JPS5928074B2 - Josefson Voltage Standard Device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a Josephson voltage standard device that utilizes the unique characteristics of Josephson elements to obtain accurate voltage values. It is possible to obtain a precision voltage standard device in a high voltage region by using superconducting closed circuits connected in sequence.

As this type of Josephson voltage standard device, the first
A direct current I and a high frequency current i are connected to one Josephson element J as shown in Figure A, or to a plurality of N Josephson elements J' sequentially connected in series as shown in Figure 1B. are simultaneously supplied, and the frequency f of the supplied high-frequency current i
A constant DC voltage V expressed by the following 11th equation proportional to .

That is, Φ0 V=n・-・f(1) 2π Here, n is a positive integer, and Φ0 is the magnetic flux quantum = 2.07×1
Each represents 0-15Wb.

However, the reason why a constant DC voltage proportional to the frequency of the supplied high-frequency current is obtained as described above is that in Josephson elements J and J', as the DC current I increases, the phase of the current in each element increases. rotation occurs, and a DC voltage proportional to the rotation speed is induced. However, when a high-frequency current i is supplied superimposed on the DC current, a phenomenon occurs in which the rotation of the phase of the current in the element synchronizes with the frequency of the high-frequency current. Therefore, a constant DC voltage proportional to the frequency of the supplied high-frequency current will be generated.

The Josephson voltage standard device is capable of obtaining a standard voltage value by utilizing this phenomenon exhibited by a Josephson element in which a thin insulating layer is interposed between superconducting metal layers. Since the stability of V can be obtained up to about 10-", the obtained DC voltage V is also about 10-"
It is possible to obtain a stability of about 12, and therefore it has been used as a national voltage standard. However, in the conventional Josephson voltage standard device of this type, the DC voltage obtained by the equation (1) described above is calculated using the frequency f of the supplied high-frequency current as f = 1010Π
z, when n = 1, the voltage value V = 201 tV, which is extremely low. In order to make the voltage value V an easy-to-use voltage value of about IV, the frequency f must be made extremely high, or the integer n Two methods have been considered in the past: how to make the value extremely large.

Since it is difficult in terms of high frequency technology to continuously and stably maintain the frequency f of the high frequency current at an extremely high value, conventionally, the integer n in equation (1) has been set to a large value. Various methods have been studied. However, the first
When one Josephson element is used alone as shown in FIG. A, the relationship between current J and voltage T is generally expressed by the following equation (a), as is well known.

Here, VO is the high-frequency voltage generated by the high-frequency current 1, % is a constant between π and 1π, Jn is the Betzel function, and ψ0, f, n are the magnetic flux quantum, respectively, as described above. The frequency of high frequency current 1 is a positive pot number.

In this equation (a), the term related to time t becomes zero, so current' becomes DC current 1, voltage g becomes constant DC voltage, and (b
) formula is expressed as V=Nl7-0 -FI, and formula (1) is obtained.

Based on the above, we actually measured the relationship between the voltage and the current obtained when a high-frequency current 1 with a frequency of 10 GHz was supplied to the Josephson element J shown in Figure 1A, and the results are shown in Figure 2. Ta.

In other words, when one di]Cefson element is used alone, when the integer n in equation (1) is set to larger values in sequence, the current-voltage characteristic curve in Fig. 2 shows the result as shown by the solid line. Another disadvantage is that as the integer n becomes larger, the range of current values in which a required constant DC voltage value V can be obtained becomes narrower, and there is no margin for operation as a voltage standard device. In addition, when N Josephson elements are connected in series as shown in Figure 1B, the voltage obtained by one Josephson element is obtained by connecting the N Josephson elements in series. Various methods have been studied to obtain a constant DC voltage having a value N times the value of
In that case, there is no guarantee that all N Josephson elements connected in series with each other will generate the same voltage value, and therefore the integer n in equation (1)
Since the value of is arbitrary, it is difficult to use it as a voltage standard device.

The object of the present invention is to eliminate the above-mentioned conventional drawbacks and difficulties, and to provide a Josephson voltage standard device that can operate accurately and stably over a wide current range even when obtaining a high output DC voltage. It is about providing.

