the soliton 31 of the first type, which is meaningful as information, is effectively improved, and information can be transmitted earlier. This is shown in FIG. In the figure, the signal of the phase for the soliton (sine = Gordon's solution) is shown as the locus of the center of gravity for the sum u of the square of the derivative and the second derivative. The thick line indicates the locus of the soliton 31 of the first type, that is, the soliton corresponding to the information,
The thin line indicates the locus of the second type soliton 32. A region 33 represented by a small circle in the figure indicates that the solitons 31 and 32 collide with each other. Due to the collision, the first-type soliton 31 is displaced toward the traveling direction side, and it is determined that the first-type soliton 31 did not collide at the position of x = L after a plurality of collisions. Type 1 soliton in the case (broken line in the figure)
It arrives earlier than ΔT.

[0038]

As described above, according to the present invention, when information is transmitted using a soliton signal on a Josephson transmission line, a soliton having a propagation speed smaller than that of a soliton used for information transmission is used as a background signal. The steady transmission on the transmission line makes it possible to effectively increase the propagation speed of the soliton used for information transmission, which has the effect of enabling information transmission at a higher speed.

[Brief description of drawings]

FIG. 1 is a perspective view illustrating an example of a Josephson transmission line.

FIG. 2 is a diagram illustrating collision of solitons on a Josephson transmission line.

FIG. 3 is a diagram illustrating collision of solitons on a Josephson transmission line.

FIG. 4 is a block diagram showing a configuration of an information transmission device according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating a threshold V _o in a soliton signal recognition circuit.

FIG. 6 is a diagram illustrating an improvement in effective transmission speed of a faster soliton.

[Explanation of symbols]

 10,90 Josephson transmission line 11,91 First superconductor 12,92 Second superconductor 13,93 Insulating layer 21,22,31,32 Soliton 14 Soliton signal transmitting circuit 15 Soliton signal receiving circuit 16 Soliton Signal recognition circuit

Claims (6)

[Claims] 1. A Josephson transmission line having a first superconductor and a second superconductor weakly coupled to each other is used, and the phase of the wave function of a Cooper pair in the first superconductor is used. An information transmission method for transmitting a signal from one end side to the other end side of the Josephson transmission line by excitation based on a phase difference of a wave function of a Cooper pair in the second superconductor, A method of transmitting information, characterized in that an excitation having a propagation velocity smaller than that of the excitation is used as a background signal, the background signal is generated, and the background signal is steadily propagated from the one end side to the other end side. 2. The information transmission method according to claim 1, wherein the excitation is a soliton. 3. The information transmission method according to claim 1, wherein an insulator thin film is sandwiched between the first superconductor and the second superconductor in the Josephson transmission line. 4. A Josephson transmission line having a first superconductor and a second superconductor weakly coupled to each other, and a Josephson transmission line provided at one end of the Josephson transmission line, wherein
Phase of the Cooper pair wavefunction and the second
In the superconductor of, a pumping means based on the difference between the phase of the wave function of the Cooper pair and the Josephson transmission line is generated, and transmitting means for transmitting to the other end side of the Josephson transmission line, and to the other end side. Provided, and having a receiving means for detecting the excitation, the transmitting means, while transmitting the excitation having a relatively large propagation velocity in accordance with the transmission signal, constantly pumping a relatively small propagation velocity An information transmission device which transmits and detects as a reception signal only when the receiving means receives the excitation having a relatively high propagation velocity. 5. The information transmission method according to claim 4, wherein the excitation is a soliton. 6. The method of transmitting information according to claim 4, wherein an insulator thin film is sandwiched between the first superconductor and the second superconductor in the Josephson transmission line.