Superconducting quantum interference device

[0002] This invention relates to superconducting quantum interference devices used for measurement of a fine magnetic field, electric current, voltage, electromagnetic wave and so on and, more particularly, to a superconducting quantum interference device enabling high sensitivity measurement and spatial resolution but low in noise.

[0003] There is shown in FIG. 7 a structural view of a superconducting quantum interference device (hereinafter abbreviated as SQUID) according to the prior art. On a substrate 5 are integrated Josephson junctions 1, washer coils 2, a detection coil 3, and a feedback-modulation coil 4. The SQUID shown in FIG. 7 is a DC-SQUID including two Josephson junctions in a superconducting loop. The feedback-modulation coil 4 is magnetically coupled to the washer coils 2. A superconducting loop is constituted by the detection coil 3 for external magnetic field detection, the washer coils 2, and a pair of the Josephson junctions 1 is connected at the respective ends of the washer coils.

[0004] The SQUID is connected to an external control system 6 and supplied with a bias current Ib. The external control system 6 feeds back to the superconducting loop a control signal Im corresponding to a SQUID output Vs. The control signal Im is delivered as a magnetic signal to the superconducting loop through the feedback-modulation coil 4 magnetically coupled to the washer coil 2. An output voltage signal Vout corresponding to an external magnetic field, which has been detected by the detection coil 3 is obtained on the external control system.

[0005] In order to obtain high spatial resolution, the detection coil has to be made small in size. Due to this, as shown in FIG. 7 a fine detection coil 3 is integrated, together with the Josephson junctions 1 and washer coils 2, on the substrate.

[0006] Conventionally, the SQUID used for measurement on high spatial resolution has integrated Josephson junctions and washer coils and the like, together with a fine detection coil, on a common substrate. A signal generated by a signal source is detected by the detection coil. Meanwhile, a magnetic field due to the signal source links similarly to the washer coil. In such a case, measurement is made on the basic of a magnetic field linked to both the detection coil and the washer coil. As a consequence, it is difficult to measure with accuracy a magnetic field linked only to the detection coil that is to be measured.

[0007] In the case where using a detection coil having a small detection area, there is a considerable affection of a magnetic field linked to portions other than the detection coil, and particularly the washer coil.

[0008] In order to solve the above problem, the present invention utilizes a superconductor for enclosing a loop formed by a washer coil that is formed at an upper portion or lower portion of the washer coil, or a washer coil formed on a ground plane of a superconducting thin film, thereby preventing a magnetic field caused by a signal source from being linked to portions other than a detection coil.

[0009] FIG. 1 is a structural view showing a superconducting quantum interference device according to Embodiment 1;

[0010] FIG. 2 is an equivalent circuit diagram showing the superconducting quantum interference device according to Embodiment 1;

[0011] FIG. 3 is a structural view showing a superconducting quantum interference device according to Embodiment 2;

[0012] FIG. 4 is an equivalent circuit diagram showing the superconducting quantum interference device according to Embodiment 2;

[0013] FIG. 5 is a structural view showing a superconducting quantum interference device according to Embodiment 3;

[0014] FIG. 6 is an equivalent circuit diagram showing the superconducting quantum interference device according to Embodiment 3; and

[0015] FIG. 7 is a structural view showing a superconducting quantum interference device according to a prior art.

[0016] Now embodiments of the present invention will be described with reference to the drawings.

[0017] FIG. 1 shows a structural view of a superconducting quantum interference device (hereinafter abbreviated as SQUID) showing Embodiment 1 of the invention, while FIG. 2 shows an equivalent circuit diagram of the same. The SQUID shown in FIG. 1 and FIG. 2 is a DC-SQUID including two Josephson junctions in a superconducting loop. On a substrate 5 are integrated Josephson junctions 1, washer coils 2, a detection coil 3, and a feedback-modulation coil 4. The feedback-modulation coil 4 is magnetically coupled to the washer coils 2. A superconducting loop is constituted by the detection coil 3 for magnetic field detection, the washer coils 2, and a pair of the Josephson junctions 1 connected at respective ends of the washer coils. A super conductor to enclose the loop formed by the washer coils 2 is formed as an upper layer or lower layer of the washer coil 2. Alternatively, the washer coils 2 are formed on a ground plane 7. A strip line structure is provided for a detection-coil-and-washer-coil connection line 8, serving as a connection part between the detection coil and washer coils. In FIG. 2, no ground plane layer is depicted.

[0018] The SQUID is connected to an external control system 6, for supplying a bias current Ib. The external control system 6 feeds back to the superconducting loop a control signal Im corresponding to a SQUID output Vs. The control signal Im is delivered as a magnetic signal to the superconducting loop through the feedback-modulation coil 4 magnetically coupled to the washer coils 2. Thus, an output voltage signal Vout corresponding to an external magnetic field, which has been detected by the detection coil 3, is obtained by the external control system 6.