That is, the Josephson voltage standard device of the present invention has a superconducting closed circuit formed by sequentially connecting at least three Josephson elements in a ring, and the voltage standard device of the present invention has a superconducting closed circuit formed by sequentially connecting at least three Josephson elements in a ring. By supplying a direct current and a high frequency current to the one Josephson element, a direct current voltage having a value proportional to the frequency of the high frequency current supplied to both ends of the one Josephson element is generated. .

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

First, to explain the operating principle of the Josephson voltage standard device of the present invention, as shown in FIG. 3, a superconducting closed circuit is formed by connecting one Josephson element J1 and N Josephson elements J2 in parallel. When DC current and high frequency current 1 are simultaneously supplied to the parallel connection point of one Josephson element J1 and NXJ2, the phase of the current flowing through Josephson elements J1 and J2 is changed to Y1 and Y2, respectively. Then, in accordance with the general property that the phase of a current must return to its original state after one circuit in a closed circuit, the following relationship holds true. In other words, the phase of the current flowing through each Josephson element in the superconducting closed circuit shown does not rotate freely;
Since it rotates under the conditions of equation (2), the DC voltages generated in each Josephson element J1 and J2 in this phase-locked state are also generated with a certain relationship between them. I decided to do it,
Therefore, even in the case of N Josephson elements J2 connected in series, the conventional method that simultaneously supplies direct current and high frequency current only to the series connection circuit of N Josephson elements as shown in FIG. As in the case of The problem will be completely resolved.

In the superconducting closed circuit according to the present invention configured as described above, due to the phase constraint under the above conditions, the relationship between the current supplied to the single-connected Josephson element and the voltage v generated in the Josephson element is expressed as follows. Unlike the formula (a) described above for the single Josephson element shown in Figure 1A,
It is expressed by the following equation (c).

In this equation (c), as in the above-mentioned equation (a), when , the term related to time t in the second term becomes zero, and constant DC voltage=NNL2·f1 is obtained.

Moreover, the variable (--1・?) of the Betzel function Jn in the second term of equation (c) changes to Betz 1 in the first term Mt
Otsu I&
It was also confirmed that a similarly wide current region can be obtained when the value is only 1 compared to the variable (1·'t/o) of the 1-cell function Jn.

On the other hand, based on the rotation of the phase of the current that flows when DC current 1 is supplied to N Josephson elements J2 connected in series, the integer n in equation (1) applied in such a case can be set to NN
Then, the range of current values in which a constant DC voltage determined by equation (1) can be obtained corresponding to the integer NN-NXN is a wide current range corresponding to the above-mentioned integer n set for Josephson element J1. A constant DC voltage will be generated.

In other words, the range in which the phase rotation of the current flowing through one Josephson element is slow, and therefore the integer n is small and (1
) In the range where the constant DC voltage V determined by the formula is low, the range of current values in which the constant DC voltage can be obtained is wide. As described above with reference to FIG. 2, in the range where the constant DC voltage determined by equation (1) is high, the range of current values in which the constant DC voltage can be obtained becomes significantly narrower. On the other hand, in the superconducting closed circuit according to the present invention, under the phase constraint condition according to equation (2), the phase rotation of the current flowing through the N Josephson elements J2 connected in series is expressed as Compared to the phase rotation of the current flowing through Josephson element J1 of Even when the phase rotation of the current flowing through Josephson element J1 reaches a large value corresponding to NXN and becomes extremely fast, determine the DC constant voltage generated in N series-connected Josephson elements J2 (1) The integer value in the equation is the integer value N The constant DC voltage generated in element J2 is generated in a wide current range corresponding to the integer value n.

Furthermore, since the phase rotation of the current flowing through the Josephson element J2 is a much slower value of 1A than the phase rotation of the current flowing through the Josephson element J1, N pieces of Josephson elements connected in series are The DC constant voltage value generated in each Josephson element J2 in the element J2 is 1/N of the DC constant voltage value generated in the single connected Josephson element J1, but the total voltage value when N pieces are connected in series is , is equal to the voltage value generated in the single-connected Josephson element J1. Therefore, by increasing the number N of Josephson elements sequentially connected in series in the superconducting closed circuit according to the present invention, it is possible to easily generate a constant DC high voltage having a wide operating range with high stability. can be generated.

