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Planar dc SQUIDs have been made and tested. They consist of a 5mm × 5mm square with a 30μm to 100μm hole, the junctions being near the center of the device. The loop inductance is in the range of 100- 200 pH. The technology uses Nb/NbOx/Pb overlapped junctions and Ta shunt resistors. Coupling is discussed ; we show that the input coupling constant k cannot be obtained by direct measurements and an efficiency constant r = I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> /nI <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> , is introduced where n is the number of turns of the input coil, I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> the input current and I <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> the induced current in the SQUID loop. An experimental efficiency of about 50% has been obtained.
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Garnier et al. (1983) studied this question.
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