Noise canceling high-sensitive magnetometer
FIELD OF THE INVENTION 
     This invention relates to magnetometers using the superconducting quantum interference device(SQUID) and particurally to magnetometers which make it possible to detect very weak magnetic flux with low noises and high sensitivity. 
     BACKGROUND OF THE INVENTION 
     The magnetometers, each of which uses the SQUID comprising a superconducting loop having one or more Josephson junctions, have been proposed(for example, see IEEE. Trans. Devices, ED27, No.10(1980) pp.1896˜1908.). The magnetometers can detect very weak magnetic flux and can be used in the fields of detection of biological magnetic field, geophysics, surveying of natural resouces and so on. 
     SUMMARY OF THE INVENTION 
     It has been desired for the SQUID magnetometer to detect magnetic flux with high sensitivity and low noises. In other words, the object of this invention is to improve the sensivity of the SQUID magnetometer and reduce the equivalent input noises thereof. 
     The above problem will be solved by this high-sensitive magnetometer which has two SQUIDs, each comprising a superconducting loop with one or more Josephson junctions and also has a coil comprising a continuous loop for providing magnetic flux of input signals to each of the SQUIDs in opposite directions. 
     As the SQUID, a DC-SQUID having a superconducting loop with two or more Josephson junctions or an RF-SQUID having a superconducting loop with one Josephson junction can be used. 
     According to this invention, magnetic flux noises present inside the SQUID loops are mutually sensed by each SQUID. These noises generate output voltages across each SQUID. But, these voltages can be added destructively. Thus the magnetic flux noises are cancelled. In contrast, signals produce constructive additions of voltages. Then the signals create large output voltages with higher output impedance, as compared with the case where only one SQUID is used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a circuit diagram of an example of this invention, and 
     FIG. 2 shows a graph showing a relation, in the example of FIG. 1, between a magnetic flux noise and output voltages with respect to the common ground (&#34;4&#34; in FIG. 1). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, we will explain an example of this invention with reference to the accompanying drawings. FIG. 1 shows a circuit diagram of a magnetometer according to this invention. In this example, the SQUIDs are DC-SQUIDs 1 and 1&#39; made by two superconducting films and and an insulation film therebetween. 
     The DC-SQUIDs 1 and 1&#39; are respectively formed by connecting superconducting films 2 and 2&#39; to another superconducting film 4 through two Josephson junctions 3 and 3&#39;. Close to the superconducting films 2 and 2&#39; of the DC-SQUIDs 1 and 1&#39;, provided are input coils 5 and 5&#39;, the winding directions of which are opposite. These input coils 5 and 5&#39; are connected in serial to form one loop and a terminal 6 is led out from the middle point of the loop. When signals caught by a pick-up coil are inputted from the terminal 6 of the input coils, these signals are equally divided and introduced into the input coils 5 and 5&#39;. Since the winding directions of the coils 5 and 5&#39; are opposite, the directions of magnetic flux passing through the respective DC-SQUIDs become opposite. The output signals of the magnetometer are taken across the terminals 7 and 7&#39; as output voltages of two DC-SQUIDs. The bias currents from two direct current sources 8 and 8&#39; are supplied to the DC-SQUIDs 1 and 1&#39; through terminals 9, 9&#39; and 9&#34;. The directions of the bias currents are set so that voltages of opposite polarities with respect to the portion 4 are generated at the terminals 7 and 7&#39;. The reference numerals 10 and 10&#39; are coils for feedback modulation or extra input coils. 
     In the magnetometer as constructed above, the magnetic flux transmittance between the input coils 5 and 5&#39; has an ability for removing magnetic flux noises inside the DC-SQUIDs 1 and 1&#39;. In this point, it is preferable that the couplings between the SQUIDs 1 and 1&#39; and the input coils 5 and 5&#39; are the tight couplings. The flux noises are removed as described below. 
     FIG. 2 shows a characteristic between magnetic flux and detection voltages in the example shown in FIG. 1. The curves A and B show the output voltages of the SQUID 1 and the SQUID 1&#39;, respectively, with respect to a voltage reference at the terminal 4. The DC-SQUIDs have to be biased into an operating point where a flux applied at the input results in a voltage variation across the terminals 7-7&#39;. An example of how the magnetometer can be biased is shown in FIG. 2. The SQUlD 1 is biased by Φ biasA and the SQUID 1&#39; by Φ biasB. 
     The respective output voltages corresponding to a magnetic flux noise Φ n inside the SQUIDs 1 and 1&#39; are indicated by n and n&#39;. By measuring voltage across 7 and 7&#39;, the voltages n and n&#39;(voltages produced by the magnetic flux noise Φ n) are cancelled. 
     On the other hand, the signal magnetic flux Φ s is applied to the SQUIDs in opposite directions. By measuring the voltage across 7 and 7&#39;, the signal voltages S and S&#39; are added constructively, resulting in a large output voltage swing and a large output impedance as compared with the case that a single DC-SQUID is used. 
     In the above example, the invention has been described concerning the magnetometer using the DC-SQUIDs. The magnetometer using the RF-SQUIDs can be also constructed according to the same principle and brings the same advantages. The output signals of the magnetometer using the RF-SQUIDs can be obtained by adding output voltages generated across resonance(tank) circuits which are magnetically connected to the respective RF-SQUIDs through output coils of the corresponding resonance circuits. Since the circumferential circuits such as the resonance circuits magnetically connected to RF-SQUIDs through the output coils and an alternative current magnetic flux bias supply means for supplying an alternative current magnetic flux bias to the RF-SQUIDs through the resonance circuits are well known to those skilled in the art, these are not explained in detail herein. If necessary, refer to &#34;Introduction to cryo-electronics&#34; written by Akira Nakamura, Published by Ohm-sha(In Japanese). 
     According to this invention, high-sensitive magnetometers can be realized. Therefore, eminent advantages can be obtained in diagnoses using the biological magnetic measurements, magnetic inspection of products and so on.