FIGS. 5A and 5B show an example of a specific configuration of the Josefson voltage standard device of the present invention, which comprises a superconducting closed circuit operating as described above using the equivalent circuit shown in FIG. 3.

The specific configuration shown is that each Josephson element Jl, J2 in the equivalent circuit shown in FIG. 3 is constructed of superconducting thin films S1 and S2 with an insulating thin layer interposed therebetween. As shown, a superconducting ground plane 2 made of, for example, a Nb thin film and having a thickness of about 1 μm is formed on a substrate 1 made of silicon, glass, etc. to form a shield against external magnetic flux and electromagnetic waves, and After electrically insulating the surface of the conductive ground plane 2 by oxidizing it to a thickness of approximately 200λ, a superconducting thin film made of, for example, lead Pb (
S1) 4 is deposited, and the superconducting thin film (S1) 4 is further deposited.
A thin insulating layer 5 for forming a Josephson junction is formed by oxidizing the surface of the superconductor to a thickness of about 20 mm, and a superconducting thin film (S2) 6 made of lead, Pb, etc. is deposited on top of the insulating thin layer 5. Therefore, each Josephson element J in the equivalent circuit shown in FIG.
, , J2 is configured. The upper superconducting thin film (S
2) As is clear from the top view shown in Figure A, 6 is
The oxidized surfaces 5 of two adjacent lower superconducting thin films (S1) 4 and the lower surfaces of those two A continuous vapor deposition film is formed along the oxidized surface 3 of the superconducting ground plane 2 exposed in the middle of the superconducting thin film (S1) 4. Therefore, each Josephson element Jl,
In order to configure J2 in a square shape with a side of 60 ttm, for example, the lower superconducting thin film (S1) 4 constituting the two elements is formed into a rectangular shape with a width of 100 μm or more and a length of 120 μm or more, and the two elements are The upper superconducting thin film (
S2) 6 is formed into a rectangular shape having a width of 60 μm and a length of 120 μm or more, for example. In the configuration example shown in FIG. The conductive thin films S1-0 are each extended and expanded as shown in the figure to constitute end plates, and voltage terminals Va, Va,
In addition to providing Vb to extract a constant DC voltage,
By supplying a DC current 1 between those end plates and, for example, connecting a coaxial feeder line CC to supply a high frequency current 1 of, for example, 20 GHz, a constant DC voltage having a desired constant value can be stably and can be easily generated. As is clear from the above description, according to the present invention, a stable high DC voltage having a wide operating current range can be easily generated using a superconducting closed circuit formed by connecting a large number of Josephson elements in a ring. As a result, a Josephson voltage standard device capable of obtaining a high voltage value suitable for practical use can be easily realized.

[Brief explanation of the drawing]

1A and 1B are equivalent circuit diagrams schematically showing the configuration of the conventional Josephson voltage standard device, FIG. 2 is a characteristic curve diagram showing the mode of operation thereof, and FIG. 3 is the Josephson voltage standard device of the present invention. Equivalent circuit diagram showing the operating principle of
The figure is also a characteristic curve diagram showing an example of the mode of operation.
Figures A and B are a plan view and a longitudinal cross-sectional view, respectively, showing an example of the specific configuration. 1... Substrate, 2... Superconducting ground plane, 3, 5... Oxide insulating film, 4, 6...
...Superconducting thin film, J, J', Jl, J2...
Josefson element...DC voltmeter, Sl, S
2, Sl-0, S2-0...Superconducting thin film, Va
, Vb... Voltage terminal, CC... Coaxial feed line.

Claims (1)

[Claims] 1 A superconducting closed circuit formed by sequentially connecting at least three Josephson elements in a ring, and direct current and high-frequency current being applied directly to one Josephson element in the superconducting closed circuit. A Josephson voltage standard device, characterized in that, by supplying the voltage to both ends of the Josephson element, a DC voltage having a value proportional to the frequency of the high-frequency current supplied to both ends of the Josephson element is generated.