[0019] The SQUID of the present embodiment has the superconductor formed at the upper portion or lower portion of the washer coil 2, to enclose the loop formed by the washer coils 2. Alternatively, the washer coils 2 are formed on the ground plane. Accordingly, the external magnetic field cannot extend through the ground plane. As a consequence, it is possible to prevent linking to the washer coils 2. Meanwhile, the strip line structure for the detection-coil-and-washer-coil connection line 8 reduces magnetic field linking at that position. Due to this, there is no magnetic field coupling at regions other than the detection coil 3. It is therefore possible to conduct measurement only on the magnetic field being linked to the detection coil 3, thus enabling measurement with higher accuracy.

[0020] As a means for high spatial resolution, the detection coil 3 is made small in size. It is particularly effective in use a detection coil with a small detection area.

[0021] FIG. 3 shows a structural view of a superconducting quantum interference device showing Embodiment 2 of the invention, while FIG. 4 shows an equivalent circuit diagram of the same.

[0022] The SQUID of this embodiment is a DC-SQUID of a double washer structure having two washer coils 2 connected oppositely in series to each other. A superconductor is formed at an upper layer or lower layer of the washer coil 2 to enclose the loop formed by the washer coils 2. Alternatively, these two washer coils 2 are formed on a ground plane 7. Respective feedback-modulation coils 4 are magnetically coupled to the washers 2. A superconducting loop is constituted by the detection coil 3, the two washer coils 2, and two Josephson junctions 1. In FIG. 4, no ground plane layer is depicted.

[0023] In this embodiment, besides the washer coils 2 formed on the ground plane and a strip line structure of a coil-and-washer-coil connection line 8, the double washer structure is provided in which the washer coils 2 themselves are in opposite directions. Due to this, it is possible. to efficiently eliminate a magnetic field linking to the washer coils 2.

[0024] FIG. 5 shows a structural view of a superconducting quantum interference device showing Embodiment 3 of the invention, while FIG. 6 shows an equivalent circuit diagram of the same. On a substrate 5 are integrated Josephson junctions 1, a washer coil 2, a detection coil 3, a feedback-modulation coil 4, and an input coil 9. A super conductor is formed as an upper layer or lower layer of the washer coil 2 to enclose a loop formed by the washer coils 2. Alternatively, the washer coils 2 are formed on a ground plane 7. A first superconducting loop is formed by the washer coil 2 and a pair of Josephson junctions 1 connected at respective ends of the washer coil 2.

[0025] The detection coil 3 for magnetic field detection and the input coil 9 are connected through coil-and-washer-coil connection line 8 which has a strip line structure, forming a second superconducting loop. Meanwhile, the input coil 3 is magnetically coupled to the washer coil 2. The magnetic field detected by the detection coil 3 is delivered to the first superconducting loop through the input coil 9. In FIG. 6, no ground plane layer is depicted.

[0026] The SQUID shown in the present embodiment has, similarly, the superconductor formed at the upper portion or lower portion of the washer coil 2, to enclose the loop formed by the washer coil 2. Alternatively, provided are the washer coil 2 formed on the ground plane and the coil-washer-coil connection lines 8 with the strip line structure. Accordingly, it is possible to eliminate magnetic field linking to the washer coil 2. The detection coil 3 and the input coil 9 form a second superconducting loop, which provides a feature to freely select a form for the detection coil. Besides a 1-turn coil, a complicated form of a detection coil can be employed, including, for example, a differential type detection coil.

[0027] The present invention is practiced in the embodied forms as explained above, and has the following effects.

[0028] By forming a superconductor for enclosing a loop to be formed by a washer coil at an upper layer or lower layer of the washer coil, or otherwise forming on a ground plane a washer coil used for delivering a signal sent from an external control system to a superconducting loop through a feedback-modulation coil, it is possible to prevent an external magnetic field from being linked to the washer coil. Due to this, there is no magnetic coupling in regions other than a detection coil, and measurement is possible only on a magnetic field linked to the detection coil. Thus, measurement is possible with higher accuracy. Although detection coil size reduction is implemented as a means to enhance spatial resolution, the invention is effective for a reduced area of a detection coil thus suited for measurement with high spatial resolution.

[0029] Also, by forming a superconductor for enclosing a washer coil in an upper layer or lower layer of the washer coil with an interlayer insulation layer sandwiched, that is, by forming a ground plane layer, it is possible to closely provide the washer coil and the shielding superconductor, enhancing the magnetic field shielding effect.

[0030] Also, by using a material having a critical magnetic field greater than a critical magnetic field of the superconducting thin film forming the Josephson junction and washer coil, it is possible to provide a magnetic field shielding effect for the washer coil. At the same time, use in an intense magnetic field becomes possible without losing superconductivity.

[0031] Furthermore, by forming the superconductor for enclosing a loop to be formed by the washer coil of a first kind superconductor material, it becomes possible to completely shield a magnetic field at and under a critical magnetic field for the first kind superconductor.